<html>

<head>
<meta http-equiv="Content-Type" content="text/html; charset=windows-1252">
<meta http-equiv="Content-Language" content="en-us">
<meta name="GENERATOR" content="Microsoft FrontPage 4.0">
<meta name="ProgId" content="FrontPage.Editor.Document">
<title>Mutatis mutandis - Using Preference Pages as Property Pages</title>
<link rel="stylesheet" href="../default_style.css">
</head>

<body LINK="#0000ff" VLINK="#800080">
<div align="right">&nbsp; <font face="Times New Roman, Times, serif" size="2">Copyright 
  &copy; 2003 Berthold Daum.</font> 
  <table border=0 cellspacing=0 cellpadding=2 width="100%">
    <tr> 
      <td align=LEFT valign=TOP colspan="2" bgcolor="#0080C0"><b><font face="Arial,Helvetica"><font color="#FFFFFF">&nbsp;Eclipse 
        Corner Article</font></font></b></td>
    </tr>
  </table>
</div>
<div align="left"> 
  <h1><img src="images/Idea.jpg" height=86 width=120 align=CENTER></h1>
</div>
<p>&nbsp;</p>

<h1 ALIGN="CENTER">Mutatis mutandis - Using Preference Pages as Property Pages</h1>

<blockquote>
<b>Summary</b>

<br>
A common problem in the implementation of applications is the implementation of
project-specific properties that override workbench-wide preferences on project or file level. The
naive approach is to implement these pages from scratch. However, writing the
same code twice is a boring task and leads to increased maintenance efforts. In
this article we show how existing preferences pages (with or without field
editors) can be easily converted into pages that can act as both preference and
property pages. We demonstrate this by implementing the abstract class <code> FieldEditorOverlayPage</code>
providing the necessary functionality. 
  <p><b>Berthold Daum, bdaum industrial communications</b> <br>
    October 24, 2003 </p>
</blockquote>

<hr width="100%">
<h2>The problem</h2>
<p>Implementing <i>Preference Pages</i> for Eclipse plug-ins is not a difficult 
  task, especially if you use field editors. Such preference pages are usually 
  implemented as subclasses of <code>org.eclipse.jface.preference.PreferencePage</code> 
  and <code>org.eclipse.jface.preference.FieldEditorPreferencePage</code>. There 
  are already two excellent articles on <i> Eclipse Corner</i> dealing with this 
  subject: <a href="../Article-Preferences/preferences.htm">Preferences in the 
  Eclipse Workbench UI</a> by Tod Creasey and <a href="../Article-Field-Editors/field_editors.html">Simplifying 
  Preference Pages with Field Editors</a> by Ryan Cooper. In this article I will 
  not go into details with the implementation of preference pages but assume that 
  you are already fairly familiar with standard implementation techniques of such 
  pages.</p>
<p>The problem that I want to discuss here begins after you already have implemented
preference pages for your plug-in. At some stage you notice that some or all of your plug-in
preferences should rather be project specific or even file specific. The usual
way to implement such project or file specific settings are <i>Property Pages</i>,
which usually are subclasses of <code>org.eclipse.ui.dialogs.PropertyPage</code>.
The Eclipse workbench already contains some examples of how property pages are used to override workbench settings on project level. One example are the <i> Java
Compiler</i> settings. These settings can be controlled in the workbench
preferences but can be overridden for each
Java project separately by project specific property pages.&nbsp;</p>
<table border="0">
  <tr>
    <td width="50%"><img border="0" src="images/PreferencePage.GIF" width="441" height="380">
      <p><i>The Java compiler settings in the workbench preferences</i>
      <p><i>&nbsp;</i></td>
  </tr>
  <tr>
    <td width="50%"><img border="0" src="images/PropertyPage.GIF" width="440" height="355">
      <p><i>The Java compiler settings as a property page of a Java project</i></td>
  </tr>
</table>
<p>As we see, both preference page and property page look, in fact, very similar. There are some differences, however,
at the top of the page. Instead of the page description, the property page
contains two radio buttons for toggling between workbench settings and project settings. It also features a button for direct access to the
corresponding workbench preference page. But its main body (in this case the
tabbed notebook) is identical with the main body of the corresponding preference
page.</p>
<h2>Possible solutions</h2>
<p>An obvious solution to this problem is to implement both pages from scratch.
However, since the main body of the both pages is identical, this would result
in a large portion of duplicated code, code that is subject to later maintenance
and thus increases maintenance costs. After all, object-oriented programming is
about reuse, isn't it?</p>
<p>Well, why don't we factor out the main body of both pages into a separate
class? This is actually the way in which both pages shown above are implemented.
(The tabbed notebook is implemented in JDT class <code>CompilerConfigurationBlock</code>.)
But this technique is not trivial. What I did not tell you is that preference
pages and property pages are based on different data models. Workbench preferences
are usually stored in plug-in specific but workbench-wide preference stores. Properties,
in contrast, are stored as resource related properties. The interface <code>org.eclipse.core.resources.IResource</code>
provides the necessary access method. When a resource is deleted, also its
properties cease to exist. This different way of data management would require
us to implement two variants of access routines, if we want to use preference
pages in the role of property pages. For property pages we would need to direct all field accesses
to the respective resource (project or file), for preference pages we would need
to direct all field accesses to the plug-in specific preference store. Unfortunately, this
strategy is not possible for preference pages utilizing field editors. Field
editors internally always access a preference store - we cannot tell them to
access resource properties instead. So we would need to re-implement the whole
property page - replacing the field editors with standard SWT widgets! Not very nice.</p>
<p>Fortunately, there is another solution and it works for field editors, too.
To get to this solution we have to look at the inheritance trees of both field
editor preference pages and property pages. (We will deal with the problem of
field editor preferences pages first and will discuss &quot;normal&quot;
preference pages later.)</p>
<table border="0">
  <tr>
    <td width="50%" valign="top"><img border="0" src="images/FieldEditorPreferenceHierarchy.GIF" width="225" height="169"></td>
    <td width="50%" valign="top"><img border="0" src="images/PropertyHierarchy.GIF" width="220" height="181"></td>
  </tr>
</table>
<p>Both types are extensions of the abstract class <code>PreferencePage</code>.
