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references_and_resolve.md: minor edits

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topics/reference_guide/custom_language_support/references_and_resolve.md
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@ -4,31 +4,29 @@
One of the most important and tricky parts in implementing a custom language PSI is resolving references.
Resolving references gives users the ability to navigate from a PSI element usage (accessing a variable, calling a method, etc.) to the declaration of that element (the variable's definition, a method declaration, and so on).
This feature is needed in order to support the <menupath>Navigate | Declaration or Usages</menupath> action invoked by <shortcut>Ctrl/Cmd+B</shortcut> or clicking the mouse button while holding <shortcut>Ctrl/Cmd</shortcut> key, and it is a prerequisite for implementing the [Find Usages](find_usages.md) action, the [Rename Refactoring](rename_refactoring.md) and [Code Completion](code_completion.md).
> The <menupath>View | Quick Definition</menupath> action is based on the same mechanism, so it becomes automatically available for all references that can be resolved by the language plugin.
>
> To customize exact document range to show in the popup (e.g., include "surrounding" code or comments), provide [`ImplementationTextSelectioner`](upsource:///platform/lang-api/src/com/intellij/codeInsight/hint/ImplementationTextSelectioner.java) registered in `com.intellij.lang.implementationTextSelectioner` extension point.
>
{type="note"}
The <menupath>View | Quick Definition</menupath> action is based on the same mechanism, so it becomes automatically available for all references that can be resolved by the language plugin.
To customize the exact document range to show in the popup (e.g., include "surrounding" code or comments), provide [`ImplementationTextSelectioner`](upsource:///platform/lang-api/src/com/intellij/codeInsight/hint/ImplementationTextSelectioner.java) registered in `com.intellij.lang.implementationTextSelectioner` extension point.
## PsiReference
All PSI elements which work as references (for which the <menupath>Navigate | Declaration or Usages</menupath> action applies) need to implement the
[`PsiElement.getReference()`](upsource:///platform/core-api/src/com/intellij/psi/PsiElement.java) method and to return a [`PsiReference`](upsource:///platform/core-api/src/com/intellij/psi/PsiReference.java) 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`](upsource:///platform/core-api/src/com/intellij/psi/HintedReferenceHost.java) to provide additional hints.
The primary method of the [`PsiReference`](upsource:///platform/core-api/src/com/intellij/psi/PsiReference.java) 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`](upsource:///platform/core-api/src/com/intellij/psi/PsiNamedElement.java) interface.
In order to enable more advanced functionality, prefer implementing [`PsiNameIdentifierOwner`](upsource:///platform/core-api/src/com/intellij/psi/PsiNameIdentifierOwner.java) over [`PsiNamedElement`](upsource:///platform/core-api/src/com/intellij/psi/PsiNamedElement.java) where possible.
> While the referencing element and the referenced element both may have a name, only the element which **introduces** the name (e.g., the definition `int x = 42`) needs to implement `PsiNamedElement`.
> The referencing element at the point of usage (e.g., the `x` in the expression `x + 1`) should not implement `PsiNamedElement` since it does not _have_ a name.
>
{type="note"}
> In order to enable more advanced functionality, prefer implementing [`PsiNameIdentifierOwner`](upsource:///platform/core-api/src/com/intellij/psi/PsiNameIdentifierOwner.java) over [`PsiNamedElement`](upsource:///platform/core-api/src/com/intellij/psi/PsiNamedElement.java) where possible.
>
{type="tip"}
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).
@ -38,16 +36,15 @@ Still, additional optimizations are possible (for example, performing the tree w
- [Reference](upsource:///plugins/properties/src/com/intellij/lang/properties/ResourceBundleReference.java) to a ResourceBundle in the [Properties language plugin](upsource:///plugins/properties)
- [Custom Language Support Tutorial: Reference Contributor](reference_contributor.md)
> To optimize `getReferences()` performance, consider implementing [`HintedReferenceHost`](upsource:///platform/core-api/src/com/intellij/psi/HintedReferenceHost.java) to provide additional hints.
> Please see also [](psi_performance.md#cache-results-of-heavy-computations).
>
{type="tip"}
## Implementing Resolve Logic
There is a set of interfaces that can be used as a base for implementing resolve support, namely the [`PsiScopeProcessor`](upsource:///platform/core-api/src/com/intellij/psi/scope/PsiScopeProcessor.java) interface and the [`PsiElement.processDeclarations()`](upsource:///platform/core-api/src/com/intellij/psi/PsiElement.java) 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 [](psi_performance.md#cache-results-of-heavy-computations).
The implementation of resolve based on the standard helper classes contains the following components:
* A class implements the [`PsiScopeProcessor`](upsource:///platform/core-api/src/com/intellij/psi/scope/PsiScopeProcessor.java) interface, which gathers the possible declarations for the reference and stops the resolve process when it has successfully completed.
@ -62,7 +59,7 @@ The implementation of resolve based on the standard helper classes contains the
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
## Resolving to Multiple Targets
An extension of the [`PsiReference`](upsource:///platform/core-api/src/com/intellij/psi/PsiReference.java) interface, which allows a reference to resolve to multiple targets, is the [`PsiPolyVariantReference`](upsource:///platform/core-api/src/com/intellij/psi/PsiPolyVariantReference.java) interface.
The targets to which the reference resolves are returned from the `multiResolve()` method.