[analysis_server] Add snippet for function definitions

Fixes https://github.com/Dart-Code/Dart-Code/issues/4017.

Change-Id: I1017be3abe73a8cfcd6354f1c2ca99ad729dcf47
Reviewed-on: https://dart-review.googlesource.com/c/sdk/+/247553
Reviewed-by: Brian Wilkerson <brianwilkerson@google.com>
Commit-Queue: Brian Wilkerson <brianwilkerson@google.com>

pkg/analysis_server/lib/src/services/snippets/dart/dart_snippet_producers.dart

@@ -166,6 +166,47 @@ class DartForLoopSnippetProducer extends DartSnippetProducer {
       DartForLoopSnippetProducer._(request);
 }
 
+/// Produces a [Snippet] that creates a function definition.
+class DartFunctionSnippetProducer extends DartSnippetProducer {
+  static const prefix = 'fun';
+  static const label = 'fun';
+
+  DartFunctionSnippetProducer._(super.request);
+
+  @override
+  Future<Snippet> compute() async {
+    final builder = ChangeBuilder(session: request.analysisSession);
+    final indent = utils.getLinePrefix(request.offset);
+
+    await builder.addDartFileEdit(request.filePath, (builder) {
+      builder.addReplacement(request.replacementRange, (builder) {
+        void writeIndented(String string) => builder.write('$indent$string');
+
+        builder.addSimpleLinkedEdit('returnType', 'void');
+        builder.write(' ');
+        builder.addSimpleLinkedEdit('name', 'name');
+        builder.write('(');
+        builder.addSimpleLinkedEdit('params', 'params');
+        builder.writeln(') {');
+        writeIndented('  ');
+        builder.selectHere();
+        builder.writeln();
+        writeIndented('}');
+      });
+    });
+
+    return Snippet(
+      prefix,
+      label,
+      'Insert a function definition.',
+      builder.sourceChange,
+    );
+  }
+
+  static DartFunctionSnippetProducer newInstance(DartSnippetRequest request) =>
+      DartFunctionSnippetProducer._(request);
+}
+
 /// Produces a [Snippet] that creates an if/else statement.
 class DartIfElseSnippetProducer extends DartSnippetProducer {
   static const prefix = 'ife';

pkg/analysis_server/lib/src/services/snippets/dart/snippet_manager.dart

@@ -29,8 +29,10 @@ class DartSnippetManager {
       FlutterStatefulWidgetWithAnimationControllerSnippetProducer.newInstance,
       FlutterStatelessWidgetSnippetProducer.newInstance,
       DartClassSnippetProducer.newInstance,
+      DartFunctionSnippetProducer.newInstance,
     ],
     SnippetContext.inBlock: [
+      DartFunctionSnippetProducer.newInstance,
       DartDoWhileLoopSnippetProducer.newInstance,
       DartForInLoopSnippetProducer.newInstance,
       DartForLoopSnippetProducer.newInstance,
@@ -42,6 +44,9 @@ class DartSnippetManager {
       DartTestBlockSnippetProducer.newInstance,
       DartTestGroupBlockSnippetProducer.newInstance,
     ],
+    SnippetContext.inClass: [
+      DartFunctionSnippetProducer.newInstance,
+    ]
   };
 
   Future<List<Snippet>> computeSnippets(

pkg/analysis_server/test/lsp/completion_dart_test.dart

@@ -2639,6 +2639,71 @@ void f() {
 ''');
   }
 
+  Future<void> test_snippets_functionClassMember() async {
+    final content = '''
+class A {
+  fun^
+}
+''';
+
+    await initializeWithSnippetSupport();
+    final updated = await expectAndApplySnippet(
+      content,
+      prefix: DartFunctionSnippetProducer.prefix,
+      label: DartFunctionSnippetProducer.label,
+    );
+
+    expect(updated, r'''
+class A {
+  ${1:void} ${2:name}(${3:params}) {
+    $0
+  }
+}
+''');
+  }
+
+  Future<void> test_snippets_functionNested() async {
+    final content = '''
+void a() {
+  fun^
+}
+''';
+
+    await initializeWithSnippetSupport();
+    final updated = await expectAndApplySnippet(
+      content,
+      prefix: DartFunctionSnippetProducer.prefix,
+      label: DartFunctionSnippetProducer.label,
+    );
+
+    expect(updated, r'''
+void a() {
+  ${1:void} ${2:name}(${3:params}) {
+    $0
+  }
+}
+''');
+  }
+
+  Future<void> test_snippets_functionTopLevel() async {
+    final content = '''
+fun^
+''';
+
+    await initializeWithSnippetSupport();
+    final updated = await expectAndApplySnippet(
+      content,
+      prefix: DartFunctionSnippetProducer.prefix,
+      label: DartFunctionSnippetProducer.label,
+    );
+
+    expect(updated, r'''
+${1:void} ${2:name}(${3:params}) {
+  $0
+}
+''');
+  }
+
   Future<void> test_snippets_if() async {
     final content = '''
 void f() {

