Annotate nullability on FlutterEngine to make swift writing more ergonomic (#11808)

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

@@ -16,6 +16,8 @@
 
 @class FlutterViewController;
 
+NS_ASSUME_NONNULL_BEGIN
+
 /**
  * The FlutterEngine class coordinates a single instance of execution for a
  * `FlutterDartProject`.  It may have zero or one `FlutterViewController` at a
@@ -56,9 +58,9 @@ FLUTTER_EXPORT
  * @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 project The `FlutterDartProject` to run.
  */
-- (instancetype)initWithName:(NSString*)labelPrefix project:(FlutterDartProject*)projectOrNil;
+- (instancetype)initWithName:(NSString*)labelPrefix project:(nullable FlutterDartProject*)project;
 
 /**
  * Initialize this FlutterEngine with a `FlutterDartProject`.
@@ -73,12 +75,12 @@ FLUTTER_EXPORT
  * @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 project 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
+                     project:(nullable FlutterDartProject*)project
       allowHeadlessExecution:(BOOL)allowHeadlessExecution NS_DESIGNATED_INITIALIZER;
 
 /**
@@ -103,7 +105,7 @@ FLUTTER_EXPORT
  *   tree-shaken by the Dart compiler.
  * @return YES if the call succeeds in creating and running a Flutter Engine instance; NO otherwise.
  */
-- (BOOL)runWithEntrypoint:(NSString*)entrypoint;
+- (BOOL)runWithEntrypoint:(nullable NSString*)entrypoint;
 
 /**
  * Runs a Dart program on an Isolate using the specified entrypoint and Dart library,
@@ -120,7 +122,7 @@ FLUTTER_EXPORT
  *   this will default to the same library as the `main()` function in the Dart program.
  * @return YES if the call succeeds in creating and running a Flutter Engine instance; NO otherwise.
  */
-- (BOOL)runWithEntrypoint:(NSString*)entrypoint libraryURI:(NSString*)uri;
+- (BOOL)runWithEntrypoint:(nullable NSString*)entrypoint libraryURI:(nullable NSString*)uri;
 
 /**
  * Destroy running context for an engine.
@@ -177,11 +179,15 @@ FLUTTER_EXPORT
 /**
  * The `FlutterMethodChannel` used for localization related platform messages, such as
  * setting the locale.
+ *
+ * Can be nil after `destroyContext` is called.
  */
-@property(nonatomic, readonly) FlutterMethodChannel* localizationChannel;
+@property(nonatomic, readonly, nullable) FlutterMethodChannel* localizationChannel;
 /**
  * The `FlutterMethodChannel` used for navigation related platform messages.
  *
+ * Can be nil after `destroyContext` is called.
+ *
  * @see [Navigation
  * Channel](https://docs.flutter.io/flutter/services/SystemChannels/navigation-constant.html)
  * @see [Navigator Widget](https://docs.flutter.io/flutter/widgets/Navigator-class.html)
@@ -191,6 +197,8 @@ FLUTTER_EXPORT
 /**
  * The `FlutterMethodChannel` used for core platform messages, such as
  * information about the screen orientation.
+ *
+ * Can be nil after `destroyContext` is called.
  */
 @property(nonatomic, readonly) FlutterMethodChannel* platformChannel;
 
@@ -198,6 +206,8 @@ FLUTTER_EXPORT
  * The `FlutterMethodChannel` used to communicate text input events to the
  * Dart Isolate.
  *
+ * Can be nil after `destroyContext` is called.
+ *
  * @see [Text Input
  * Channel](https://docs.flutter.io/flutter/services/SystemChannels/textInput-constant.html)
  */
@@ -207,6 +217,8 @@ FLUTTER_EXPORT
  * The `FlutterBasicMessageChannel` used to communicate app lifecycle events
  * to the Dart Isolate.
  *
+ * Can be nil after `destroyContext` is called.
+ *
  * @see [Lifecycle
  * Channel](https://docs.flutter.io/flutter/services/SystemChannels/lifecycle-constant.html)
  */
@@ -216,6 +228,8 @@ FLUTTER_EXPORT
  * The `FlutterBasicMessageChannel` used for communicating system events, such as
  * memory pressure events.
  *
+ * Can be nil after `destroyContext` is called.
+ *
  * @see [System
  * Channel](https://docs.flutter.io/flutter/services/SystemChannels/system-constant.html)
  */
@@ -224,6 +238,8 @@ FLUTTER_EXPORT
 /**
  * The `FlutterBasicMessageChannel` used for communicating user settings such as
  * clock format and text scale.
+ *
+ * Can be nil after `destroyContext` is called.
  */
 @property(nonatomic, readonly) FlutterBasicMessageChannel* settingsChannel;
 
@@ -234,7 +250,7 @@ FLUTTER_EXPORT
  * observatory service is ready. In release mode or before the observatory has
  * started, it returns `nil`.
  */
-@property(nonatomic, readonly) NSURL* observatoryUrl;
+@property(nonatomic, readonly, nullable) NSURL* observatoryUrl;
 
 /**
  * The `FlutterBinaryMessenger` associated with this FlutterEngine (used for communicating with
@@ -247,8 +263,10 @@ FLUTTER_EXPORT
  *
  * This property will be nil if the engine is not running.
  */
-@property(nonatomic, readonly, copy) NSString* isolateId;
+@property(nonatomic, readonly, copy, nullable) NSString* isolateId;
 
 @end
 
+NS_ASSUME_NONNULL_END
+
 #endif  // FLUTTER_FLUTTERENGINE_H_

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

@@ -69,12 +69,12 @@
   FlutterBinaryMessengerRelay* _binaryMessenger;
 }
 
-- (instancetype)initWithName:(NSString*)labelPrefix project:(FlutterDartProject*)projectOrNil {
-  return [self initWithName:labelPrefix project:projectOrNil allowHeadlessExecution:YES];
+- (instancetype)initWithName:(NSString*)labelPrefix project:(FlutterDartProject*)project {
+  return [self initWithName:labelPrefix project:project allowHeadlessExecution:YES];
 }
 
 - (instancetype)initWithName:(NSString*)labelPrefix
-                     project:(FlutterDartProject*)projectOrNil
+                     project:(FlutterDartProject*)project
       allowHeadlessExecution:(BOOL)allowHeadlessExecution {
   self = [super init];
   NSAssert(self, @"Super init cannot be nil");
@@ -85,10 +85,10 @@
 
   _weakFactory = std::make_unique<fml::WeakPtrFactory<FlutterEngine>>(self);
 
-  if (projectOrNil == nil)
+  if (project == nil)
     _dartProject.reset([[FlutterDartProject alloc] init]);
   else
-    _dartProject.reset([projectOrNil retain]);
+    _dartProject.reset([project retain]);
 
   _pluginPublications = [NSMutableDictionary new];
   _platformViewsController.reset(new flutter::FlutterPlatformViewsController());