Adding a setup function for hot restart (#18921)

* Adding a setup function for hot restart that can be overwritten to perform any necessary initialization before a restart is started.

* Adding a test for setupHotRestart

* Removing local pubspec change

* Fixing some type annotation errors

* Responding to comments

packages/flutter_tools/lib/src/run_hot.dart

@@ -27,6 +27,11 @@ class HotRunnerConfig {
   bool computeDartDependencies = true;
   /// Should the hot runner assume that the minimal Dart dependencies do not change?
   bool stableDartDependencies = false;
+  /// A hook for implementations to perform any necessary initialization prior
+  /// to a hot restart. Should return true if the hot restart should continue.
+  Future<bool> setupHotRestart() async {
+    return true;
+  }
 }
 
 HotRunnerConfig get hotRunnerConfig => context[HotRunnerConfig];
@@ -477,6 +482,10 @@ class HotRunner extends ResidentRunner {
       );
       try {
         final Stopwatch timer = new Stopwatch()..start();
+        if (!(await hotRunnerConfig.setupHotRestart())) {
+          status.cancel();
+          return new OperationResult(1, 'setupHotRestart failed');
+        }
         await _restartFromSources();
         timer.stop();
         status.cancel();

packages/flutter_tools/test/hot_test.dart

@@ -2,7 +2,13 @@
 // Use of this source code is governed by a BSD-style license that can be
 // found in the LICENSE file.
 
+import 'dart:async';
+
+import 'package:flutter_tools/src/device.dart';
+import 'package:flutter_tools/src/resident_runner.dart';
 import 'package:flutter_tools/src/run_hot.dart';
+import 'package:meta/meta.dart';
+import 'package:mockito/mockito.dart';
 import 'package:test/test.dart';
 
 import 'src/context.dart';
@@ -83,4 +89,49 @@ void main() {
       }), true);
     });
   });
+
+  group('hotRestart', () {
+    final List<FlutterDevice> devices = <FlutterDevice>[];
+
+    setUp(() {
+      devices.add(new FlutterDevice(new MockDevice(),
+          previewDart2: false, trackWidgetCreation: false));
+    });
+
+    testUsingContext('no setup', () async {
+      expect((await new HotRunner(devices).restart(fullRestart: true)).isOk,
+          true);
+    });
+
+    testUsingContext('setup function succeeds', () async {
+      expect((await new HotRunner(devices).restart(fullRestart: true)).isOk,
+          true);
+    }, overrides: <Type, Generator>{
+      HotRunnerConfig: () => new TestHotRunnerConfig(successfulSetup: true),
+    });
+
+    testUsingContext('setup function fails', () async {
+      expect((await new HotRunner(devices).restart(fullRestart: true)).isOk,
+          false);
+    }, overrides: <Type, Generator>{
+      HotRunnerConfig: () => new TestHotRunnerConfig(successfulSetup: false),
+    });
+  });
+}
+
+class MockDevice extends Mock implements Device {
+  MockDevice() {
+    when(isSupported()).thenReturn(true);
+  }
+}
+
+class TestHotRunnerConfig extends HotRunnerConfig {
+  bool successfulSetup;
+
+  TestHotRunnerConfig({@required this.successfulSetup});
+
+  @override
+  Future<bool> setupHotRestart() async {
+    return successfulSetup;
+  }
 }