The class <code>FieldEditorPreferencePage</code> implements, in addition, the
interface <code>IPropertyChangeListener</code>, while the class <code>PropertyPage</code>
additionally implements the interface <code>IWorkbenchPropertyPage</code>. Since
the extension from <code>PreferencePage</code> to <code>PropertyPage</code> is
minimal (the implementation of <code>IWorkbenchPropertyPage</code> is
trivial), it makes sense to base our solution on class <code>FieldEditorPreferencePage</code>
and simply add the implementation of <code>IWorkbenchPropertyPage</code>.</p>
<p>But how do we deal with the problem of different data models for property
pages and preference pages? Since field editors can only access preference
stores, we simply create a temporary preference store (which we call <i>overlay store</i>
within this article)
when a property page is initialized. We implement this store by extending class <code>PreferenceStore</code>
but provide different semantics to its access methods. In particular, when
fetching a value from the store, we look into the resource properties first if
we can find an identically named value there. Only if such a value is not found,
we refer to the underlying workbench preference store. The <code>save()</code>
methods of this store have different semantics, too: they must store the values
contained in the overlay store into the resource properties instead of saving them as preferences. So,
our field editors have only to deal with a preference store and are perfectly
happy.</p>
<h2>Implementing class PropertyStore</h2>
<p>The class <code> PropertyStore</code> implements the overlay store and extends the class
<code>PreferenceStore</code>. Each instance of <code>
PropertyStore</code> represents the properties of a specific resource in context
of a specific
property page but as an incarnation of a property store. Because the store is
resource and page specific, we pass the <img src="images/tag_1.gif" height=13 width=24 align=CENTER>
resource and a <img src="images/tag_3.gif" height=13 width=24 align=CENTER>
page identification in the constructor:</p>
<pre><b><font COLOR="#7f0055">	public</font></b> <b><font COLOR="#7f0055">class</font></b> PropertyStore <b><font COLOR="#7f0055">extends</font></b> PreferenceStore {
<img src="images/tag_1.gif" height=13 width=24 align=CENTER>	  <b><font COLOR="#7f0055">private</font></b> IResource resource;
<img src="images/tag_2.gif" height=13 width=24 align=CENTER>	  <b><font COLOR="#7f0055">private</font></b> IPreferenceStore workbenchStore;
<img src="images/tag_3.gif" height=13 width=24 align=CENTER>	  <b><font COLOR="#7f0055">private</font></b> String pageId;</pre>
<pre>	  <b><font COLOR="#7f0055">public</font></b> PropertyStore(IResource resource, 
                               IPreferenceStore workbenchStore, 
                               String pageId) {
	    <font COLOR="#7f0055"><b>this</b></font>.resource = resource;
	    <b><font COLOR="#7f0055">this</font></b>.workbenchStore = workbenchStore;
	    <b><font COLOR="#7f0055">this</font></b>.pageId = pageId;
	}</pre>
<p>We also pass the underlying <img src="images/tag_2.gif" height=13 width=24 align=CENTER>
workbench preference store. When a preference value is not available as a
resource property value, we will route accesses through to this
store.</p>
<h5>Getters and Setters</h5>
<p>Now, let's first discuss the <code>get...()</code> accessors. The class <code>PreferenceStore</code>
has an awful lot of them, one for each primitive Java type and one for <code>java.lang.String</code>. Here, we show only
the accessor for type <code>String</code> but leave the others to your
imagination. Let's start with the accessor for the default values:</p>
<pre><b><font COLOR="#7f0055">	public</font></b> String getDefaultString(String name) {
	  <b><font COLOR="#7f0055">return</font></b> workbenchStore.getDefaultString(name);
	}</pre>
<p>Well, that was easy enough. Because we don't store any default values in <code>PropertyStore</code>
we just fetch the default values from the workbench preference store. Next are
the accessors for non-default values:</p>
<pre><b><font COLOR="#7f0055">	public</font></b> String getString(String name) {
	  insertValue(name);
	  <font COLOR="#7f0055"><b>return</b></font> <b><font COLOR="#7f0055">super</font></b>.getString(name);
	}</pre>
<p>That seems easy enough, too, but what's in <code>insertValue()</code> ?</p>
<pre>	<b><font COLOR="#7f0055">private</font></b> <b><font COLOR="#7f0055">boolean</font></b> inserting = <b><font COLOR="#7f0055">false</font></b>;</pre>
<pre><b><font COLOR="#7f0055">	private</font></b> <b><font COLOR="#7f0055">synchronized</font></b> <b><font COLOR="#7f0055">void</font></b> insertValue(String name) {
<img src="images/tag_1.gif" height=13 width=24 align=CENTER>	  <b><font COLOR="#7f0055">if</font></b> (inserting)
	    <b><font COLOR="#7f0055">return</font></b>;
<img src="images/tag_2.gif" height=13 width=24 align=CENTER>	  <b><font COLOR="#7f0055">if</font></b> (<b><font COLOR="#7f0055">super</font></b>.contains(name))
	    <b><font COLOR="#7f0055">return</font></b>;
<img src="images/tag_1.gif" height=13 width=24 align=CENTER>	  inserting = <b><font COLOR="#7f0055">true</font></b>;
	  String prop = <b><font COLOR="#7f0055">null</font></b>;
	  <b><font COLOR="#7f0055">try</font></b> {
<img src="images/tag_3.gif" height=13 width=24 align=CENTER>	    prop = getProperty(name);
	  } <b><font COLOR="#7f0055">catch</font></b> (CoreException e) {
	  }
	  <font COLOR="#7f0055"><b>if</b></font> (prop == <b><font COLOR="#7f0055">null</font></b>)
<img src="images/tag_4.gif" height=13 width=24 align=CENTER>	    prop = workbenchStore.getString(name);
	  <font COLOR="#7f0055"><b>if</b></font> (prop != <b><font COLOR="#7f0055">null</font></b>)
<img src="images/tag_5.gif" height=13 width=24 align=CENTER>	    setValue(name, prop);
	  inserting = <b><font COLOR="#7f0055">false</font></b>;
	}</pre>
<pre><b><font COLOR="#7f0055">	private</font></b> String getProperty(String name) <b><font COLOR="#7f0055">
                              throws</font></b> CoreException {
<img src="images/tag_6.gif" height=13 width=24 align=CENTER>	  <b><font COLOR="#7f0055">return</font></b> resource.getPersistentProperty(<b><font COLOR="#7f0055">
                          new</font></b> QualifiedName(pageId, name));
	}</pre>
<p>Not easy at all! It starts with a synchronized method and a <img src="images/tag_1.gif" height=13 width=24 align=CENTER>
semaphore (<code>inserting</code>). This is necessary to avoid recursions.
Method calls inside the <code>insertValue()</code> body internally call <code>getString()</code>
which would lead to unlimited recursion and a stack overflow. The semaphore
inhibits just this. To make this logic thread safe we have declared this method
as synchronized.</p>
<p>After we have made sure that this is not a recursive call we first look if the value is already stored in the <img src="images/tag_2.gif" height=13 width=24 align=CENTER>
 local store instance. If yes, we do
nothing. If not, we try to <img src="images/tag_3.gif" height=13 width=24 align=CENTER>
read a property with the same name. We always <img src="images/tag_6.gif" height=13 width=24 align=CENTER>
 qualify the names of properties with the page identification to avoid name clashes between
properties from different property pages. If such a property value does not
exist, we get the value from the <img src="images/tag_4.gif" height=13 width=24 align=CENTER>
workbench preference store. Then we <img src="images/tag_5.gif" height=13 width=24 align=CENTER>
cache this value in the local store.&nbsp;Sooner or later, the value of all
fields in a given page will end up in this local core: all field editors will
call a <code>get...()</code> method in order to display a value on the
page.&nbsp;</p>
<p>What about the corresponding <code>set...() </code>methods? Fortunately, we
don't have to override these methods because all changes are applied directly to
the local store. However, a few other <code>PreferenceStore</code> methods need
consideration.</p>
<h5>More methods</h5>
<p>In particular, we have to modify the methods <code>contains()</code>, <code>isDefault()</code>
and <code>setToDefault()</code>. Let's start with method <code>contains()</code>:</p>
<pre><b><font COLOR="#7f0055">	public</font></b> <b><font COLOR="#7f0055">boolean</font></b> contains(String name) {
<img src="images/tag_1.gif" height=13 width=24 align=CENTER>	  <b><font COLOR="#7f0055">return</font></b> workbenchStore.contains(name);
	}</pre>
<p>Here, we just <img src="images/tag_1.gif" height=13 width=24 align=CENTER> delegate to the workbench preference store assuming that
the property page is an exact replica of the preference page.</p>
<p>Default values need a bit more attention. Above, we had simply delegated <code>getDefault...()</code>
calls to the workbench preference store. But what if we want to reset a
property to its default value? Standard preference stores internally contain two
sets of properties: one for the default values and one for the non-default
values. The second set only contains a value if it is unequal to the default value.
