References and Resolve

PSI References

All PSI elements which work as references (for which the Navigate | Declaration or Usages action applies) need to implement the PsiElement.getReference() method and to return a PsiReference implementation from that method. The PsiReference can be implemented by the same class as PsiElement, or by a different class. An element can also contain multiple references (for example, a string literal can contain multiple substrings which are valid fully-qualified class names), in which case it can implement PsiElement.getReferences() and return the references as an array. To optimize PsiElement.getReferences() performance, consider implementing HintedReferenceHost to provide additional hints.

The primary method of the PsiReference interface is resolve(), which returns the element to which the reference points, or null if it was not possible to resolve the reference to a valid element (for example, should it point to an undefined class). The resolved element should implement the PsiNamedElement interface. In order to enable more advanced functionality, prefer implementing PsiNameIdentifierOwner over PsiNamedElement where possible.

A counterpart to the resolve() method is isReferenceTo(), which checks if the reference resolves to the specified element. The latter method can be implemented by calling resolve() and comparing the result with the passed PSI element. Still, additional optimizations are possible (for example, performing the tree walk only if the element text is equal to the text of the reference).

Examples:

• Reference to a ResourceBundle in the Properties language plugin

• Custom Language Support Tutorial: Reference Contributor

Implementing Resolve Logic

There is a set of interfaces that can be used as a base for implementing resolve support, namely the PsiScopeProcessor interface and the PsiElement.processDeclarations() method. These interfaces have several extra complexities that are unnecessary for most custom languages (like support for substituting Java generics types). Still, they are required if the custom language can have references to Java code. If Java interoperability is not required, the plugin can forgo the standard interfaces and provide its own, different implementation of resolve.

Please see also Cache Results of Heavy Computations.

The implementation of resolve based on the standard helper classes contains the following components:

• A class implements the PsiScopeProcessor interface, which gathers the possible declarations for the reference and stops the resolve process when it has successfully completed. The primary method which needs to be implemented is execute(), which is called to process every declaration encountered during the resolve, and returns true if the resolve needs to be continued or false if the declaration has been found. The methods getHint() and handleEvent() are used for internal optimizations and can be left empty in the PsiScopeProcessor implementations for custom languages.

• A function which walks the PSI tree up from the reference location until the resolve has successfully completed or until the end of the resolve scope has been reached. If the target of the reference is located in a different file, the file can be located, for example, using FilenameIndex.getFilesByName() (if the file name is known) or by iterating through all custom language files in the project (iterateContent() in the ProjectFileIndex interface obtained from ProjectRootManager.getFileIndex()).

• The individual PSI elements, on which the processDeclarations() method is called during the PSI tree walk. If a PSI element is a declaration, it passes itself to the execute() method of the PsiScopeProcessor passed to it. Also, if necessary, according to the language scoping rules, a PSI element can pass the PsiScopeProcessor to its child elements.

Resolving to Multiple Targets

An extension of the PsiReference interface, which allows a reference to resolve to multiple targets, is the PsiPolyVariantReference interface. The targets to which the reference resolves are returned from the multiResolve() method. The Navigate | Declaration or Usages action for such references allows the user to choose a navigation target in a popup. The implementation of multiResolve() can be also based on PsiScopeProcessor, and can collect all valid targets for the reference instead of stopping when the first valid target is found.

Additional Highlighting

Implement HighlightedReference to add additional highlighting for non-obvious places (e.g., inside String literals). Such references will automatically be highlighted using String | Highlighted reference text attributes from Settings | Editor | Color Scheme | Language Defaults.