## Implementing a content viewer

The compare plug-in allows you to supply specialized viewers for viewing and merging content differences between unstructured elements.

### Simple content viewers

A content viewer is used in places where only a single input is available and therefore no compare is necessary. A typical example for this is the "Restore from Local History" function. The org.eclipse.compare.contentViewers extension point allows you to define a specialized content viewer that does not compare its inputs.

    <extension
    	point="org.eclipse.compare.contentViewers">
    	<viewer
    		extensions="java,java2"
    		class="org.eclipse.jdt.internal.ui.compare.JavaTextViewerCreator"
    		id="org.eclipse.jdt.internal.ui.compare.JavaTextViewerCreator">
    	</viewer>
    	<contentTypeBinding
    		contentTypeId="org.eclipse.jdt.core.javaSource"
    		contentViewerId="org.eclipse.jdt.internal.ui.compare.JavaTextViewerCreator">
    	</contentTypeBinding>
    </extension>

Specialized viewers contributed by your plug-in are designated in the viewer element. You must specify the id of the viewer and the class that creates it. You may also specify any file extensions for which the content viewer should be used.

You may also use the contentTypeBinding element to associate a content type with a content viewer.

### Content merge viewers

A content merge viewer performs a two-way or three-way compare of its inputs and presents the result side-by-side or in any other suitable way. The viewer lets the user merge between the inputs. Content merge viewers are common for text or images.

If the standard merge viewers are not appropriate for your plug-in's function, you may choose to implement your own content merge viewer. Your content merge viewer should be registered with the platform using the org.eclipse.compare.contentMergeViewers extension point. The following markup shows the definition of specialized content merge viewers for viewing Java files and properties files in the Java IDE:

    <extension
    	point="org.eclipse.compare.contentMergeViewers">
    	<viewer
    		extensions="java,java2"
    		class="org.eclipse.jdt.internal.ui.compare.JavaContentViewerCreator"
    		id="org.eclipse.jdt.internal.ui.compare.JavaContentViewerCreator">
    	</viewer>
    	<contentTypeBinding
    		contentTypeId="org.eclipse.jdt.core.javaProperties"
    		contentMergeViewerId="org.eclipse.compare.TextMergeViewerCreator">
    	</contentTypeBinding>
    	<contentTypeBinding
    		contentTypeId="org.eclipse.jdt.core.javaSource"
    		contentMergeViewerId="org.eclipse.jdt.internal.ui.compare.JavaContentViewerCreator">
    	</contentTypeBinding>
    </extension>

Similar to content viewers, specialized merge viewers contributed by your plug-in are designated in the viewer element. You must specify the id of the viewer and the class that creates it. You may also specify any file extensions for which the content merge viewer should be used.

Also similar to content viewers, you can use contentTypeBinding to associate a content type with a merge viewer. The JDT plug-in binds content merge viewers to two different content types: Java source and Java properties files.

ContentMergeViewer is an abstract compare and merge viewer with two side-by-side content areas and an optional content area for a common ancestor (for three-way compare). Because the implementation makes no assumptions about the content type, the subclass is responsible for dealing with the specific content type.

ImageMergeViewer in org.eclipse.compare.internal shows how to implement a simple merge viewer for images using a ContentMergeViewer. A ContentMergeViewer accesses its model by means of a content provider which must implement the IMergeViewerContentProvider interface.

#### Text merging

If your viewer uses text, additional classes that compare and merge text content can be used.

TextMergeViewer is the concrete subclass of ContentMergeViewer used for comparing and merging text content. A text merge viewer uses the RangeDifferencer to perform a textual, line-by-line comparison of two (or three) input documents.

For text lines that differ, the TextMergeViewer uses an ITokenComparator to find the longest sequences of matching and non-matching tokens. The TextMergeViewer's default token compare works on characters separated by white space. If a different strategy is needed (for example, Java tokens in a Java-aware merge viewer), clients can create their own token comparators by implementing the ITokenComparator interface.

TextMergeViewer works on whole documents and on sub ranges of documents. For partial documents, the viewer's input must be an IDocumentRange instead of an IDocument.

#### Range differencing

RangeDifferencer finds the longest sequences of matching and non-matching comparable entities in text content. Its implementation is based on an objectified version of the algorithm described in: A File Comparison Program, by Webb Miller and Eugene W. Myers, Software Practice and Experience, Vol. 15, Nov. 1985. Clients must supply an input to the differencer that implements the IRangeComparator interface. IRangeComparator breaks the input data into a sequence of entities and provides a method for comparing one entity with the entity in another IRangeComparator.

For example, to compare two text documents and find the longest common sequences of matching and non-matching lines, the implementation of IRangeComparator must break the document into lines and provide a method for testing whether two lines are considered equal. See org.eclipse.compare.internal.DocLineComparator for an example of how this can be done.

The differencer returns the differences among these sequences as an array of RangeDifference objects. Every single RangeDifference describes the kind of difference (no change, change, addition, deletion) and the corresponding ranges of the underlying comparable entities in the two or three inputs.