So, when a value is reset to its default value, it is sufficient to remove it
from the set of non-default values. In our case, however, we need a different
implementation. We must explicitly <img src="images/tag_1.gif" height=13 width=24 align=CENTER>
store property values that equal the default value. So we are able to set
resource level properties to their default while on workbench level they still
have a non-default value:</p>
<pre><b><font COLOR="#7f0055">	public</font></b> <b><font COLOR="#7f0055">void</font></b> setToDefault(String name) {
<img src="images/tag_1.gif" height=13 width=24 align=CENTER>	    setValue(name, getDefaultString(name));
	}</pre>
<p>This has implications for the method <code>isDefault()</code>. The standard
implementation just tests if the specified property name is contained in the set
of non-default values. In our case, however, this is not sufficient because this
value set may contain default values, too. We must therefore explicitly compare <img src="images/tag_2.gif" height=13 width=24 align=CENTER>
 non-default values with <img src="images/tag_1.gif" height=13 width=24 align=CENTER>
default values:</p>
<font>
<pre><b><font COLOR="#7f0055">	public</font></b> <b><font COLOR="#7f0055">boolean</font></b> isDefault(String name) {
<img src="images/tag_1.gif" height=13 width=24 align=CENTER>	  String defaultValue = getDefaultString(name);
	  <font COLOR="#7f0055"><b>if</b></font> (defaultValue == <b><font COLOR="#7f0055">null</font></b>) <b><font COLOR="#7f0055">return</font></b> <b><font COLOR="#7f0055">false</font></b>;
<img src="images/tag_2.gif" height=13 width=24 align=CENTER>	  <font COLOR="#7f0055"><b>return</b></font> defaultValue.equals(getString(name));
	}</pre>
<h5>Updating Resource Properties</h5>
<p>What remains to do is overriding both <code>save()</code>
 methods of class <code>PreferenceStore</code>. In contrast to the standard save operation we write all
cached values to
the resource properties:</p>
<pre><b><font COLOR="#7f0055">	public</font></b> <b><font COLOR="#7f0055">void</font></b> save() <b><font COLOR="#7f0055">throws</font></b> IOException {
	  writeProperties();
	}</pre>
<pre><font COLOR="#7f0055">	<b>public</b></font> <b><font COLOR="#7f0055">void</font></b> save(OutputStream out, String header) <b><font COLOR="#7f0055">
                                         throws</font></b> IOException {
	  writeProperties();
	}</pre>
<pre><font COLOR="#7f0055">	<b>private</b></font> <b><font COLOR="#7f0055">void</font></b> writeProperties() <b><font COLOR="#7f0055">throws</font></b> IOException {
<img src="images/tag_1.gif" height=13 width=24 align=CENTER>	  String[] preferences = <b><font COLOR="#7f0055">super</font></b>.preferenceNames();
	  <b><font COLOR="#7f0055">for</font></b> (<b><font COLOR="#7f0055">int</font></b> i = 0; i &lt; preferences.length; i++) {
	    String name = preferences[i];
	    <font COLOR="#7f0055"><b>try</b></font> {
<img src="images/tag_2.gif" height=13 width=24 align=CENTER>	      setProperty(name, getString(name));
	    } <b><font COLOR="#7f0055">catch</font></b> (CoreException e) {
	      <b><font COLOR="#7f0055">throw</font></b> <b><font COLOR="#7f0055">new</font></b> IOException(<font COLOR="#2a00ff">
                    &quot;Cannot write resource property &quot;</font> + name);
	    }
	  }
	}</pre>
<pre><b><font COLOR="#7f0055">	private</font></b> <b><font COLOR="#7f0055">void</font></b> setProperty(String name, String value) <b><font COLOR="#7f0055">
                                         throws</font></b> CoreException {
<img src="images/tag_3.gif" height=13 width=24 align=CENTER>	  resource.setPersistentProperty(<b><font COLOR="#7f0055">
                       new</font></b> QualifiedName(pageId, name), value);
	}</pre>
<p>Here, we first get the <img src="images/tag_1.gif" height=13 width=24 align=CENTER>
 names of all values in the local preference store. Then we <img src="images/tag_2.gif" height=13 width=24 align=CENTER>
 write each of them into the resource properties as shown above. Again, we need to <img src="images/tag_3.gif" height=13 width=24 align=CENTER>
 qualify the property name with the page identification.</p>
<p>We are now done with the implementation of class <code>PropertyStore</code>
and turn our attention to GUI issues.</p>
<h2>Implementing class FieldEditorOverlayPage</h2>
<p>The class <code>FieldEditorOverlayPage</code> can act as a common superclass for all field editor preference pages that
also
want to behave as property pages.</p>
<h5>A place in the type hierarchy</h5>
<p> As we have already discussed above this
class <img src="images/tag_1.gif" height=13 width=24 align=CENTER> extends class <code>FieldEditorPreferencePage</code> and additionally
<img src="images/tag_2.gif" height=13 width=24 align=CENTER> implements <code>IWorkbenchPropertyPage</code>.
We
<img src="images/tag_3.gif" height=13 width=24 align=CENTER> replicate all constructors of class <code>FieldEditorPreferencePage</code> but
<img src="images/tag_4.gif" height=13 width=24 align=CENTER>	     save the image passed in
the third constructor for later use:</p>
<pre><b><font COLOR="#7f0055">	public</font></b> <b><font COLOR="#7f0055">abstract</font></b> <b><font COLOR="#7f0055">class</font></b> FieldEditorOverlayPage
<img src="images/tag_1.gif" height=13 width=24 align=CENTER>	                <font COLOR="#7f0055"><b>extends</b></font> FieldEditorPreferencePage
<img src="images/tag_2.gif" height=13 width=24 align=CENTER>                     <b><font COLOR="#7f0055">implements</font></b> IWorkbenchPropertyPage</pre>
<pre><img src="images/tag_3.gif" height=13 width=24 align=CENTER><b><font COLOR="#7f0055">	public</font></b> FieldEditorOverlayPage(<b><font COLOR="#7f0055">int</font></b> style) {
	  <b><font COLOR="#7f0055">super</font></b>(style);
	}</pre>
<pre><b><font COLOR="#7f0055">	public</font></b> FieldEditorOverlayPage(String title, <b><font COLOR="#7f0055">int</font></b> style) {
	  <b><font COLOR="#7f0055">super</font></b>(title, style);
	}</pre>
<pre><b><font COLOR="#7f0055">	private</font></b> ImageDescriptor image;</pre>
<pre><b><font COLOR="#7f0055">	public</font></b> FieldEditorOverlayPage(String title, 
                                      ImageDescriptor image, <b><font COLOR="#7f0055">
                                      int</font></b> style) {
	  <b><font COLOR="#7f0055">super</font></b>(title, image, style);
<img src="images/tag_4.gif" height=13 width=24 align=CENTER>	  <b><font COLOR="#7f0055">this</font></b>.image = image;
	}</pre>
<p>The implementation of interface <code>IWorkbenchPropertyPage</code>  is
trivial, indeed:</p>
<pre><img src="images/tag_1.gif" height=13 width=24 align=CENTER>	<b><font COLOR="#7f0055">private</font></b> IAdaptable element;</pre>
<pre><img src="images/tag_2.gif" height=13 width=24 align=CENTER>	<font COLOR="#7f0055"><b>public</b></font> <b><font COLOR="#7f0055">void</font></b> setElement(IAdaptable element) {
	  <b><font COLOR="#7f0055">this</font></b>.element = element;
      	}</pre>
<pre>   	<b><font COLOR="#7f0055">public</font></b> IAdaptable getElement() {
	  <font COLOR="#7f0055"><b>return</b></font> element;
      	}</pre>
<p>The method <img src="images/tag_2.gif" height=13 width=24 align=CENTER>
<code>setElement()</code> is called when a property page is opened. In our
case, when the property page is called on a project, folder, or file, the <code>
IAdaptable</code> that is passed as an argument is actually of type <code>IResource</code>.
