Writing assists

This package gives analyzer plugin authors the ability to write “quick assists,” which can make local changes to source code, like small refactorings, from a developer's IDE. This document describes briefly how to write such an assist, and how to register it in an analyzer plugin.

The ResolvedCorrectionProducer class

A quick assist is specified by subclassing the ResolvedCorrectionProducer class. Here is our example:

import 'package:analysis_server_plugin/edit/dart/correction_producer.dart';
import 'package:analyzer/dart/ast/ast.dart';
import 'package:analyzer_plugin/utilities/change_builder/change_builder_core.dart';
import 'package:analyzer_plugin/utilities/assist/assist.dart';
import 'package:analyzer_plugin/utilities/range_factory.dart';

class RemoveAwait extends ResolvedCorrectionProducer {
  static const _removeAwaitKind = AssistKind(
      'dart.assist.removeAwait', 30 /* default */, "Remove the 'await' keyword");

  RemoveAwait({required super.context});

  @override
  CorrectionApplicability get applicability =>
      CorrectionApplicability.singleLocation;

  @override
  AssistKind get assistKind => _removeAwaitKind;

  @override
  Future<void> compute(ChangeBuilder builder) async {
    var awaitExpression = node;
    if (awaitExpression is AwaitExpression) {
      var awaitToken = awaitExpression.awaitKeyword;
      await builder.addDartFileEdit(file, (builder) {
        builder.addDeletion(range.startStart(awaitToken, awaitToken.next!));
      });
    }
  }
}

Let's look at each declaration individually:

• class RemoveAwait - A quick assist is a class that extends ResolvedCorrectionProducer. The name of the base class indicates that an instance of this class can produce “corrections” (a set of edits) for a resolved library.

• static const _removeAwaitKind = AssistKind(...) - Each quick assist must have an associated AssistKind which has a unique id ('dart.assist.removeAwait'), a priority (DartFixKindPriority.standard is a fine default), and a message which is displayed in the IDE ("Remove the 'await' keyword").

• RemoveAwait({required super.context}); - A standard constructor that accepts a CorrectionProducerContext and passes it up to the super-constructor.

• CorrectionApplicability get applicability => - the applicability field describes how widely an assist can be applied safely and sensibly. Currently, only CorrectionApplicability.singleLocation should be used.

• AssistKind get assistKind => _removeAwaitKind; - each instance of this class can refer to the static field for it's assistKind.

• Future<void> compute(ChangeBuilder builder) - This method is called when the client has requested quick assists at a specific location, and we want a possible correction from this correction producer. This is the code that looks at the node field and surrounding code and determines what correction to offer, if any.

  • await builder.addDartFileEdit(...) - Once we have determined that we want to offer an assist, we call this method, and specify code deletions, insertions, and/or replacements inside the callback function. If there are cases where this correction producer will not offer any quick assists (such as the source code having certain properties), then those cases should be checked so that we don't call this method in such cases.
  • builder.addDeletion(...) - For this assist (removing an await keyword), we can use addDeletion to specify a range of source code text to delete. The DartFileEditBuilder class has many utilities for adding various edits.

  Writing a quick assist can be non-trivial, even for changes which are conceptually simple. It may be helpful to see examples that are similar to a desired assist. See the assists that are offered by Dart Analysis Server for hundreds of examples.

Instances of the correction producer class are short-lived, and they can contain state related to the source-code-under-analysis. Indeed, the CorrectionProducerContext, which is passed into the constructor, and available as a field in the super-class, contains information specific to the code-under-analysis.

Registering a quick assist

In order for a quick assist to be used in an analyzer plugin, it must be registered. Register the quick assist‘s constructor inside a plugin’s register method:

import 'package:analysis_server_plugin/plugin.dart';
import 'package:analysis_server_plugin/registry.dart';

final plugin = SimplePlugin();

class SimplePlugin extends Plugin {
  @override
  void register(PluginRegistry registry) {
    registry.registerAssist(RemoveAwait.new);
  }
}

Instances of correction producers contain state related to the specific source-code-under-analysis, which is why the constructor is given here, instead of a long-lived instance.

See writing a plugin for information about the Plugin class.