Add a viewController property to the ios/macOS FlutterPluginRegistrar protocol (flutter#174168)

flutter#104117

Adds a `viewController` (per
flutter#104117 (comment),
presumably because iOS plugins want to presenting new view controllers)
getter that makes the single-view assumption. ~This is a method instead
of a weak + readonly property like the macOS registrar protocol because
existing getters are also methods, e.g.,~
```objc
- (NSObject<FlutterBinaryMessenger>*)messenger;
- (NSObject<FlutterTextureRegistry>*)textures;
```

~Unfortunately swift callers will have to use a preprocess macro and add
different code paths for macOS and iOS.~

#### Single view assumption

I've considered adding a `- (nullable
UIViewController*)viewControllerForViewID: ViewIDType;` method but that
will force plugins to make breaking changes in order to migrate off of
the deprecated `keyWindow` property. Take the text input plugin as an
example, it has to add a `viewID` argument to the `setTextInputClient`
call when establishing a new text input connection, so the text input
plugin knows which view hierarchy to add the text input view to. It is
unclear to me whether more `FlutterPluginRegistrar` changes are needed
to completely get rid of the single-view assumption in the protocol (the
views `FlutterPlatformViewFactory` creates are always added as subviews
to `flutterView` so I guess that interface may have to change in the
future? According to according to [Flutter’s multi-window
status](https://docs.google.com/document/d/13E27tD8_9f6lDgwg3MpGNTV8XIRCZH3ByI-t9kI9IUM/edit?tab=t.0#heading=h.8lxwn1tiky70),
only a `viewForIdentifier` method will be added to registrar), so IMO it
would be a better experience to not introduce the `viewID` concept right
now and force plugins to make break changes, since they may have to make
more breaking changes in the future.

Alternatively, we could also ignore the `viewID` args for now and always
return the view controller of the implicit view in the implementation,
so that plugin authors can use made-up view IDs to get the implicit view
controller. But that's going to make future multiview migrations
difficult since the compiler won't be able to catch unmigrated code if
the plugin is using a bogus view ID.

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above.
- [ ] I updated/added relevant documentation (doc comments with `///`).
- [ ] I added new tests to check the change I am making, or this PR is
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Fixes] where supported.
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Author: LongCatIsLooong

engine/src/flutter/shell/platform/darwin/ios/framework/Headers/FlutterPlatformViews.h

@@ -34,7 +34,7 @@ FLUTTER_DARWIN_EXPORT
  * @param frame The rectangle for the newly created `UIView` measured in points.
  * @param viewId A unique identifier for this `UIView`.
  * @param args Parameters for creating the `UIView` sent from the Dart side of the Flutter app.
- *   If `createArgsCodec` is not implemented, or if no creation arguments were sent from the Dart
+ *   If `createArgsCodec` is not implemented, or if no creation arguments were sent from the Dart
  *   code, this will be null. Otherwise this will be the value sent from the Dart code as decoded by
  *   `createArgsCodec`.
  */

engine/src/flutter/shell/platform/darwin/ios/framework/Headers/FlutterPlugin.h

@@ -297,6 +297,20 @@ typedef enum {
  */
 - (NSObject<FlutterTextureRegistry>*)textures;
 
+/**
+ * The `UIViewController` whose view is displaying Flutter content.
+ *
+ * The plugin typically should not store a strong reference to this view
+ * controller.
+ *
+ * This property is provided for backwards compatibility for apps that assume
+ * a single view, and will eventually be replaced by the multi-view API variant.
+ *
+ * This property may be |nil|, for instance in a headless environment, or when
+ * the underlying Flutter engine is deallocated.
+ */
+@property(nullable, readonly) UIViewController* viewController;
+
 /**
  * Registers a `FlutterPlatformViewFactory` for creation of platform views.
  *

engine/src/flutter/shell/platform/darwin/ios/framework/Source/FlutterEngine.mm

@@ -2,6 +2,7 @@
 // Use of this source code is governed by a BSD-style license that can be
 // found in the LICENSE file.
 