We simply store this element in an <img src="images/tag_1.gif" height=13 width=24 align=CENTER>
 instance variable. Obviously, when this variable is not <code>null</code>, the
current instance of <code>FieldEditorOverlayPage</code>
represents a property page, otherwise a preference page. We express this fact in
the following method:</p>
<pre><b><font COLOR="#7f0055">	public</font></b> <b><font COLOR="#7f0055">boolean</font></b> isPropertyPage() {
	  <b><font COLOR="#7f0055">return</font></b> element != <b><font COLOR="#7f0055">null</font></b>;
	}</pre>
<p>We can use this method, for example, in subclasses that want to vary the page
content depending on if it is a property page or a preference page.</p>
<h5>Additional GUI elements</h5>
<p>Also the GUI elements of class <code>FieldEditorOverlayPage</code> are
variable. If the page instance represents a property page, we want the button
group for project/workbench selection at the top. If it is a preference page, we
don't. Depending on the state of these buttons we must enable or
disable the field editors, too.</p>
<p>Let's start with this button group. To add these buttons to a property page we
extend the method <code>createContents()</code>:</p>
<pre><b><font COLOR="#7f0055">	public static</font></b> <b><font COLOR="#7f0055">final</font></b> String USEPROJECTSETTINGS = <font COLOR="#2a00ff">
                                     &quot;useProjectSettings&quot;</font>;</pre>
<pre><b><font COLOR="#7f0055">	private</font></b> Button useWorkspaceSettingsButton, 
	               useProjectSettingsButton,
	               configureButton;</pre>
<pre><b><font COLOR="#7f0055">	protected</font></b> Control createContents(Composite parent) {
<img src="images/tag_1.gif" height=13 width=24 align=CENTER>	  <font COLOR="#7f0055"><b>if</b></font> (isPropertyPage())
	    createSelectionGroup(parent);
	  <font COLOR="#7f0055"><b>return</b></font> <b><font COLOR="#7f0055">super</font></b>.createContents(parent);
	}</pre>
<pre><b><font COLOR="#7f0055">	private</font></b> void createSelectionGroup(Composite parent) {
	  Composite comp = <b><font COLOR="#7f0055">new</font></b> Composite(parent, SWT.NONE);
	  GridLayout layout = <b><font COLOR="#7f0055">new</font></b> GridLayout(2, <b><font COLOR="#7f0055">false</font></b>);
	  layout.marginHeight = 0;
	  layout.marginWidth = 0;
	  comp.setLayoutData(<b><font COLOR="#7f0055">new</font></b> GridData(GridData.FILL_HORIZONTAL));
<img src="images/tag_2.gif" height=13 width=24 align=CENTER>	  Composite radioGroup = <b><font COLOR="#7f0055">new</font></b> Composite(comp, SWT.NONE);
	  radioGroup.setLayout(<b><font COLOR="#7f0055">new</font></b> GridLayout());
	  radioGroup.setLayoutData(<b><font COLOR="#7f0055">new</font></b> GridData(GridData.FILL_HORIZONTAL));
	  useWorkspaceSettingsButton =
	           createRadioButton(radioGroup, <font COLOR="#2a00ff">&quot;Use workspace settings&quot;</font>);
	  useProjectSettingsButton =
	           createRadioButton(radioGroup, <font COLOR="#2a00ff">&quot;Use project settings&quot;</font>);
<img src="images/tag_3.gif" height=13 width=24 align=CENTER>	  configureButton = <b><font COLOR="#7f0055">new</font></b> Button(comp, SWT.PUSH);
	  configureButton.setText(<font COLOR="#2a00ff">&quot;Configure Workspace Settings ...&quot;</font>);
	  configureButton.addSelectionListener(<b><font COLOR="#7f0055">new</font></b> SelectionAdapter() {
	    <b><font COLOR="#7f0055">public</font></b> <b><font COLOR="#7f0055">void</font></b> widgetSelected(SelectionEvent e) {
<img src="images/tag_4.gif" height=13 width=24 align=CENTER>	      configureWorkspaceSettings();
	    }
	  });
<b><font COLOR="#7f0055">	  try</font></b> {
<img src="images/tag_5.gif" height=13 width=24 align=CENTER>	    String use = ((IResource) element).getPersistentProperty(
	                   <font COLOR="#7f0055"><b>new</b></font> QualifiedName(pageId, USEPROJECTSETTINGS));
<img src="images/tag_6.gif" height=13 width=24 align=CENTER>	    <b><font COLOR="#7f0055">if</font></b> (&quot;true&quot;.equals(use)) {
	      useProjectSettingsButton.setSelection(<b><font COLOR="#7f0055">true</font></b>);
	      configureButton.setEnabled(<b><font COLOR="#7f0055">false</font></b>);
	    } <b><font COLOR="#7f0055">else
</font></b>	      useWorkspaceSettingsButton.setSelection(<b><font COLOR="#7f0055">true</font></b>);
	  } <b><font COLOR="#7f0055">catch</font></b> (CoreException e) {
	    useWorkspaceSettingsButton.setSelection(<b><font COLOR="#7f0055">true</font></b>);
	  }
	}</pre>
<p>This code is straightforward. If the current instance is a <img src="images/tag_1.gif" height=13 width=24 align=CENTER>
property page, we create a group of <img src="images/tag_2.gif" height=13 width=24 align=CENTER>
two radio buttons and a <img src="images/tag_3.gif" height=13 width=24 align=CENTER>
push button. When this button is pressed,
the <img src="images/tag_4.gif" height=13 width=24 align=CENTER> method <code>configureWorkspaceSettings()</code>
 is called for configuring the corresponding workspace settings. (We will discuss this method later.)&nbsp;</p>
<p>After the controls have been created they are <img src="images/tag_6.gif" height=13 width=24 align=CENTER>
initialized. To do so we <img src="images/tag_5.gif" height=13 width=24 align=CENTER>
fetch the resource property <code>USEPROJECTSETTINGS</code>.
Because this setting may be different for each single property page, we qualify
the name of this property with the page identification.</p>
<p>Both radio buttons are created via the convenience method <code>createRadioButton()</code>:</p>
<pre><b><font COLOR="#7f0055">	private</font></b> Button createRadioButton(Composite parent, String label) {
	  <font COLOR="#7f0055"><b>final</b></font> Button button = <b><font COLOR="#7f0055">new</font></b> Button(parent, SWT.RADIO);
	  button.setText(label);
	  button.addSelectionListener(<b><font COLOR="#7f0055">new</font></b> SelectionAdapter() {
	    <b><font COLOR="#7f0055">public</font></b> <b><font COLOR="#7f0055">void</font></b> widgetSelected(SelectionEvent e) {
<img src="images/tag_1.gif" height=13 width=24 align=CENTER>	      configureButton.setEnabled(button == useWorkspaceSettingsButton);
<img src="images/tag_2.gif" height=13 width=24 align=CENTER>	      updateFieldEditors();
	    }
	  });
	  <b><font COLOR="#7f0055">return</font></b> button;
	}</pre>
<p>When a radio button is pressed, we <img src="images/tag_1.gif" height=13 width=24 align=CENTER>
enable or disable
the button for the workbench configuration (enabled when we use the workbench
settings), and we <img src="images/tag_2.gif" height=13 width=24 align=CENTER> enable or disable the field editors (disabled when the
workbench settings are activated).&nbsp;</p>
<p> However, this confronts us with a problem. We have to know
the page's field editors. Our parent class <code>FieldEditorPreferencePage</code>
has this knowledge, but unfortunately it keeps this knowledge close to itself.