pkg/analysis_server/test/services/snippets/dart/dart_snippet_producers_test.dart

@@ -23,6 +23,7 @@ void main() {
     defineReflectiveTests(DartTryCatchSnippetProducerTest);
     defineReflectiveTests(DartWhileLoopSnippetProducerTest);
     defineReflectiveTests(DartClassSnippetProducerTest);
+    defineReflectiveTests(DartFunctionSnippetProducerTest);
     defineReflectiveTests(DartTestBlockSnippetProducerTest);
     defineReflectiveTests(DartTestGroupBlockSnippetProducerTest);
   });
@@ -215,6 +216,160 @@ void f() {
   }
 }
 
+@reflectiveTest
+class DartFunctionSnippetProducerTest extends DartSnippetProducerTest {
+  @override
+  final generator = DartFunctionSnippetProducer.newInstance;
+
+  @override
+  String get label => DartFunctionSnippetProducer.label;
+
+  @override
+  String get prefix => DartFunctionSnippetProducer.prefix;
+
+  Future<void> test_classMethod() async {
+    var code = r'''
+class A {
+  ^
+}''';
+    final snippet = await expectValidSnippet(code);
+    expect(snippet.prefix, prefix);
+    expect(snippet.label, label);
+    expect(snippet.change.edits, hasLength(1));
+    code = withoutMarkers(code);
+    for (var edit in snippet.change.edits) {
+      code = SourceEdit.applySequence(code, edit.edits);
+    }
+    expect(code, '''
+class A {
+  void name(params) {
+    
+  }
+}''');
+    expect(snippet.change.selection!.file, testFile);
+    expect(snippet.change.selection!.offset, 36);
+    expect(snippet.change.linkedEditGroups.map((group) => group.toJson()), [
+      {
+        'positions': [
+          {'file': testFile, 'offset': 12},
+        ],
+        'length': 4,
+        'suggestions': []
+      },
+      {
+        'positions': [
+          {'file': testFile, 'offset': 17},
+        ],
+        'length': 4,
+        'suggestions': []
+      },
+      {
+        'positions': [
+          {'file': testFile, 'offset': 22},
+        ],
+        'length': 6,
+        'suggestions': []
+      },
+    ]);
+  }
+
+  Future<void> test_nested() async {
+    var code = r'''
+void a() {
+  ^
+}''';
+    final snippet = await expectValidSnippet(code);
+    expect(snippet.prefix, prefix);
+    expect(snippet.label, label);
+    expect(snippet.change.edits, hasLength(1));
+    code = withoutMarkers(code);
+    for (var edit in snippet.change.edits) {
+      code = SourceEdit.applySequence(code, edit.edits);
+    }
+    expect(code, '''
+void a() {
+  void name(params) {
+    
+  }
+}''');
+    expect(snippet.change.selection!.file, testFile);
+    expect(snippet.change.selection!.offset, 37);
+    expect(snippet.change.linkedEditGroups.map((group) => group.toJson()), [
+      {
+        'positions': [
+          {'file': testFile, 'offset': 13},
+        ],
+        'length': 4,
+        'suggestions': []
+      },
+      {
+        'positions': [
+          {'file': testFile, 'offset': 18},
+        ],
+        'length': 4,
+        'suggestions': []
+      },
+      {
+        'positions': [
+          {'file': testFile, 'offset': 23},
+        ],
+        'length': 6,
+        'suggestions': []
+      },
+    ]);
+  }
+
+  Future<void> test_topLevel() async {
+    var code = r'''
+class A {}
+  
+^
+
+class B {}''';
+    final snippet = await expectValidSnippet(code);
+    expect(snippet.prefix, prefix);
+    expect(snippet.label, label);
+    expect(snippet.change.edits, hasLength(1));
+    code = withoutMarkers(code);
+    for (var edit in snippet.change.edits) {
+      code = SourceEdit.applySequence(code, edit.edits);
+    }
+    expect(code, '''
+class A {}
+  
+void name(params) {
+  
+}
+
+class B {}''');
+    expect(snippet.change.selection!.file, testFile);
+    expect(snippet.change.selection!.offset, 36);
+    expect(snippet.change.linkedEditGroups.map((group) => group.toJson()), [
+      {
+        'positions': [
+          {'file': testFile, 'offset': 14},
+        ],
+        'length': 4,
+        'suggestions': []
+      },
+      {
+        'positions': [
+          {'file': testFile, 'offset': 19},
+        ],
+        'length': 4,
+        'suggestions': []
+      },
+      {
+        'positions': [
+          {'file': testFile, 'offset': 24},
+        ],
+        'length': 6,
+        'suggestions': []
+      },
+    ]);
+  }
+}
+
 @reflectiveTest
 class DartIfElseSnippetProducerTest extends DartSnippetProducerTest {
   @override