+#include <UIKit/UIKit.h>
 #include "common/settings.h"
 #define FML_USED_ON_EMBEDDER
 
@@ -1521,6 +1522,10 @@ - (instancetype)initWithPlugin:(NSString*)pluginKey flutterEngine:(FlutterEngine
   return _flutterEngine.textureRegistry;
 }
 
+- (nullable UIViewController*)viewController {
+  return _flutterEngine.viewController;
+}
+
 - (void)publish:(NSObject*)value {
   _flutterEngine.pluginPublications[_pluginKey] = value;
 }

engine/src/flutter/shell/platform/darwin/ios/framework/Source/FlutterEngineTest.mm

@@ -189,6 +189,17 @@ - (void)testNotifyPluginOfDealloc {
   OCMVerify([plugin detachFromEngineForRegistrar:[OCMArg any]]);
 }
 
+- (void)testGetViewControllerFromRegistrar {
+  FlutterDartProject* project = [[FlutterDartProject alloc] init];
+  FlutterEngine* engine = [[FlutterEngine alloc] initWithName:@"engine" project:project];
+  id mockEngine = OCMPartialMock(engine);
+  NSObject<FlutterPluginRegistrar>* registrar = [mockEngine registrarForPlugin:@"plugin"];
+
+  // Verify accessing the viewController getter calls FlutterEngine.viewController.
+  (void)[registrar viewController];
+  OCMVerify(times(1), [mockEngine viewController]);
+}
+
 - (void)testSetBinaryMessengerToSameBinaryMessenger {
   FakeBinaryMessengerRelay* fakeBinaryMessenger = [[FakeBinaryMessengerRelay alloc] init];
 

engine/src/flutter/shell/platform/darwin/macos/framework/Headers/FlutterPluginRegistrarMacOS.h

@@ -49,6 +49,19 @@ FLUTTER_DARWIN_EXPORT
  */
 @property(nullable, readonly) NSView* view;
 
+/**
+ * The `NSViewController` that hosts |view|.
+ *
+ * The plugin typically should not store a strong reference to this view
+ * controller.
+ *
+ * This property is provided for backwards compatibility for apps that assume
+ * a single view, and will eventually be replaced by the multi-view API variant.
+ *
+ * This property is |nil| when |view| is |nil|.
+ */
+@property(nullable, readonly) NSViewController* viewController;
+
 /**
  * Registers |delegate| to receive handleMethodCall:result: callbacks for the given |channel|.
  */

engine/src/flutter/shell/platform/darwin/macos/framework/Source/FlutterEngine.mm

@@ -366,6 +366,10 @@ - (NSView*)viewForIdentifier:(FlutterViewIdentifier)viewIdentifier {
   return controller.flutterView;
 }
 
+- (NSViewController*)viewController {
+  return [_flutterEngine viewControllerForIdentifier:kFlutterImplicitViewId];
+}
+
 - (void)addMethodCallDelegate:(nonnull id<FlutterPlugin>)delegate
                       channel:(nonnull FlutterMethodChannel*)channel {
   [channel setMethodCallHandler:^(FlutterMethodCall* call, FlutterResult result) {

engine/src/flutter/shell/platform/darwin/macos/framework/Source/FlutterEngineTest.mm

@@ -744,6 +744,21 @@ @implementation MockableFlutterEngine
   EXPECT_EQ([engine valuePublishedByPlugin:pluginName], secondValue);
 }
 
+TEST_F(FlutterEngineTest, RegistrarForwardViewControllerLookUpToEngine) {
+  NSString* fixtures = @(flutter::testing::GetFixturesPath());
+  FlutterDartProject* project = [[FlutterDartProject alloc]
+      initWithAssetsPath:fixtures
+             ICUDataPath:[fixtures stringByAppendingString:@"/icudtl.dat"]];
+  FlutterEngine* engine = [[FlutterEngine alloc] initWithName:@"test" project:project];
+
+  FlutterViewController* viewController = [[FlutterViewController alloc] initWithEngine:engine
+                                                                                nibName:nil
+                                                                                 bundle:nil];
+  id<FlutterPluginRegistrar> registrar = [engine registrarForPlugin:@"MyPlugin"];
+
+  EXPECT_EQ([registrar viewController], viewController);
+}
+
 // If a channel overrides a previous channel with the same name, cleaning
 // the previous channel should not affect the new channel.
 //

engine/src/flutter/shell/platform/darwin/macos/framework/Source/FlutterMenuPluginTest.mm

@@ -58,6 +58,10 @@ - (nonnull NSString*)lookupKeyForAsset:(nonnull NSString*)asset
                            fromPackage:(nonnull NSString*)package {
   return @"";
 }
+
+- (NSViewController*)viewController {
+  return nil;
+}
 @end
 
 namespace flutter::testing {