We must therefore keep track of the field editor ourselves. To do so, we
override the method <img src="images/tag_2.gif" height=13 width=24 align=CENTER>
<code>addField()</code> and add each field editor added to the page to a <img src="images/tag_1.gif" height=13 width=24 align=CENTER>
list:</p>
<pre><img src="images/tag_1.gif" height=13 width=24 align=CENTER><b><font COLOR="#7f0055">	</font><font COLOR="#7f0055">private</font></b> List editors = <b><font COLOR="#7f0055">new</font></b> ArrayList();</pre>
<pre><img src="images/tag_2.gif" height=13 width=24 align=CENTER><b><font COLOR="#7f0055">	protected</font></b> <b><font COLOR="#7f0055">void</font></b> addField(FieldEditor editor) {
	  editors.add(editor);
	  <font COLOR="#7f0055"><b>super</b></font>.addField(editor);
	}</pre>
<p>Now, we can implement the method <code>updateFieldEditors()</code>:</p>
<pre><b><font COLOR="#7f0055">	private</font></b> <b><font COLOR="#7f0055">void</font></b> updateFieldEditors() {
	  <font COLOR="#7f0055"><b>boolean</b></font> enabled = useProjectSettingsButton.getSelection();
	  updateFieldEditors(enabled);
	}</pre>
<pre><b><font COLOR="#7f0055">	protected</font></b> <b><font COLOR="#7f0055">void</font></b> updateFieldEditors(<b><font COLOR="#7f0055">boolean</font></b> enabled) {
	  Composite parent = getFieldEditorParent();
<img src="images/tag_1.gif" height=13 width=24 align=CENTER>	  Iterator it = editors.iterator();
	  <font COLOR="#7f0055"><b>while</b></font> (it.hasNext()) {
	    FieldEditor editor = (FieldEditor) it.next();
<img src="images/tag_2.gif" height=13 width=24 align=CENTER>	    editor.setEnabled(enabled, parent);
	  }
	}</pre>
<p>We simply <img src="images/tag_1.gif" height=13 width=24 align=CENTER> 
iterate through the list of field editors and
<img src="images/tag_2.gif" height=13 width=24 align=CENTER>
 tell each field editor if it is enabled or not. Subclasses may override this
method if special treatment is required. This may be the case when the
enablement of field editors depends on the state of other field editors or on
the current state of the application.</p>
<h5>Modifying the access layer</h5>
<p>We have nearly completed the GUI part of class <code>FieldEditorOverlayPage</code>.
We only must enable or disable the field editors appropriately after they have
been created. The best place to do this is the method <code>createControl()</code>
where we <img src="images/tag_4.gif" height=13 width=24 align=CENTER> call <code>updateFieldEditors()</code>
after the <img src="images/tag_3.gif" height=13 width=24 align=CENTER> complete
page content has been created:</p>
<pre><b><font COLOR="#7f0055">	private</font></b> IPreferenceStore overlayStore;</pre>
<pre><b><font COLOR="#7f0055">	private</font></b> String pageId;</pre>
<pre><b><font COLOR="#7f0055">	public</font></b> <b><font COLOR="#7f0055">void</font></b> createControl(Composite parent) {
	  <b><font COLOR="#7f0055">if</font></b> (isPropertyPage()) {
<img src="images/tag_1.gif" height=13 width=24 align=CENTER>	    pageId = getPageId();
<img src="images/tag_2.gif" height=13 width=24 align=CENTER>	    overlayStore = <b><font COLOR="#7f0055">new</font></b> PropertyStore((IResource) getElement(), <b><font COLOR="#7f0055">
                                             super</font></b>.getPreferenceStore(), 
                                             pageId);
	  }
<img src="images/tag_3.gif" height=13 width=24 align=CENTER>	  <font COLOR="#7f0055"><b>super</b></font>.createControl(parent);
	  <b><font COLOR="#7f0055">if</font></b> (isPropertyPage())
<img src="images/tag_4.gif" height=13 width=24 align=CENTER>	    updateFieldEditors();
	}</pre>
<pre><img src="images/tag_5.gif" height=13 width=24 align=CENTER><b><font COLOR="#7f0055">	protected</font></b> <b><font COLOR="#7f0055">abstract</font></b> String getPageId();</pre>
<p>The method <code>createControl()</code> is also the best place to <img src="images/tag_2.gif" height=13 width=24 align=CENTER>
create an instance of class <code>PropertyStore</code> (see above). This
instance will act as our local overlay store. This <code>PropertyStore</code>
instance is supplied with a <code>pageId</code> used for qualifying property
names. We <img src="images/tag_1.gif" height=13 width=24 align=CENTER> obtain
this value via the <img src="images/tag_5.gif" height=13 width=24 align=CENTER>
method <code>getPageId()</code>. As this method is abstract, subclasses of <code>FieldEditorOverlayPage</code>
are required to implement it.</p>
<p>What remains to do is to inform clients about our overlay store. We do this
by overriding the method <code>getPreferenceStore()</code>:</p>
<pre><b><font COLOR="#7f0055">	public</font></b> IPreferenceStore getPreferenceStore() {
	  <b><font COLOR="#7f0055">if</font></b> (isPropertyPage())
<img src="images/tag_1.gif" height=13 width=24 align=CENTER>	    <b><font COLOR="#7f0055">return</font></b> overlayStore;
<img src="images/tag_2.gif" height=13 width=24 align=CENTER>	  <b><font COLOR="#7f0055">return</font></b> <b><font COLOR="#7f0055">super</font></b>.getPreferenceStore();
	}</pre>
<p>If the page is a property page we return the <img src="images/tag_1.gif" height=13 width=24 align=CENTER>
overlay store, otherwise we just return the <img src="images/tag_2.gif" height=13 width=24 align=CENTER>
standard preference store. All clients that can work with preference stores
(such as field editors) will be happy.</p>
<h5>Handling button events</h5>
<p>The button group from above, needs some special treatment when the <i>OK</i>
button or the <i>Restore Defaults</i> button is pressed. When the <i>OK</i>
button is pressed, we need to save the selection state of the radio buttons, and
when the <i>Restore Defaults</i> button is pressed, we need to set the selection
state of these buttons:</p>
<p>To implement this <i> OK</i> button behavior, we override the method <code>performOk()</code>.
Pay close attention here because things are getting a bit tricky:</p>
<pre><b><font COLOR="#7f0055">	public</font></b> <b><font COLOR="#7f0055">boolean</font></b> performOk() {
<img src="images/tag_1.gif" height=13 width=24 align=CENTER>	  <b><font COLOR="#7f0055">boolean</font></b> result = <b><font COLOR="#7f0055">super</font></b>.performOk();
	  <b><font COLOR="#7f0055">if</font></b> (result &amp;&amp; isPropertyPage()) {
<font color="#3f7f5f">	    </font>IResource resource = (IResource) element;
	    <b><font COLOR="#7f0055">try</font></b> {
<img src="images/tag_2.gif" height=13 width=24 align=CENTER>	      String value = (useProjectSettingsButton.getSelection()) ? 
