Allow FlutterViewController to be released when not initialized with an engine (#6879)

* Break cycle between FlutterViewController and FlutterEngine

shell/platform/darwin/ios/framework/Headers/Flutter.h

@@ -8,6 +8,9 @@
 /**
  BREAKING CHANGES:
 
+ December 17, 2018:
+  - Changed designated initializer on FlutterEngine
+
  October 5, 2018:
   - Removed FlutterNavigationController.h/.mm
   - Changed return signature of `FlutterDartHeadlessCodeRunner.run*` from void

shell/platform/darwin/ios/framework/Headers/FlutterEngine.h

@@ -55,8 +55,28 @@ FLUTTER_EXPORT
  *   the threads used by this FlutterEngine.
  * @param projectOrNil The `FlutterDartProject` to run.
  */
+- (instancetype)initWithName:(NSString*)labelPrefix project:(FlutterDartProject*)projectOrNil;
+
+/**
+ * Initialize this FlutterEngine with a `FlutterDartProject`.
+ *
+ * If the FlutterDartProject is not specified, the FlutterEngine will attempt to locate
+ * the project in a default location (the flutter_assets folder in the iOS application
+ * bundle).
+ *
+ * A newly initialized engine will not run the `FlutterDartProject` until either
+ * `-runWithEntrypoint:` or `-runWithEntrypoint:libraryURI:` is called.
+ *
+ * @param labelPrefix The label prefix used to identify threads for this instance. Should
+ *   be unique across FlutterEngine instances, and is used in instrumentation to label
+ *   the threads used by this FlutterEngine.
+ * @param projectOrNil The `FlutterDartProject` to run.
+ * @param allowHeadlessExecution Whether or not to allow this instance to continue
+ *   running after passing a nil `FlutterViewController` to `-setViewController:`.
+ */
 - (instancetype)initWithName:(NSString*)labelPrefix
-                     project:(FlutterDartProject*)projectOrNil NS_DESIGNATED_INITIALIZER;
+                     project:(FlutterDartProject*)projectOrNil
+      allowHeadlessExecution:(BOOL)allowHeadlessExecution NS_DESIGNATED_INITIALIZER;
 
 /**
  * The default initializer is not available for this object.

shell/platform/darwin/ios/framework/Headers/FlutterHeadlessDartRunner.h

@@ -46,8 +46,25 @@ FLUTTER_DEPRECATED("FlutterEngine should be used rather than FlutterHeadlessDart
  * be unique across FlutterEngine instances
  * @param projectOrNil The `FlutterDartProject` to run.
  */
+- (instancetype)initWithName:(NSString*)labelPrefix project:(FlutterDartProject*)projectOrNil;
+
+/**
+ * Iniitalize this FlutterHeadlessDartRunner with a `FlutterDartProject`.
+ *
+ * If the FlutterDartProject is not specified, the FlutterHeadlessDartRunner will attempt to locate
+ * the project in a default location.
+ *
+ * A newly initialized engine will not run the `FlutterDartProject` until either
+ * `-runWithEntrypoint:` or `-runWithEntrypoint:libraryURI` is called.
+ *
+ * @param labelPrefix The label prefix used to identify threads for this instance. Should
+ * be unique across FlutterEngine instances
+ * @param projectOrNil The `FlutterDartProject` to run.
+ * @param allowHeadlessExecution Must be set to `YES`.
+ */
 - (instancetype)initWithName:(NSString*)labelPrefix
-                     project:(FlutterDartProject*)projectOrNil NS_DESIGNATED_INITIALIZER;
+                     project:(FlutterDartProject*)projectOrNil
+      allowHeadlessExecution:(BOOL)allowHeadlessExecution NS_DESIGNATED_INITIALIZER;
 
 /**
  * Not recommended for use - will initialize with a default label ("io.flutter.headless")

shell/platform/darwin/ios/framework/Headers/FlutterViewController.h

@@ -151,6 +151,11 @@ FLUTTER_EXPORT
  */
 @property(nonatomic, getter=isViewOpaque) BOOL viewOpaque;
 
+/**
+ * The `FlutterEngine` instance for this view controller.
+ */
+@property(weak, nonatomic, readonly) FlutterEngine* engine;
+
 @end
 
 #endif  // FLUTTER_FLUTTERVIEWCONTROLLER_H_

shell/platform/darwin/ios/framework/Source/FlutterEngine.mm

@@ -60,12 +60,22 @@
   fml::scoped_nsobject<FlutterBasicMessageChannel> _settingsChannel;
 
   int64_t _nextTextureId;
+
+  BOOL _allowHeadlessExecution;
 }
 
 - (instancetype)initWithName:(NSString*)labelPrefix project:(FlutterDartProject*)projectOrNil {
+  return [self initWithName:labelPrefix project:projectOrNil allowHeadlessExecution:YES];
+}
+
+- (instancetype)initWithName:(NSString*)labelPrefix
+                     project:(FlutterDartProject*)projectOrNil
+      allowHeadlessExecution:(BOOL)allowHeadlessExecution {
   self = [super init];
   NSAssert(self, @"Super init cannot be nil");
   NSAssert(labelPrefix, @"labelPrefix is required");
+
+  _allowHeadlessExecution = allowHeadlessExecution;
   _labelPrefix = [labelPrefix copy];
 
   _weakFactory = std::make_unique<fml::WeakPtrFactory<FlutterEngine>>(self);
@@ -76,7 +86,6 @@
     _dartProject.reset([projectOrNil retain]);
 
   _pluginPublications = [NSMutableDictionary new];
-  _publisher.reset([[FlutterObservatoryPublisher alloc] init]);
   _platformViewsController.reset(new shell::FlutterPlatformViewsController());
 
