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# Providing Outlines
Outline information is used by clients to allow users to see the structure of
their code.
## Implementation details
Outline information is available through a subscription. If the server has
subscribed for outline information in some set of files, then the plugin should
send the information in an `analysis.outline` notification whenever the
information needs to be updated.
When a notification needs to be sent, the method `sendOutlineNotification` will
be invoked. This method is responsible for sending the notification.
The easiest way to add support for this notification is by adding the classes
`OutlineMixin` and `DartOutlineMixin` (from
`package:analyzer_plugin/plugin/outline_mixin.dart`) to the list of mixins
for your subclass of `ServerPlugin`. This will leave you with one abstract
method that you need to implement: `getOutlineContributors`. That method is
responsible for returning a list of `OutlineContributor`s. It is the outline
contributors that produce the actual outline information. (Most plugins will
only need a single outline contributor.)
To write an outline contributor, create a class that implements
`OutlineContributor`. The interface defines a single method named
`computeOutline`. The method has two arguments: an `OutlineRequest` that
describes the file for which outline information is being requested and an
`OutlineCollector` through which outline information is to be added.
If you mix in the class `DartOutlineMixin`, then the request will be an instance
of `DartOutlineRequest`, which also has analysis results.
## Example
Start by creating a class that implements `OutlineContributor`, then
implement the method `computeOutline`. This method is typically implemented
by creating a visitor (such as an AstVisitor) that can visit the results of the
analysis (such as a CompilationUnit) and extract the outline information from
the analysis result.
For example, your contributor might look something like the following:
```dart
class MyOutlineContributor implements OutlineContributor {
@override
void computeOutline(
OutlineRequest request, OutlineCollector collector) {
if (request is DartOutlineRequest) {
OutlineVisitor visitor = new OutlineVisitor(collector);
request.result.unit.accept(visitor);
}
}
}
class OutlineVisitor extends RecursiveAstVisitor {
final OutlineCollector collector;
OutlineVisitor(this.collector);
@override
void visitClassDeclaration(ClassDeclaration node) {
// ...
}
}
```
Given a contributor like the one above, you can implement your plugin similar to
the following:
```dart
class MyPlugin extends ServerPlugin with OutlineMixin, DartOutlineMixin {
// ...
@override
List<OutlineContributor> getOutlineContributors(String path) {
return <OutlineContributor>[new MyOutlineContributor()];
}
}
```