                             TRUE : FALSE;
	      resource.setPersistentProperty(<font COLOR="#7f0055"><b>
                    new</b></font> QualifiedName(pageId, USEPROJECTSETTINGS), value);
            } <b><font COLOR="#7f0055">catch</font></b> (CoreException e) {
            }
	  }
	  <font COLOR="#7f0055"><b>return</b></font> result;
	}</pre>
<p>In method <code>performOk()</code> we first <img src="images/tag_1.gif" height=13 width=24 align=CENTER>
execute the <code>performOk()</code> method of the super class, then save <img src="images/tag_2.gif" height=13 width=24 align=CENTER>
 the state of the radio buttons into a resource property.</p>
<pre><b><font COLOR="#7f0055">	protected</font></b> <b><font COLOR="#7f0055">void</font></b> performDefaults() {
	  <b><font COLOR="#7f0055">if</font></b> (isPropertyPage()) {
<img src="images/tag_1.gif" height=13 width=24 align=CENTER>	    useWorkspaceSettingsButton.setSelection(<b><font COLOR="#7f0055">true</font></b>);
	    useProjectSettingsButton.setSelection(<b><font COLOR="#7f0055">false</font></b>);
	    configureButton.setEnabled(<b><font COLOR="#7f0055">true</font></b>);
<img src="images/tag_2.gif" height=13 width=24 align=CENTER>	    updateFieldEditors();
	  }
	  <font COLOR="#7f0055"><b>super</b></font>.performDefaults();
	}</pre>
<p>Here, we
<img src="images/tag_1.gif" height=13 width=24 align=CENTER> reset all buttons
to <i>Use workspace settings</i> and <img src="images/tag_2.gif" height=13 width=24 align=CENTER>
invoke method <code>updateFieldEditors()</code> to disable the field editors,
too.</p>
<p>To conclude the definition of this class, we implement
the invocation of the corresponding workbench preference page when the <i>Configure
Workbench Settings...</i> button is pressed. This is done in method <code>configureWorkspaceSettings()</code>:</p>
<pre><b><font COLOR="#7f0055">	protected</font></b> <b><font COLOR="#7f0055">void</font></b> configureWorkspaceSettings() {
	  <font COLOR="#7f0055"><b>try</b></font> {
<img src="images/tag_1.gif" height=13 width=24 align=CENTER>	    IPreferencePage page = 
                   (IPreferencePage) <b><font COLOR="#7f0055">this</font></b>.getClass().newInstance();
<img src="images/tag_2.gif" height=13 width=24 align=CENTER>	    page.setTitle(getTitle());
	    page.setImageDescriptor(image);
<img src="images/tag_3.gif" height=13 width=24 align=CENTER>	    showPreferencePage(pageId, page);
	  } <b><font COLOR="#7f0055">catch</font></b> (InstantiationException e) {
	    e.printStackTrace();
	  } <b><font COLOR="#7f0055">catch</font></b> (IllegalAccessException e) {
	    e.printStackTrace();
	  }
	}</pre>
<p>This method first <img src="images/tag_1.gif" height=13 width=24 align=CENTER>
 creates a sibling of the current instance, <img src="images/tag_2.gif" height=13 width=24 align=CENTER>
 then completes its definition with title and image (remember, we had saved the
required <code>ImageDescriptor</code> instance in one of the constructors), and <img src="images/tag_3.gif" height=13 width=24 align=CENTER>
calls method <code>showPreferencePage()</code> on
this new page:</p>
<pre><b><font COLOR="#7f0055">	protected</font></b> <b><font COLOR="#7f0055">void</font></b> showPreferencePage(String id, IPreferencePage page) {
<img src="images/tag_1.gif" height=13 width=24 align=CENTER>	  <b><font COLOR="#7f0055">final</font></b> IPreferenceNode targetNode = <b><font COLOR="#7f0055">new</font></b> PreferenceNode(id, page);
<img src="images/tag_2.gif" height=13 width=24 align=CENTER>	  PreferenceManager manager = <b><font COLOR="#7f0055">new</font></b> PreferenceManager();
	  manager.addToRoot(targetNode);
<img src="images/tag_3.gif" height=13 width=24 align=CENTER>	  <font COLOR="#7f0055"><b>final</b></font> PreferenceDialog dialog = <b><font COLOR="#7f0055">
                new</font></b> PreferenceDialog(getControl().getShell(), manager);
	  BusyIndicator.showWhile(getControl().getDisplay(), <b><font COLOR="#7f0055">new</font></b> Runnable() {
	    <b><font COLOR="#7f0055">public</font></b> <b><font COLOR="#7f0055">void</font></b> run() {
<img src="images/tag_4.gif" height=13 width=24 align=CENTER>	      dialog.create();
	      dialog.setMessage(targetNode.getLabelText());
<img src="images/tag_5.gif" height=13 width=24 align=CENTER>	      dialog.open();
	    }
	  });
	}</pre>
<p>The code for method <code>showPreferencePage()</code> is actually pinched from an existing
workbench component (to be precise: from the property page implementation for the
Java compiler settings). It <img src="images/tag_1.gif" height=13 width=24 align=CENTER>
 creates a solitary preference node, <img src="images/tag_2.gif" height=13 width=24 align=CENTER>
 adds it as the root object to a new preference manager, then constructs a <img src="images/tag_3.gif" height=13 width=24 align=CENTER> 
new
preference dialog with this manager. This dialog is then <img src="images/tag_4.gif" height=13 width=24 align=CENTER>
 created and <img src="images/tag_5.gif" height=13 width=24 align=CENTER> opened.</p>
<p>This concludes the definition of class <code>FieldEditorOverlayPage</code>.</p>
<h2>Using class FieldEditorOverlayPage</h2>
<p>Using (i.e. subclassing) this class is very simple. Let us assume that we
already have implemented a subclass of class <code>FieldEditorPreferencePage</code>.
All what we have to do is to:</p>
<ol>
  <li>modify the <code>extends</code> declaration of this subclass (exchange <code>FieldEditorPreferencePage</code>
    against <code>FieldEditorOverlayPage</code>).</li>
  <li>provide an implementation of method <code>getPageId()</code>.</li>
  <li>optionally we may implement layout variations depending on the page type
    (preference page or property page).</li>
</ol>
<p>Afterwards we can use this subclass in both roles: as a property page and as
a preference page.</p>
<p>Let us look at an example. Here is the original field editor preference page:</p>
<pre><b><font COLOR="#7f0055">	public</font></b> <b><font COLOR="#7f0055">class</font></b> DefaultSpellCheckerPreferencePage <b><font COLOR="#7f0055">
	       extends</font></b> FieldEditorPreferencePage <b><font COLOR="#7f0055">
               implements</font></b> IWorkbenchPreferencePage {</pre>
<pre>	  <font COLOR="#7f0055"><b>public</b></font> DefaultSpellCheckerPreferencePage() {
	    <b><font COLOR="#7f0055">super</font></b>(GRID);
	  }</pre>
<pre><b><font COLOR="#7f0055">	  public</font></b> IPreferenceStore doGetPreferenceStore() {
	    <b><font COLOR="#7f0055">return</font></b> SpellCheckerPlugin.getDefault().getPreferenceStore();
	  }</pre>
<pre><b><font COLOR="#7f0055">	  public</font></b> <b><font COLOR="#7f0055">void</font></b> init(IWorkbench workbench) {
	    setDescription(<font COLOR="#2a00ff">&quot;All changes will take effect ...&quot;</font>);
	  }</pre>
<pre><b><font COLOR="#7f0055">	  public</font></b> <b><font COLOR="#7f0055">void</font></b> createFieldEditors() {
	    Composite composite = getFieldEditorParent();
	    addField(<b><font COLOR="#7f0055">new</font></b> IntegerFieldEditor(
	       Configuration.SPELL_THRESHOLD,
	       <font COLOR="#2a00ff">&quot;Spell &amp;Threshold&quot;</font>,
	       composite));
	    addField(<b><font COLOR="#7f0055">new</font></b> BooleanFieldEditor(
	       Configuration.SPELL_IGNOREDIGITWORDS,
	       <font COLOR="#2a00ff">&quot;&amp;Ignore Numbers&quot;</font>,
	       composite));
	    ...
	    addField(<b><font COLOR="#7f0055">new</font></b> BooleanFieldEditor(
	       SpellCheckerPreferences.CHECKWHILETYPING,
	       <font COLOR="#2a00ff">&quot;Check &amp;while Typing&quot;</font>,
	       composite));
	    ...