   [self setupChannels];
@@ -135,7 +144,14 @@
   FML_DCHECK(self.iosPlatformView);
   _viewController = [viewController getWeakPtr];
   self.iosPlatformView->SetOwnerViewController(_viewController);
-  [self maybeSetupPlatformViewChannels];
+  if (!viewController && !_allowHeadlessExecution) {
+    [self resetChannels];
+
+    _shell.reset();
+    _threadHost.Reset();
+  } else {
+    [self maybeSetupPlatformViewChannels];
+  }
 }
 
 - (FlutterViewController*)viewController {
@@ -176,6 +192,20 @@
   return _settingsChannel.get();
 }
 
+- (void)resetChannels {
+  _localizationChannel.reset();
+  _navigationChannel.reset();
+  _platformChannel.reset();
+  _platformViewsChannel.reset();
+  _textInputChannel.reset();
+  _lifecycleChannel.reset();
+  _systemChannel.reset();
+  _settingsChannel.reset();
+}
+
+// If you add a channel, be sure to also update `resetChannels`.
+// Channels get a reference to the engine, and therefore need manual
+// cleanup for proper collection.
 - (void)setupChannels {
   _localizationChannel.reset([[FlutterMethodChannel alloc]
          initWithName:@"flutter/localization"
@@ -221,8 +251,6 @@
   _textInputPlugin.get().textInputDelegate = self;
 
   _platformPlugin.reset([[FlutterPlatformPlugin alloc] initWithEngine:[self getWeakPtr]]);
-
-  [self maybeSetupPlatformViewChannels];
 }
 
 - (void)maybeSetupPlatformViewChannels {
@@ -348,6 +376,11 @@
     FML_LOG(ERROR) << "Could not start a shell FlutterEngine with entrypoint: "
                    << entrypoint.UTF8String;
   } else {
+    [self setupChannels];
+    if (!_platformViewsController) {
+      _platformViewsController.reset(new shell::FlutterPlatformViewsController());
+    }
+    _publisher.reset([[FlutterObservatoryPublisher alloc] init]);
     [self maybeSetupPlatformViewChannels];
   }
 

shell/platform/darwin/ios/framework/Source/FlutterHeadlessDartRunner.mm

@@ -28,7 +28,17 @@
 }
 
 - (instancetype)initWithName:(NSString*)labelPrefix project:(FlutterDartProject*)projectOrNil {
-  return [super initWithName:labelPrefix project:projectOrNil];
+  return [self initWithName:labelPrefix project:projectOrNil allowHeadlessExecution:YES];
+}
+
+- (instancetype)initWithName:(NSString*)labelPrefix
+                     project:(FlutterDartProject*)projectOrNil
+      allowHeadlessExecution:(BOOL)allowHeadlessExecution {
+  NSAssert(allowHeadlessExecution == YES,
+           @"Cannot initialize a FlutterHeadlessDartRunner without headless execution.");
+  return [super initWithName:labelPrefix
+                     project:projectOrNil
+      allowHeadlessExecution:allowHeadlessExecution];
 }
 - (instancetype)init {
   return [self initWithName:@"io.flutter.headless" project:nil];

shell/platform/darwin/ios/framework/Source/FlutterViewController.mm

@@ -67,7 +67,9 @@
   if (self) {
     _viewOpaque = YES;
     _weakFactory = std::make_unique<fml::WeakPtrFactory<FlutterViewController>>(self);
-    _engine.reset([[FlutterEngine alloc] initWithName:@"io.flutter" project:projectOrNil]);
+    _engine.reset([[FlutterEngine alloc] initWithName:@"io.flutter"
+                                              project:projectOrNil
+                               allowHeadlessExecution:NO]);
     _flutterView.reset([[FlutterView alloc] initWithDelegate:_engine opaque:self.isViewOpaque]);
     [_engine.get() createShell:nil libraryURI:nil];
     _engineNeedsLaunch = YES;
@@ -116,8 +118,8 @@
   [self setupNotificationCenterObservers];
 }
 
-- (fml::scoped_nsobject<FlutterEngine>)engine {
-  return _engine;
+- (FlutterEngine*)engine {
+  return _engine.get();
 }
 
 - (fml::WeakPtr<FlutterViewController>)getWeakPtr {
@@ -389,9 +391,9 @@
 
   if (_engineNeedsLaunch) {
     [_engine.get() launchEngine:nil libraryURI:nil];
-    [_engine.get() setViewController:self];
     _engineNeedsLaunch = NO;
   }
+  [_engine.get() setViewController:self];
 
   // Only recreate surface on subsequent appearances when viewport metrics are known.
   // First time surface creation is done on viewDidLayoutSubviews.
@@ -423,7 +425,7 @@
   TRACE_EVENT0("flutter", "viewDidDisappear");
   [self surfaceUpdated:NO];
   [[_engine.get() lifecycleChannel] sendMessage:@"AppLifecycleState.paused"];
-
+  [_engine.get() setViewController:nil];
   [super viewDidDisappear:animated];
 }
 

shell/platform/darwin/ios/framework/Source/FlutterViewController_Internal.h

@@ -17,8 +17,6 @@
 - (fml::WeakPtr<FlutterViewController>)getWeakPtr;
 - (shell::FlutterPlatformViewsController*)platformViewsController;
 
-@property(readonly) fml::scoped_nsobject<FlutterEngine> engine;
-
 @end
 
 #endif  // FLUTTER_SHELL_PLATFORM_DARWIN_IOS_FRAMEWORK_SOURCE_FLUTTERVIEWCONTROLLER_INTERNAL_H_