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/******************************************************************************
 * Copyright (c) 2005 IBM Corporation and others.
 * All rights reserved. This program and the accompanying materials
 * are made available under the terms of the Eclipse Public License v1.0
 * which accompanies this distribution, and is available at
 * http://www.eclipse.org/legal/epl-v10.html
 *
 * Contributors:
 *    IBM Corporation - initial API and implementation 
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<p>Provides a set of classes to help in evaluating expressions for
Eclipse <i>plug-in</i> action enablement and visibility. The {@link
org.eclipse.gmf.runtime.common.ui.services.action.filter.ActionFilterService}
allows clients to extend the action filters supported by objects
selected in workbench. It also permits the entire workbench selection 
to be considered so that an action can be enabled if it is supported on one 
or more elements in the selection.</p>

<p>To add a new attribute to an object for action filtering, the object
must implement the <code>IAdaptable</code> interface and an adapter
factory must be registered that returns the 
{@link org.eclipse.gmf.runtime.common.ui.services.action.filter.ActionFilterService}
when the object is adapted to <code>IActionFilter</code>. An 
{@link org.eclipse.gmf.runtime.common.ui.services.action.filter.IActionFilterProvider} 
can then be registered to provide the test for the attribute using the 
<code>org.eclipse.gmf.runtime.common.ui.services.action.actionFilterProviders</code>
extension point.</p>

<p>Such attributes are named in <code>objectState</code> elements which
evaluate the attribute state of each object in the current selection for
enablement or visibility criteria (see the org.eclipse.ui.popupMenus extension point for details).
Eclipse documentation states that if each object in the selection has 
the specified attribute state, the expression is evaluated as true. However, this is
not necessarily the case for attributes defined by an action filter provider, which can 
optionally evaluate the expression to true if one or more of the objects in the selection
has the specified attribute state.  The meaning of the value of the attribute (the <code>objectState</code> value) 
is defined by the action filter provider.</p>

<p>Action filter providers should be implemented by extending the {@link
org.eclipse.gmf.runtime.common.ui.services.action.filter.AbstractActionFilterProvider}
class in this package.</p>

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