	  }
	}</pre>
<p>And this is how we have to modify this class to be able using it as
both a preference page and a property page. We have printed these changes in bold
type:</p>
<pre><b><font COLOR="#7f0055">	public</font></b> <b><font COLOR="#7f0055">class</font></b> DefaultSpellCheckerPreferencePage <b><font COLOR="#7f0055">
	       extends</font></b> <b>FieldEditorOverlayPage</b> <b><font COLOR="#7f0055">
               implements</font></b> IWorkbenchPreferencePage {</pre>
<pre>	  <font COLOR="#7f0055"><b>public</b></font> DefaultSpellCheckerPreferencePage() {
	    <b><font COLOR="#7f0055">super</font></b>(GRID);
	  }</pre>
<pre><b><font COLOR="#7f0055">	  public</font></b> IPreferenceStore doGetPreferenceStore() {
	    <b><font COLOR="#7f0055">return</font></b> SpellCheckerPlugin.getDefault().getPreferenceStore();
	  }</pre>
<pre><b><font COLOR="#7f0055">	  public</font></b> <b><font COLOR="#7f0055">void</font></b> init(IWorkbench workbench) {
	    setDescription(<font COLOR="#2a00ff">&quot;All changes will take effect ...&quot;</font>);
	  }</pre>
<pre><b><font COLOR="#7f0055">	  protected</font> String getPageId() {
</b><img src="images/tag_1.gif" height=13 width=24 align=CENTER><b>	    <font COLOR="#7f0055">return</font> &quot;com.bdaum.SpellChecker.preferences.defaultPreferences&quot;;
	  }</b></pre>
<pre><b><font COLOR="#7f0055">	  public</font></b> <b><font COLOR="#7f0055">void</font></b> createFieldEditors() {
	    Composite composite = getFieldEditorParent();
	    addField(<b><font COLOR="#7f0055">new</font></b> IntegerFieldEditor(
	       Configuration.SPELL_THRESHOLD,
	       <font COLOR="#2a00ff">&quot;Spell &amp;Threshold&quot;</font>,
	       composite));
	    addField(<b><font COLOR="#7f0055">new</font></b> BooleanFieldEditor(
	       Configuration.SPELL_IGNOREDIGITWORDS,
	       <font COLOR="#2a00ff">&quot;&amp;Ignore Numbers&quot;</font>,
	       composite));
	    ...
	    <b><font COLOR="#7f0055">if</font> (!isPropertyPage()) {</b>
<img src="images/tag_2.gif" height=13 width=24 align=CENTER>	      addField(<b><font COLOR="#7f0055">new</font></b> BooleanFieldEditor(
	         SpellCheckerPreferences.CHECKWHILETYPING,
	         <font COLOR="#2a00ff">&quot;Check &amp;while Typing&quot;</font>,
	         composite));
	      ...
	    <b>}</b></pre>
<p>Note, that we display the <img src="images/tag_2.gif" height=13 width=24 align=CENTER>
last field editor only in preference pages, not in property pages.</p>
<p>In the plug-in manifest file <code>plugin.xml</code> the
necessary declarations could look like this:</p>
<pre><img src="images/tag_3.gif" height=13 width=24 align=CENTER><font COLOR="#000080">	&lt;extension id=</font><font COLOR="#008000">&quot;com.bdaum.aoModeling.preferences&quot;</font><font COLOR="#000080">
</font>		   <font COLOR="#000080">name=</font><font COLOR="#008000">&quot;SpellChecker Preferences&quot;
</font>	           <font COLOR="#000080">point=</font><font COLOR="#008000">&quot;org.eclipse.ui.preferencePages&quot;</font><font COLOR="#000080">&gt;
</font>	  <font COLOR="#000080">&lt;page name=</font><font COLOR="#008000">&quot;Spelling&quot; </font><font COLOR="#000080">class=</font><font COLOR="#008000">
</font><img src="images/tag_4.gif" height=13 width=24 align=CENTER><font COLOR="#008000">   &quot;com.bdaum.SpellChecker.preferences.DefaultSpellCheckerPreferencePage&quot;
</font><img src="images/tag_5.gif" height=13 width=24 align=CENTER>	        <font COLOR="#000080">id= </font><font COLOR="#008000">&quot;com.bdaum.SpellChecker.preferences.defaultPreferences&quot;</font><font COLOR="#000080">&gt;
</font>	  <font COLOR="#000080">&lt;/page&gt;
</font>	<font COLOR="#000080">&lt;/extension&gt;</font></pre>
<pre><font COLOR="#000080">	&lt;extension id=</font><font COLOR="#008000">&quot;com.bdaum.aoModeling.properties&quot;
</font>		   <font COLOR="#000080">name=</font><font COLOR="#008000">&quot;SpellChecker Properties&quot;
</font>	           <font COLOR="#000080">point=</font><font COLOR="#008000">&quot;org.eclipse.ui.propertyPages&quot;</font><font COLOR="#000080">&gt;
</font><img src="images/tag_6.gif" height=13 width=24 align=CENTER>	  <font COLOR="#000080">&lt;page objectClass=</font><font COLOR="#008000">&quot;org.eclipse.core.resources.IProject&quot;
</font>	        <font COLOR="#000080">adaptable=</font><font COLOR="#008000">&quot;true&quot;
</font>	        <font COLOR="#000080">name=</font><font COLOR="#008000">&quot;Spell Default&quot; </font><font COLOR="#000080">class=</font><font COLOR="#008000">
</font><img src="images/tag_4.gif" height=13 width=24 align=CENTER><font COLOR="#008000">   &quot;com.bdaum.SpellChecker.preferences.DefaultSpellCheckerPreferencePage&quot;
</font><img src="images/tag_7.gif" height=13 width=24 align=CENTER>	        <font COLOR="#000080">id=</font><font COLOR="#008000">&quot;com.bdaum.SpellChecker.propertyPage.project&quot;</font><font COLOR="#000080">&gt;
</font>	  <font COLOR="#000080">&lt;/page&gt;
</font>	<font COLOR="#000080">&lt;/extension&gt;</font></pre>
<p>Note, that the page identification returned by <img src="images/tag_1.gif" height=13 width=24 align=CENTER>
 method <code>getPageId()</code> matches the <img src="images/tag_3.gif" height=13 width=24 align=CENTER>
page identification specified in the preference page extension point. Note also, that we used the same
<img src="images/tag_4.gif" height=13 width=24 align=CENTER> page implementation in both
<img src="images/tag_3.gif" height=13 width=24 align=CENTER> the preference page extension point and
<img src="images/tag_6.gif" height=13 width=24 align=CENTER> the property page extension point.
