Languages can choose to support IntelliPrompt completion lists and respond to Ctrl+Space.
Basic Concepts
The ICompletionProvider interface defines a single method: RequestSession. This method has a parameter that indicates the IEditorView in which to open the session and a boolean parameter that indicates whether to auto-complete if a single match is found.
SyntaxEditor is wired up to automatically call this method if a completion provider is registered as a service on the current language when Ctrl+Space is typed by the end user.
RequestSession can also be called directly in your code if you wish to generically initiate a completion session, assuming the language supports it.
Implementors of the method should use code to examine the context of the caret in the view. This may involve token scanning and/or AST examination if available. Based on the context (in a method declaration, in a type declaration, in a documentation comment, etc.), a completion session should be opened if appropriate.
The CompletionProviderBase Base Class
The easiest way to implement an ICompletionProvider for a language is to inherit the CompletionProviderBase class. Then just override its abstract RequestSession method.
Registering with a Language
Any object that implements ICompletionProvider can be associated with a syntax language by registering it as an ICompletionProvider service on the language.
This code registers a completion provider in the completionProvider variable with the syntax language that is already declared in the language variable:
It is a common practice for a language itself to implement the ICompletionProvider interface and register itself as a completion provider service.
Ordering Providers
The ICompletionProvider interface inherits the IOrderable interface. When multiple language services are registered that implement ICompletionProvider, they can be ordered. The highest priority provider will be used first. If it indicates that a session was opened, no other providers will be used. If it indicates that a session was not opened, the next provider is checked, and so on. This allows for layering of multiple providers.