As basically every part of Eclipse Xtext, the content assist API provides a reasonable default behavior and is greatly customizable by means of dependency injection. This blog post demonstrates, how easy it is to create own completion proposals with this API.
For the sake of simplicity, I'll use a grammar that fits in a single line:
It allows to define exactly one color literal and the syntax is not really strict about the literal's format. Valid input files are considered to look like color "0,0,0" or color "255,255,255".
Now comes the fun part: The proposal list for the rgb string should provide some frequently used values, e.g. literals for black or white.
Therefore I have to customize the proposal provider. Xtext already provides an empty stub implementation that will take the custom code. In order to change the proposals for the rgb-value in the parser rule Color, the method completeColor_Rgb(..) has to be overridden. JDT's content assist or the quick outline in the proposal provider implementation helps to find all suitable entry points.
Two lines of code later, I can already insert literals for black or white:
createCompletionProposal(..) returns an instance of the ConfigurableCompletionProposal which has a quite powerful API. It allows to change the appearance of the display string, I can set additional hover information, enable linked edit modes for the custom proposal or make it auto-insertable to name only a few.
The next example uses an own ReplacementTextApplier to hook into the actual moment, when the proposal is applied:
You'll guess that already from the code: This implementation allows to pick the color by means of a color chooser after the proposal has been selected, but see yourself:
There are plenty of other interesting APIs and services around the code completion in Xtext that are worthy of talking about (e.g. the prefix matcher, which Alex covered in his recent post), but I'll leave them for another post.