Extending JSDT: adding your own content assist

Extending the JSDT content assist is a  topic asked several times on stackoverflow(20738788, 20779899). As I needed it for the Eclipse EASE project, I decided to come up with a short tutorial.

Source code for this tutorial is available on googlecode as a single zip archive, as a Team Project Set or you can checkout the SVN projects directly. 

Step 1: Preparations

As we will have dependencies to JSDT you either need to check out the source code from gerrit or add JSDT to your target platform.

Step 2: Defining the extension point

Create a new Plug-in Project com.codeandme.jsdt.contentassist and add following dependencies:

• org.eclipse.wst.jsdt.ui
• org.eclipse.core.runtime
• org.eclipse.jface.text

Afterwards create a new Extension for org.eclipse.wst.jsdt.ui.javaCompletionProposalComputer. Provide an ID and Name. The ID will be needed as reference to a category, the name will be displayed in Preferences/JavaScript/Editor/Content Assist/Advanced. Create a proposalCategory node and provide a nice icon for your category.

Create a second extension of the same type for our implementation. Again provide an ID. Create a javaCompletionProposalComputer subnode with a class and activate set to true. The categoryID consists of your plugin name followed by the category ID we provided earlier.

If you use a target definition containing JSDT plugins you might not be able to use the Plug-in Manifest Editor for creating these extensions. In that case you have to edit the xml code directly:

<?xml version="1.0" encoding="UTF-8"?>
<?eclipse version="3.4"?>
<plugin>
   <extension point="org.eclipse.wst.jsdt.ui.javaCompletionProposalComputer"
   id="codeandme_category"
   name="Code and me proposals">
   <proposalCategory/>
 </extension>

 <extension
       id="codeandme_proposal"
       point="org.eclipse.wst.jsdt.ui.javaCompletionProposalComputer">
   <javaCompletionProposalComputer
      class="com.codeandme.jsdt.contentassist.CustomCompletionProposalComputer"
      categoryId="com.codeandme.jsdt.contentassist.codeandme_category"
      activate="true">
   </javaCompletionProposalComputer>
 </extension>

</plugin>

Step 3: Proposal computer implementation

Now for the easy part:create a new class CustomCompletionProposalComputer with following content:

package com.codeandme.jsdt.contentassist;

import java.util.ArrayList;
import java.util.List;

import org.eclipse.core.runtime.IProgressMonitor;
import org.eclipse.jface.text.contentassist.CompletionProposal;
import org.eclipse.wst.jsdt.ui.text.java.ContentAssistInvocationContext;
import org.eclipse.wst.jsdt.ui.text.java.IJavaCompletionProposalComputer;

public class CustomCompletionProposalComputer implements IJavaCompletionProposalComputer {

 @Override
 public void sessionStarted() {
 }

 @Override
 public List computeCompletionProposals(ContentAssistInvocationContext context, IProgressMonitor monitor) {

  ArrayList<CompletionProposal> proposals = new ArrayList<CompletionProposal>();

  proposals.add(new CompletionProposal("codeandme.blogspot.com", context.getInvocationOffset(), 0, "codeandme.blogspot.com".length()));
  proposals.add(new CompletionProposal("<your proposal here>", context.getInvocationOffset(), 0, "<your proposal here>".length()));

  return proposals;
 }

 @Override
 public List computeContextInformation(ContentAssistInvocationContext context, IProgressMonitor monitor) {
  return null;
 }

 @Override
 public String getErrorMessage() {
  return null;
 }

 @Override
 public void sessionEnded() {
 }
}

CompletionProposals could contain more information like attached documentation or ContextInformation for variables and other dynamic content. See this post for some information on the Contextinformation.

Optional: Helper methods

Typically you would need to evaluate the last characters before the cursor position to filter your proposals. You also might be interested in the content of the current line left of the cursor position. Maybe these helper methods are of some interest:

 private static final Pattern LINE_DATA_PATTERN = Pattern.compile(".*?([^\\p{Alnum}]?)(\\p{Alnum}*)$");

 /**
  * Extract context relevant information from current line. The returned matcher locates the last alphanumeric word in the line and an optional non
  * alphanumeric character right before that word. result.group(1) contains the last non-alphanumeric token (eg a dot, brackets, arithmetic operators, ...),
  * result.group(2) contains the alphanumeric text. This text can be used to filter content assist proposals.
  * 
  * @param context
  *            content assist context
  * @return matcher containing content assist information
  * @throws BadLocationException
  */
 protected Matcher matchLastToken(final ContentAssistInvocationContext context) throws BadLocationException {
  String data = getCurrentLine(context);
  return LINE_DATA_PATTERN.matcher(data);
 }

 /**
  * Extract text from current line up to the cursor position
  * 
  * @param context
  *            content assist context
  * @return current line data
  * @throws BadLocationException
  */
 protected String getCurrentLine(final ContentAssistInvocationContext context) throws BadLocationException {
  IDocument document = context.getDocument();
  int lineNumber = document.getLineOfOffset(context.getInvocationOffset());
  IRegion lineInformation = document.getLineInformation(lineNumber);

  return document.get(lineInformation.getOffset(), context.getInvocationOffset() - lineInformation.getOffset());
 }