View source for FAQ How do I create a compare editor?

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Compare dialogs are typically used in simple contexts that ask the
user to select from a list of available editions.  For
richer comparisons, a compare editor is typically used.  The advantage
of using an editor is that the user can take as long as  needed to
browse, modify, and merge the contents.


Compare editors display a tree of <tt>DiffNode</tt> elements,
where each node represents a logical entity, such as a file or
programming-language element.  These nodes represent either a two-way
or a three-way comparison,  where the optional third element is the
common ancestor of the two elements being compared. Each <tt>DiffNode</tt>
references a left- and right-side element and, possibly, a third element representing
the common ancestor. As with compare dialogs, these compare elements
should implement <tt>ITypedElement</tt> and <tt>IStreamContentAccessor</tt>.
You can construct these node trees manually or use the supplied <tt>Differencer</tt>
class to help you construct it.


The <tt>DiffNode</tt> tree is computed by a 
<tt>CompareEditorInput</tt> subclass that is passed as an input to the editor.
The subclass must implement the <tt>prepareInput</tt> method to
return the tree represented by the <tt>DiffNode</tt>. 
The following example illustrates a compare editor
input that uses the <tt>CompareItem</tt> class described in 
[[FAQ_How_do_I_create_a_Compare_dialog%3F]]
<pre>
   class CompareInput extends CompareEditorInput {
      public CompareInput() {
         super(new CompareConfiguration());
      }
      protected Object prepareInput(IProgressMonitor pm) {
         CompareItem ancestor = 
            new CompareItem("Common", "contents");
         CompareItem left = 
            new CompareItem("Left", "new contents");
         CompareItem right = 
            new CompareItem("Right", "old contents");
         return new DiffNode(null, Differencer.CONFLICTING, 
            ancestor, left, right);
      }
   }
</pre>

Once you have a compare editor input, opening a compare editor
on that input is trivial.  Here is an example action that opens a
compare editor, using the preceding input:
<pre>
   public class CompareEditorAction implements 
    IWorkbenchWindowActionDelegate {
      public void run(IAction action) {
         CompareUI.openCompareEditor(new CompareInput());
      }
   }
</pre>


If you want to support merging as well as
comparing, two extra steps are involved. First, you need
to specify which of the elements is editable.  This is done by the 
<tt>CompareConfiguration</tt> object that is passed to the 
<tt>CompareEditorInput</tt> constructor.  Use the 
<tt>setLeftEditable</tt> and <tt>setRightEditable</tt> methods
to specify which of the comparison panes should support modification.
Second, your editor input class should override the <tt>save</tt> method
to perform the save of the editor contents.

<hr><font size=-2>This FAQ was originally published in [http://www.eclipsefaq.org Official Eclipse 3.0 FAQs]. Copyright 2004, Pearson Education, Inc. All rights reserved. This text is made available here under the terms of the [http://www.eclipse.org/legal/epl-v10.html Eclipse Public License v1.0].</font>