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| <td class="back"><a id="top"></a><a href="s4.1.html" rel="prev"><< §4.1 Callin method binding</a></td> |
| <td class="top"><a href="index.html" rel="contents">↑ Table of Contents ↑</a></td> |
| <td class="next"><a href="s4.3.html" rel="next">§4.3 Base calls >></a></td> |
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| <div class="breadcrumb"><a class="nav" href="s4.html" rel="section">§4 Callin Binding</a></div> |
| <div class="sect depth2" id="s4.2"> |
| <h2 class="sect">§4.2 Callin modifiers (before, after, replace)<a class="img" href="s4.2.html" |
| title="PermaLink to §4.2 Callin modifiers (before, after, replace)"><img style="vertical-align:text-top;margin-left:5px;" src="../images/permalink.png" |
| alt="" /></a></h2> |
| <div class="subsect depth3" id="s4.2.a"> |
| <h4 class="subsect">(a) <span class="title">Method composition</span><a class="img" href="s4.2.a.html" |
| title="PermaLink to (a) Method composition"><img style="vertical-align:text-top;margin-left:5px;" src="../images/permalink.png" |
| alt="" /></a></h4> |
| <p>The kind of method composition is controlled by adding one |
| of the modifiers <strong>before, after</strong> or <strong>replace</strong> after the |
| "<code><-</code>" token of the binding declaration. |
| |
| </p> |
| </div> |
| <div class="subsect depth3" id="s4.2.b"> |
| <h4 class="subsect">(b) <span class="title">Additive composition</span><a class="img" href="s4.2.b.html" |
| title="PermaLink to (b) Additive composition"><img style="vertical-align:text-top;margin-left:5px;" src="../images/permalink.png" |
| alt="" /></a></h4> |
| <p>The <code>before</code> and <code>after</code> modifiers have the |
| effect of adding a call to the role method at the beginning or end |
| of the base method, resp.<br /> |
| In this case no data are transferred from the role to the base, |
| so if the role method has a result, this will always be ignored. |
| |
| </p> |
| <h5 class="listing">Example code (Callin):</h5> |
| <div class="listing example frame"> |
| <table class="listing"> |
| <tr class="line odd"> |
| <td class="ln">1</td> |
| <td><pre><b>team</b> <b>class</b> Company {</pre></td> |
| </tr> |
| <tr class="line even"> |
| <td class="ln">2</td> |
| <td><pre> <b>protected</b> <b>class</b> Employee <b>playedBy</b> Person {</pre></td> |
| </tr> |
| <tr class="line odd"> |
| <td class="ln">3</td> |
| <td><pre> <b>public</b> <b>void</b> recalculateIncome() { ... }</pre></td> |
| </tr> |
| <tr class="line even"> |
| <td class="ln">4</td> |
| <td><pre> recalculateIncome <em><b><-</b> after</em> haveBirthday; <span class="comment">// callin binding</span></pre></td> |
| </tr> |
| <tr class="line odd"> |
| <td class="ln">5</td> |
| <td><pre> }</pre></td> |
| </tr> |
| <tr class="line even"> |
| <td class="ln">6</td> |
| <td><pre>}</pre></td> |
| </tr> |
| </table> |
| </div> |
| <div class="codecomment"> |
| <p>Line 4 declares a callin binding for the role method <code>recalculateIncome()</code> |
| defined in line 3. In combination with the role binding in line 2 this has the following effect: |
| |
| </p> |
| <ul> |
| <li><strong>After</strong> every call of the method <code>Person.haveBirthday</code> |
| the method <code>Company.recalculateIncome</code> is called. |
| </li> |
| </ul> |
| </div> |
| </div> |
| <div class="subsect depth3" id="s4.2.c"> |
| <h4 class="subsect">(c) <span class="title">Replacing composition</span><a class="img" href="s4.2.c.html" |
| title="PermaLink to (c) Replacing composition"><img style="vertical-align:text-top;margin-left:5px;" src="../images/permalink.png" |
| alt="" /></a></h4> |
| <p>The <code>replace</code> modifier causes <em>only</em> the role method to be |
| invoked, replacing the base method. |
| <br /> |
| In this case, if the base method declares a result, this should be provided by the role method. |
| Special cases of return values in callin bindings are discussed in <a href="s4.3.e.html" title="§4.3.(e) Fragile callin binding" |
| class="sect">§4.3.(e)</a></p> |
| </div> |
| <div class="subsect depth3" id="s4.2.d"> |
| <h4 class="subsect">(d) <span class="title">Callin methods</span><a class="img" href="s4.2.d.html" title="PermaLink to (d) Callin methods"><img style="vertical-align:text-top;margin-left:5px;" src="../images/permalink.png" |
| alt="" /></a></h4> |
| <p>Role methods to be bound by a callin replacement binding must have |
| the modifier <code>callin</code>. This modifier is only allowed for methods |
| of a role class.<br /> |
| A method with the <code>callin</code> modifier can only be called |
| |
| </p> |
| <ul> |
| <li>via a callin replace binding</li> |
| <li>by a <code>super</code> or <code>tsuper</code> call from an overriding callin method. |
| </li> |
| </ul> |
| <p>It is illegal for a <code>callin</code> method |
| </p> |
| <ul> |
| <li>to be called directly,</li> |
| <li>to be bound using a callout binding, and</li> |
| <li>to be bound to a base method using a <code>before</code> or <code>after</code> callin binding. |
| </li> |
| </ul> |
| <p>Despite these rules a second level role — which is played by the current role — |
| can intercept the execution of a callin method using any form of callin binding.</p> |
| <p>A callin method cannot override a regular method and vice versa, however, |
| overriding one callin method with another callin method is legal and |
| dynamic binding applies to callin method just like regular methods. |
| <br /> |
| A callin method must not declare its visibility using any of the modifiers <code>public</code>, |
| <code>protected</code> or <code>private</code>. Since callin methods can only be invoked via callin bindings |
| such visibility control would not be useful. |
| |
| </p> |
| </div> |
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| <td class="back"><a href="s4.1.html" rel="prev"><< §4.1 Callin method binding</a></td> |
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