Two main types of code completion can be provided by custom language plugins: reference completion and contributor-based completion.
Reference completion is easier to implement, but supports only the basic completion action. Contributor-based completion provides more features, supports all three completion types (basic, smart, and class name), and can be used, for example, to implement keyword completion.
To fill the completion list, the IDE calls
PsiReference.getVariants() either on the reference at the caret location or on a dummy reference that would be placed at the caret. This method needs to return an array of objects containing either strings,
PsiElement instances or instances of the
LookupElement class (see Lookup Items below). If a
PsiElement instance is returned in the array, the completion list shows the icon for the element.
The most common way to implement
getVariants() is to use the same function for walking up the tree as in
PsiReference.resolve(), and a different implementation of
PsiScopeProcessor which collects all declarations passed to its
execute() method and returns them as an array for filling the completion list.
Symbol Reference Completion
To provide completion variants by a
CompletionContributor interface gives you the greatest control over the operation of code completion for your language. Register in
com.intellij.completion.contributor extension point and specify
language attribute (unless it works on any supported language).
The core scenario of using
CompletionContributor consists of calling the
extend() method and passing in the Element Pattern specifying the context in which this completion variant is applicable, as well as a completion provider which generates the items to show in the completion list.
Keep in mind that the pattern is checked against the leaf PSI element. If you want to match a composite element, use
CompletionContributorfor completing keywords in MANIFEST.MF files.
For every lookup element, you can specify the following attributes:
Text Shown left-aligned.
Tail text Shown next to the main item text, is not used for prefix matching, and can be used, for example, to show the parameter list of the method.
Type text Shown right-aligned in the lookup list and can be used to show the return type or containing class of a method, for example.
Text attributes Bold, Strikeout, etc.
Insert handler The insert handler is a callback which is called when the item is selected and can be used to perform additional modifications of the text (for example, to put in the parentheses for a method call)