(The <img src="images/tag_7.gif" height=13 width=24 align=CENTER>
 identification of the property page actually may differ from the <img src="images/tag_3.gif" height=13 width=24 align=CENTER>
preference page id.)</p>
<table border="0">
  <tr>
    <td width="50%" valign="top"><img border="0" src="images/spellPreferences.GIF" width="452" height="383">
      <p><i>Preferences page for a spell checker plug-in</i>
      <p><i>&nbsp;</i></td>
  </tr>
  <tr>
    <td width="50%" valign="top"><img border="0" src="images/spellProperties.GIF" width="452" height="365">
      <p><i>Corresponding properties page for the same plug-in. The last group
      of widgets&nbsp;<br>
        is deliberately suppressed for property pages by this page implementation.</i></td>
  </tr>
</table>
<h2>Retrieving overlayed preference values</h2>
<p>Reading such overlayed preference values is a multi-stage process. First we
must <img src="images/tag_1.gif" height=13 width=24 align=CENTER> interrogate
the resource, if this property page uses the project settings or the workbench
settings. If it uses the project settings, we must <img src="images/tag_2.gif" height=13 width=24 align=CENTER>
read the value from the resource properties, if not we fall back to the <img src="images/tag_3.gif" height=13 width=24 align=CENTER>
values defined in the workbench preference store. Here again, we have to <img src="images/tag_4.gif" height=13 width=24 align=CENTER>
qualify the field name with the page identification when we access the resource
properties.</p>
<pre><b><font COLOR="#7f0055">	public</font></b> <b><font COLOR="#7f0055">static</font></b> String getOverlayedPreferenceValue(
                                             IPreferenceStore store, 
                                             IResource resource, 
                                             String pageId, 
                                             String name) {
	  IProject project = resource.getProject();
	  String value = <b><font COLOR="#7f0055">null</font></b>;
<img src="images/tag_1.gif" height=13 width=24 align=CENTER>	  <b><font COLOR="#7f0055">if</font></b> (useProjectSettings(project, pageId)) {
<img src="images/tag_2.gif" height=13 width=24 align=CENTER>	    value = getProperty(resource, pageId, key);
	  }
	  <font COLOR="#7f0055"><b>if</b></font> (value != <b><font COLOR="#7f0055">null</font></b>)
	    <b><font COLOR="#7f0055">return</font></b> value;
<img src="images/tag_3.gif" height=13 width=24 align=CENTER>	  <b><font COLOR="#7f0055">return</font></b> store.getString(key);
	}</pre>
<pre><b><font COLOR="#7f0055">	private</font></b> <b><font COLOR="#7f0055">static</font></b> <b><font COLOR="#7f0055">boolean</font></b> useProjectSettings(IResource resource, 
                                                  String pageId) {
	  String use = getProperty(
                             resource, 
                             pageId, 
                             FieldEditorOverlayPage.USEPROJECTSETTINGS);
	  <font COLOR="#7f0055"><b>return</b></font> <font COLOR="#2a00ff">&quot;true&quot;</font>.equals(use);
	}</pre>
<pre><b><font COLOR="#7f0055">	private</font></b> <b><font COLOR="#7f0055">static</font></b> String getProperty(IResource resource, 
                                          String pageId, 
                                          String key) {
	  <font COLOR="#7f0055"><b>try</b></font> {
<img src="images/tag_4.gif" height=13 width=24 align=CENTER>	    <font COLOR="#7f0055"><b>return</b></font> resource.getPersistentProperty(<b><font COLOR="#7f0055">
                             new</font></b> QualifiedName(pageId, key));
	  } <b><font COLOR="#7f0055">catch</font></b> (CoreException e) {
	  }
	  <b><font COLOR="#7f0055">return</font></b> <b><font COLOR="#7f0055">null</font></b>;
	}</pre>
<h2>Technology transfer</h2>
<p>What works well for field editor preferences should also work for
&quot;normal&quot; preference pages. However, some things are different when
creating the class <code>OverlayPage</code>, since we now extend a <code>PreferencePage</code>
and not a <code>FieldEditorPreferencePage</code>:</p>
<ol>
  <li>We can directly extend the class <code>PropertyPage</code> because
    this class is a subclass of <code>PreferencePage</code>. This saves us from
    implementing the interface <code>IWorkbenchPropertyPage</code> with its <code>getElement()</code>
    and <code>setElement()</code> methods.<br>
  </li>
  <li>We need to modify the constructors (no <code>style</code>
    parameter required).<br>
  </li>
  <li>We need a different implementation for method <code>createContents()</code>.
    Since <code>PreferencePage</code> only defines this method as an abstract
    method, we need to implement it from scratch. We do so by creating two
    containers in the parent composite: One for our own button group and one for
    the contents created by subclasses. We return this second container as the
    method's result.<br>
  </li>
  <li>There is no <code>addField()</code> method that we can override to
    keep track of field editors. In fact, there are no field editors at all.
    Consequently, we cannot rely on the knowledge encapsulated in the field
    editors. Therefore, we drop the methods <code>addField()</code> and <code>updateFieldEditors()</code>.
    Instead, we implement a method <code>setControlsEnabled()</code>. This
    method walks through the whole tree of child controls created by subclasses
    enabling or disabling them. (We spare tabbed notebooks and the like to allow
    for user navigation.) Again, subclasses may want to override this method
    when the enablement of controls depends on the state of other controls or on
    the state of the application.<br>
  </li>
  <li>Subclasses must play it safe: If they
    override method <code>performOk()</code> they must invoke <code>super.performOk()</code>;
    and the implementation of <code>createContents()</code> must invoke <code>super.createContents()</code>
    and use the composite returned by this method as container for all controls
    created in the subclass.</li>
</ol>
<p>At this point we leave it with these hints. For implementation details please 
  see class <code>OverlayPage</code> in the <a href="overlayPages.zip"> source 
  code zip</a>.</p>
<h2>Summary</h2>
<p>We have implemented two abstract classes <code>FieldEditorOverlayPage</code>
and <code>OverlayPage</code>. Subclasses extending these classes can act as both
preference pages and property pages leading to an improved application
consistency and lower maintenance cost. With only minimal modifications, already
existing preference pages can be made by extending&nbsp; <code>FieldEditorOverlayPage</code>
or <code>OverlayPage</code> and can then be reused in the role of property
pages. We achieved this by implementing an abstract access layer in form of
class <code>PropertyStore</code> that encapsulates the properties of a resource
but behaves as a preference store. In addition, both classes <code>FieldEditorOverlayPage</code>
and <code>OverlayPage</code> provide the additional GUI elements required by
property pages.</p>
<h2>Source Code</h2>
<p>To use these classes within your own plug-in, download the <a href="overlayPages.zip">source 
  code zip</a> and import its contents of into the source folder of your Eclipse 
  project.</p>

<h2>About the author </h2>

<p>Berthold Daum is an independent consultant and writer based in Germany. His 
  best-selling book &quot;Java Entwicklung mit Eclipse 2&quot; (dpunkt verlag) 
  will appear in an English version as &quot;Eclipse 2 for Java Developers&quot; 
  in November 2003 (John Wiley &amp; Sons). </p>

<p><small>Java and all Java-based trademarks and logos are trademarks or registered trademarks of Sun 
Microsystems, Inc. in the United States, other countries, or both.</small></p>

</body>
</html>