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Content Assist

The Xtext generator, amongst other things, generates the following two content assist related artifacts:

  • An abstract proposal provider class named Abstract{MyLang}ProposalProvider generated into the src-gen folder within the ui project, and
  • a concrete subclass in the src-folder of the ui project called {MyLang}ProposalProvider

First we will investigate the generated Abstract{MyLang}ProposalProvider with methods that look like this:

public void complete{TypeName}_{FeatureName}(
  EObject model, Assignment assignment, 
  ContentAssistContext context, ICompletionProposalAcceptor acceptor) {
  // clients may override
}

public void complete_{RuleName}(
  EObject model, RuleCall ruleCall, 
  ContentAssistContext context, ICompletionProposalAcceptor acceptor) {
  // clients may override
}

The snippet above indicates that the generated class contains a complete*-method for each assigned feature in the grammar and for each rule. The braces in the snippet are place-holders that should give a clue about the naming scheme used to create the various entry points for implementors. The generated proposal provider falls back to some default behavior for cross-references and keywords. Furthermore it inherits the logic that was introduced in grammars that were mixed into the current language.

Clients who want to customize the behavior may override the methods from the AbstractJavaBasedContentProposalProvider (src) or introduce new methods with a specialized first parameter. The framework inspects the type of the model object and dispatches method calls to the most concrete implementation, that can be found.

It is important to know, that for a given offset in a model file, many possible grammar elements exist. The framework dispatches to the method declarations for any valid element. That means, that a bunch of complete* methods may be called.

To provide a dummy proposal for the code of an event instance, you may introduce a specialization of the generated method and implement it as follows. This will propose ZonkID for an event with the name Zonk.

public void completeEvent_Code(
  Event event, Assignment assignment, 
  ContentAssistContext context, ICompletionProposalAcceptor acceptor) {
  // call implementation of superclass
  super.completeEvent_Code(model, assignment, context, acceptor);

  // compute the plain proposal
  String proposal = event.getName() + "ID";

  // Create and register the completion proposal:
  // The proposal may be null as the createCompletionProposal(..) 
  // methods check for valid prefixes and terminal token conflicts.
  // The acceptor handles null-values gracefully.
  acceptor.accept(createCompletionProposal(proposal, context));
}