Eclipse CDT plugin - action restricted to function name

Question

Currently I am developing a plugin for Eclipse CDT. I have successfully made an action declaration in my plugin.xml file.

<extension
     point="org.eclipse.ui.popupMenus">
  <objectContribution
        id="MyOwnPlugin.contribution1"
        nameFilter="*.c"
        objectClass="org.eclipse.ui.IEditorInput">
     <action
           class="myownplugin.popup.actions.DoTestsAction"
           enablesFor="1"
           id="myownplugin.doTests"
           label="Do Tests"
           menubarPath="additions">
     </action>
  </objectContribution>
</extension>

This successfully declares a popup action when I right-click on the editor and I can select the action.

However, what I would like to achieve is that the action would only appear when I right-click on a function name/function declaration. Is there a way how to achieve this? I have been trying the filter XML tag in the plugin.xml, but with no success.

Thanks.

Answer 1

I haven't tried this myself, but I believe what you want is doable (modulo the click position vs. caret position issue described in the comments). You are on the right track with using the <filter> element, but there are some other pieces required to get this to work.

---

The first thing to realize is that a context menu pertains to an object, and in the case of the editor's context menu, that object is the IEditorInput representing the contents of the editor tab.

Any mechanism to make the presence of a context menu action dependent on some condition, will only have that object available as input. It follows that the condition must be based on the state of the object (only). This is why we can base it on caret position but not on the location of the click itself: "the current caret position in the editor" is part of the state of the IEditorInput, but "the location of the current click within the editor" is not (that I know of).

---

The documentation of the <filter> element says:

This element is used to evaluate the attribute state of each object in the current selection. A match only if each object in the selection has the specified attribute state. Each object in the selection must implement, or adapt to, org.eclipse.ui.IActionFilter.

The reason this talks about a "selection" is that in some views a context menu can be invoked with multiple objects selected (e.g. in the Project Explorer view, with multiple files / folders selected). In an editor context that doesn't apply; there will only be one object in the "selection", of type IEditorInput.

Unfortunately, IEditorInput does not implement IActionFilter. However, it does implement IAdaptable, so we can use the adapter mechanism to have our plugin support adapting it to IActionFilter.

That will involve adding something like this to your plugin.xml:

  <extension point="org.eclipse.core.runtime.adapters">
     <factory 
        class="your.plugin.EditorInputAdapterFactory" 
        adaptableType="org.eclipse.ui.IEditorInput">
        <adapter type="org.eclipse.ui.IActionFilter"/>
     </factory>
  </extension>

where your.plugin.EditorInputAdapterFactory is a type you'll write in your plugin. Its implementation will look something like this:

public class EditorInputAdapterFactory implements IAdapterFactory {
    @Override
    public <T> T getAdapter(Object adaptable, Class<T> adapterType) {
        if (adaptable instanceof IEditorInput && adapterType.equals(IActionFilter.class)) {
            return new EditorInputActionFilter((IEditorInput) adaptable);
        }
        return null;
    }
}

where EditorInputActionFilter is, again, a type we'll write.

---

Ok, now we have an action filter that works with IEditorInput objects, enabling us to use the <filter> element.

The <filter> element uses an "attribute name" and an "attribute value", which will be passed it to the IActionFilter. As the author of the action filter, we get to invent these. For example, we can invent an attribute name called selectedElementType (where by "selected element" I mean the type of C++ element the caret is currently over), and a value named function.

Then our filter declaration will look like this:

<filter name="selectedElementType" value="function" />

---

Finally, we need to implement our action filter so it evaluates the property we defined for an IEditorInput object. I won't write up the complete implementation here, but in broad strokes:

• Use CDTUITools.getWorkingCopyManager() to map the IEditorInput to an IWorkingCopy, which implements ITranslationUnit.
• Get the editor's current caret position, using something like CUIPlugin.getActivePage().findEditor(editorInput).getEditorSite().getSelectionProvider().getSelection() (with appropriate null checks in between). There may be an easier way to do this, but that's what comes to mind. Since you're in an editor, the returned selection should have type ITextSelection.
• Use SharedASTJob to get access to the editor's shared AST (IASTTranslationUnit). Note that you'll need to block on the job, and the action filter will (I assume) be invoked on the UI thread, which is not ideal. (More on that below.)
• Use IASTTranslationUnit.getNodeSelector(null).findEnclosingName(offset, length), with the offset and length from the ITextSelection, to get an IASTName representing the name under the caret (if any).
• Use IASTName.resolveBinding() to get the binding (C++ semantic model object) that the name refers to.
• Check if the binding implements IFunction.

All of this will go in your implementation of IActionFilter.testAttribute(). The target parameter to that function will be the IEditorInput. For good measure, you should check that the name and value parameters correspond to the attribute names you invented (selectedElementType and function) before doing any of this (initially, your action filter will only be invoked by your <filter> element, so they'll always match, but you can imagine extending this mechanism in the future to e.g. support other selected element types.)

---

Finally, a note on performance: what you're doing here is conditioning the responsiveness of a UI element (the appearance of a popup) on a property of C++ code, which can be slow to parse and analyze. This necessarily means your popup may take longer to appear as a result (which is reflected in your action filter needing to block on the SharedASTJob). By using SharedASTJob, you are minimizing this effect by re-using an already-parsed AST if there is one, but e.g. if you've just opened an editor and you right-click, and the initial AST takes several seconds to build, your popup will take several seconds to show up. Caveat emptor.