Add a mixin for assists similar to that for fixes

R=scheglov@google.com

Review-Url: https://codereview.chromium.org/2956353002 .

pkg/analysis_server/lib/plugin/edit/assist/assist_core.dart

@@ -8,6 +8,7 @@ import 'package:analyzer/src/dart/analysis/driver.dart';
 import 'package:analyzer/src/generated/source.dart';
 import 'package:analyzer_plugin/protocol/protocol_common.dart'
     show SourceChange;
+import 'package:analyzer_plugin/utilities/assist/assist.dart';
 
 /**
  * A description of a single proposed assist.
@@ -26,7 +27,7 @@ class Assist {
    */
   static final Comparator<Assist> SORT_BY_RELEVANCE =
       (Assist firstAssist, Assist secondAssist) =>
-          firstAssist.kind.relevance - secondAssist.kind.relevance;
+          firstAssist.kind.priority - secondAssist.kind.priority;
 
   /**
    * A description of the assist being proposed.
@@ -87,37 +88,3 @@ abstract class AssistContributor {
    */
   Future<List<Assist>> computeAssists(AssistContext context);
 }
-
-/**
- * A description of a class of assists. Instances are intended to hold the
- * information that is common across a number of assists and to be shared by
- * those assists.
- *
- * Clients may not extend, implement or mix-in this class.
- */
-class AssistKind {
-  /**
-   * The name of this kind of assist, used for debugging.
-   */
-  final String name;
-
-  /**
-   * The relevance of this kind of assist for the kind of error being addressed.
-   */
-  final int relevance;
-
-  /**
-   * A human-readable description of the changes that will be applied by this
-   * kind of assist.
-   */
-  final String message;
-
-  /**
-   * Initialize a newly created kind of assist to have the given [name],
-   * [relevance] and [message].
-   */
-  const AssistKind(this.name, this.relevance, this.message);
-
-  @override
-  String toString() => name;
-}

pkg/analysis_server/lib/src/services/correction/assist.dart

@@ -5,6 +5,7 @@
 import 'package:analysis_server/plugin/edit/assist/assist_core.dart';
 import 'package:analyzer/src/dart/analysis/driver.dart';
 import 'package:analyzer/src/generated/source.dart';
+import 'package:analyzer_plugin/utilities/assist/assist.dart';
 
 /**
  * The implementation of [AssistContext].

pkg/analysis_server/lib/src/services/correction/assist_internal.dart

@@ -26,6 +26,8 @@ import 'package:analyzer/src/generated/java_core.dart';
 import 'package:analyzer/src/generated/resolver.dart';
 import 'package:analyzer/src/generated/source.dart';
 import 'package:analyzer_plugin/protocol/protocol_common.dart' hide Element;
+import 'package:analyzer_plugin/utilities/assist/assist.dart'
+    hide AssistContributor;
 import 'package:analyzer_plugin/utilities/change_builder/change_builder_dart.dart';
 import 'package:analyzer_plugin/utilities/range_factory.dart';
 import 'package:path/path.dart';

pkg/analysis_server/test/services/correction/assist_test.dart

@@ -20,6 +20,7 @@ import 'package:analyzer/src/dart/element/ast_provider.dart';
 import 'package:analyzer/src/generated/engine.dart';
 import 'package:analyzer/src/generated/source.dart';
 import 'package:analyzer_plugin/protocol/protocol_common.dart';
+import 'package:analyzer_plugin/utilities/assist/assist.dart';
 import 'package:plugin/manager.dart';
 import 'package:plugin/plugin.dart';
 import 'package:test/test.dart';

pkg/analyzer_plugin/lib/utilities/assist/assist.dart

@@ -74,6 +74,42 @@ class AssistGenerator {
   }
 }
 
+/**
+ * A description of a class of assists. Instances are intended to hold the
+ * information that is common across a number of assists and to be shared by
+ * those assists.
+ *
+ * Clients may not extend, implement or mix-in this class.
+ */
+class AssistKind {
+  /**
+   * The name of this kind of assist, used for debugging.
+   */
+  final String name;
+
+  /**
+   * The priority of this kind of assist for the kind of error being addressed.
+   */
+  final int priority;
+
+  /**
+   * A human-readable description of the changes that will be applied by this
+   * kind of assist. The message can contain parameters, where each parameter is
+   * represented by a zero-based index inside curly braces. For example, the
+   * message `"Create a component named '{0}' in '{1}'"` contains two parameters.
+   */
+  final String message;
+
+  /**
+   * Initialize a newly created kind of assist to have the given [name],
+   * [relevance] and [message].
+   */
+  const AssistKind(this.name, this.priority, this.message);
+
+  @override
+  String toString() => name;
+}
+
 /**
  * The information about a requested set of assists.
  *

pkg/analyzer_plugin/lib/utilities/assist/assist_contributor_mixin.dart

@@ -0,0 +1,40 @@
+// Copyright (c) 2017, the Dart project authors.  Please see the AUTHORS file
+// for details. All rights reserved. Use of this source code is governed by a
+// BSD-style license that can be found in the LICENSE file.
+
+import 'package:analyzer/src/generated/java_core.dart';
+import 'package:analyzer_plugin/protocol/protocol_common.dart';
+import 'package:analyzer_plugin/protocol/protocol_generated.dart';
+import 'package:analyzer_plugin/utilities/assist/assist.dart';
+import 'package:analyzer_plugin/utilities/change_builder/change_builder_core.dart';
+
+/**
+ * A partial implementation of an [AssistContributor] that provides a utility
+ * method to make it easier to add assists.
+ *
+ * Clients may not extend or implement this class, but are allowed to use it as
+ * a mix-in when creating a subclass of [AssistContributor].
+ */
+abstract class AssistContributorMixin implements AssistContributor {
+  /**
+   * The collector to which assists should be added.
+   */
+  AssistCollector get collector;
+
+  /**
+   * Add an assist. Use the [kind] of the assist to get the message and priority,
+   * and use the change [builder] to get the edits that comprise the assist. If
+   * the message has parameters, then use the list of [args] to populate the
+   * message.
+   */
+  void addAssistFromBuilder(ChangeBuilder builder, AssistKind kind,
+      {List<Object> args}) {
+    SourceChange change = builder.sourceChange;
+    if (change.edits.isEmpty) {
+      return;
+    }
+    change.message = formatList(kind.message, args);
+    collector.addAssist(
+        new PrioritizedSourceChange(kind.priority, builder.sourceChange));
+  }
+}

pkg/analyzer_plugin/lib/utilities/fixes/fix_contributor_mixin.dart

@@ -35,7 +35,7 @@ abstract class FixContributorMixin implements FixContributor {
    * [args] to populate the message.
    */
   void addFix(AnalysisError error, FixKind kind, ChangeBuilder builder,
-      {List<Object> args: null}) {
+      {List<Object> args}) {
     SourceChange change = builder.sourceChange;
     if (change.edits.isEmpty) {
       return;