[macos] FlutterKeyboardManager memory leak fix (flutter#48824)

We are embedding Flutter into MacOS app, and noticed that there is a
leak. `leaks` tool says there is a cycle reference:

```
12 (2.11K) ROOT CYCLE: <FlutterKeyboardManager 0x29ec55f40> [80]
       8 (432 bytes) __strong _primaryResponders --> ROOT CYCLE: <NSMutableArray 0x29ec560a0> [64]
          7 (368 bytes) ROOT CYCLE: <NSMutableArray (Storage) 0x29ec56140> [32]
             4 (224 bytes) ROOT CYCLE: <FlutterEmbedderKeyResponder 0x29ec56310> [80]
                1 (48 bytes) __strong _sendEvent --> ROOT CYCLE: <__NSMallocBlock__ 0x29ec56360> [48]
                   __strong [capture] --> CYCLE BACK TO <FlutterKeyboardManager 0x29ec55f40> [80]
                1 (48 bytes) __strong _pendingResponses --> <NSMutableDictionary 0x29ec563c0> [48]
                1 (48 bytes) __strong _pressingRecords --> <NSMutableDictionary 0x29ec56390> [48]
             2 (112 bytes) <FlutterChannelKeyResponder 0x29ec56450> [48]
                1 (64 bytes) __strong _channel --> <FlutterBasicMessageChannel 0x29ec564e0> [64]
       2 (1.55K) __strong _layoutMap --> <NSMutableDictionary 0x29ec56630> [48]
          1 (1.50K) <NSMutableDictionary (Storage) 0x123c34a00> [1536]
       1 (64 bytes) __strong _pendingEvents --> <NSMutableArray 0x29ec565f0> [64]
```

This patch uses `weak` pointer to `self` instead of implicit `strong`.

## Pre-launch Checklist

- [x] I read the [Contributor Guide] and followed the process outlined
there for submitting PRs.
- [x] I read the [Tree Hygiene] wiki page, which explains my
responsibilities.
- [x] I read and followed the [Flutter Style Guide] and the [C++,
Objective-C, Java style guides].
- [x] I listed at least one issue that this PR fixes in the description
above.
- [x] I added new tests to check the change I am making or feature I am
adding, or the PR is [test-exempt]. See [testing the engine] for
instructions on writing and running engine tests.
- [x] I updated/added relevant documentation (doc comments with `///`).
- [x] I signed the [CLA].
- [x] All existing and new tests are passing.

Author: sergiy-sc

shell/platform/darwin/macos/framework/Source/FlutterKeyboardManager.mm

@@ -123,18 +123,24 @@ - (nonnull instancetype)initWithViewDelegate:(nonnull id<FlutterKeyboardViewDele
         [FlutterMethodChannel methodChannelWithName:@"flutter/keyboard"
                                     binaryMessenger:[_viewDelegate getBinaryMessenger]
                                               codec:[FlutterStandardMethodCodec sharedInstance]];
+
     [keyboardChannel setMethodCallHandler:^(FlutterMethodCall* call, FlutterResult result) {
       [self handleKeyboardMethodCall:call result:result];
     }];
+
     _primaryResponders = [[NSMutableArray alloc] init];
+
+    __weak __typeof__(self) weakSelf = self;
     [self addPrimaryResponder:[[FlutterEmbedderKeyResponder alloc]
                                   initWithSendEvent:^(const FlutterKeyEvent& event,
                                                       FlutterKeyEventCallback callback,
                                                       void* userData) {
-                                    [_viewDelegate sendKeyEvent:event
-                                                       callback:callback
-                                                       userData:userData];
+                                    __strong __typeof__(weakSelf) strongSelf = weakSelf;
+                                    [strongSelf.viewDelegate sendKeyEvent:event
+                                                                 callback:callback
+                                                                 userData:userData];
                                   }]];
+
     [self
         addPrimaryResponder:[[FlutterChannelKeyResponder alloc]
                                 initWithChannel:[FlutterBasicMessageChannel
@@ -143,14 +149,14 @@ - (nonnull instancetype)initWithViewDelegate:(nonnull id<FlutterKeyboardViewDele
                                                                                getBinaryMessenger]
                                                                      codec:[FlutterJSONMessageCodec
                                                                                sharedInstance]]]];
+
     _pendingEvents = [[NSMutableArray alloc] init];
     _layoutMap = [NSMutableDictionary<NSNumber*, NSNumber*> dictionary];
     [self buildLayout];
     for (id<FlutterKeyPrimaryResponder> responder in _primaryResponders) {
       responder.layoutMap = _layoutMap;
     }
 
-    __weak __typeof__(self) weakSelf = self;
     [_viewDelegate subscribeToKeyboardLayoutChange:^() {
       [weakSelf buildLayout];
     }];

shell/platform/darwin/macos/framework/Source/FlutterKeyboardManagerTest.mm

@@ -426,9 +426,11 @@ - (bool)getPressedState;
 - (bool)keyboardChannelGetPressedState;
 - (bool)racingConditionBetweenKeyAndText;
 - (bool)correctLogicalKeyForLayouts;
+- (bool)shouldNotHoldStrongReferenceToViewDelegate;
 @end
 
 namespace flutter::testing {
+
 TEST(FlutterKeyboardManagerUnittests, SinglePrimaryResponder) {
   ASSERT_TRUE([[FlutterKeyboardManagerUnittestsObjC alloc] singlePrimaryResponder]);
 }
@@ -461,6 +463,11 @@ - (bool)correctLogicalKeyForLayouts;
   ASSERT_TRUE([[FlutterKeyboardManagerUnittestsObjC alloc] correctLogicalKeyForLayouts]);
 }
 
+TEST(FlutterKeyboardManagerUnittests, ShouldNotHoldStrongReferenceToViewDelegate) {
+  ASSERT_TRUE(
+      [[FlutterKeyboardManagerUnittestsObjC alloc] shouldNotHoldStrongReferenceToViewDelegate]);
+}
+
 }  // namespace flutter::testing
 
 @implementation FlutterKeyboardManagerUnittestsObjC
@@ -809,4 +816,30 @@ - (bool)correctLogicalKeyForLayouts {
   return TRUE;
 }
 
+- (bool)shouldNotHoldStrongReferenceToViewDelegate {
+  __strong FlutterKeyboardManager* strongKeyboardManager;
+  __weak id weakViewDelegate;
+
+  @autoreleasepool {
+    id binaryMessengerMock = OCMStrictProtocolMock(@protocol(FlutterBinaryMessenger));
+    OCMStub([binaryMessengerMock setMessageHandlerOnChannel:[OCMArg any]
+                                       binaryMessageHandler:[OCMArg any]]);
+
+    id viewDelegateMock = OCMStrictProtocolMock(@protocol(FlutterKeyboardViewDelegate));
+    OCMStub([viewDelegateMock getBinaryMessenger]).andReturn(binaryMessengerMock);
+    OCMStub([viewDelegateMock subscribeToKeyboardLayoutChange:[OCMArg any]]);
+
+    LayoutClue layoutClue;
+    OCMStub([viewDelegateMock lookUpLayoutForKeyCode:0 shift:NO])
+        .ignoringNonObjectArgs()
+        .andReturn(layoutClue);
+    FlutterKeyboardManager* keyboardManager =
+        [[FlutterKeyboardManager alloc] initWithViewDelegate:viewDelegateMock];
+    strongKeyboardManager = keyboardManager;
+    weakViewDelegate = viewDelegateMock;
+  }
+
+  return weakViewDelegate == nil;
+}
+
 @end