Migrate SemanticsObject and FlutterSemanticsScrollView to ARC (flutter/engine#52824)

On top of the `SemanticsObject` refactor https://github.com/flutter/engine/pull/52729 migrate `SemanticsObject` and `FlutterSemanticsScrollView` to ARC.

Part of https://github.com/flutter/flutter/issues/137801.

engine/src/flutter/shell/platform/darwin/ios/BUILD.gn

@@ -83,6 +83,8 @@ source_set("flutter_framework_source_arc") {
     "framework/Source/FlutterPluginAppLifeCycleDelegate.mm",
     "framework/Source/FlutterRestorationPlugin.h",
     "framework/Source/FlutterRestorationPlugin.mm",
+    "framework/Source/FlutterSemanticsScrollView.h",
+    "framework/Source/FlutterSemanticsScrollView.mm",
     "framework/Source/FlutterSpellCheckPlugin.h",
     "framework/Source/FlutterSpellCheckPlugin.mm",
     "framework/Source/FlutterTextInputDelegate.h",
@@ -100,6 +102,8 @@ source_set("flutter_framework_source_arc") {
     "framework/Source/FlutterViewResponder.h",
     "framework/Source/KeyCodeMap.g.mm",
     "framework/Source/KeyCodeMap_Internal.h",
+    "framework/Source/SemanticsObject.h",
+    "framework/Source/SemanticsObject.mm",
     "framework/Source/UIViewController+FlutterScreenAndSceneIfLoaded.h",
     "framework/Source/UIViewController+FlutterScreenAndSceneIfLoaded.mm",
     "framework/Source/connection_collection.h",
@@ -173,12 +177,8 @@ source_set("flutter_framework_source") {
     "framework/Source/FlutterHeadlessDartRunner.mm",
     "framework/Source/FlutterPlatformPlugin.h",
     "framework/Source/FlutterPlatformPlugin.mm",
-    "framework/Source/FlutterSemanticsScrollView.h",
-    "framework/Source/FlutterSemanticsScrollView.mm",
     "framework/Source/FlutterViewController.mm",
     "framework/Source/FlutterViewController_Internal.h",
-    "framework/Source/SemanticsObject.h",
-    "framework/Source/SemanticsObject.mm",
     "framework/Source/accessibility_bridge.h",
     "framework/Source/accessibility_bridge.mm",
     "framework/Source/accessibility_text_entry.h",

engine/src/flutter/shell/platform/darwin/ios/framework/Source/FlutterSemanticsScrollView.h

@@ -20,7 +20,7 @@ NS_ASSUME_NONNULL_BEGIN
  */
 @interface FlutterSemanticsScrollView : UIScrollView
 
-@property(nonatomic, assign, nullable) SemanticsObject* semanticsObject;
+@property(nonatomic, weak, nullable) SemanticsObject* semanticsObject;
 
 - (instancetype)init NS_UNAVAILABLE;
 - (instancetype)initWithFrame:(CGRect)frame NS_UNAVAILABLE;

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

@@ -4,8 +4,11 @@
 
 #import "flutter/shell/platform/darwin/ios/framework/Source/FlutterSemanticsScrollView.h"
 
+#import "flutter/shell/platform/darwin/common/framework/Headers/FlutterMacros.h"
 #import "flutter/shell/platform/darwin/ios/framework/Source/SemanticsObject.h"
 
+FLUTTER_ASSERT_ARC
+
 @implementation FlutterSemanticsScrollView
 
 - (instancetype)initWithSemanticsObject:(SemanticsObject*)semanticsObject {

engine/src/flutter/shell/platform/darwin/ios/framework/Source/SemanticsObject.h

@@ -38,7 +38,7 @@ constexpr float kScrollExtentMaxForInf = 1000;
  * The parent of this node in the node tree. Will be nil for the root node and
  * during transient state changes.
  */
-@property(nonatomic, assign, readonly) SemanticsObject* parent;
+@property(nonatomic, weak, readonly) SemanticsObject* parent;
 
 /**
  * The accessibility bridge that this semantics object is attached to. This
@@ -231,7 +231,7 @@ constexpr float kScrollExtentMaxForInf = 1000;
                                  bridge:(fml::WeakPtr<flutter::AccessibilityBridgeIos>)bridge
     NS_DESIGNATED_INITIALIZER;
 
-@property(nonatomic, assign) SemanticsObject* semanticsObject;
+@property(nonatomic, weak) SemanticsObject* semanticsObject;
 
 @end
 

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

@@ -7,6 +7,8 @@
 #import "flutter/shell/platform/darwin/ios/framework/Source/FlutterPlatformViews_Internal.h"
 #import "flutter/shell/platform/darwin/ios/framework/Source/FlutterSemanticsScrollView.h"
 
+FLUTTER_ASSERT_ARC
+
 namespace {
 
 flutter::SemanticsAction GetSemanticsActionForScrollDirection(
@@ -107,11 +109,6 @@ CGRect ConvertRectToGlobal(SemanticsObject* reference, CGRect local_rect) {
   return self;
 }
 
-- (void)dealloc {
-  [_nativeSwitch release];
-  [super dealloc];
-}
-
 - (NSMethodSignature*)methodSignatureForSelector:(SEL)sel {
   NSMethodSignature* result = [super methodSignatureForSelector:sel];
   if (!result) {
@@ -145,7 +142,7 @@ CGRect ConvertRectToGlobal(SemanticsObject* reference, CGRect local_rect) {
 @end  // FlutterSwitchSemanticsObject
 
 @interface FlutterScrollableSemanticsObject ()
-@property(nonatomic, retain) FlutterSemanticsScrollView* scrollView;
+@property(nonatomic) FlutterSemanticsScrollView* scrollView;
 @end
 
 @implementation FlutterScrollableSemanticsObject
@@ -164,9 +161,6 @@ CGRect ConvertRectToGlobal(SemanticsObject* reference, CGRect local_rect) {
 
 - (void)dealloc {
   [_scrollView removeFromSuperview];
-  _scrollView.semanticsObject = nil;
-  [_scrollView release];
-  [super dealloc];
 }
 
 - (void)accessibilityBridgeDidFinishUpdate {
@@ -249,10 +243,10 @@ CGRect ConvertRectToGlobal(SemanticsObject* reference, CGRect local_rect) {
 @end
 
 @interface SemanticsObject ()
-@property(nonatomic, retain) SemanticsObjectContainer* container;
+@property(nonatomic) SemanticsObjectContainer* container;
 
 /** Should only be called in conjunction with setting child/parent relationship. */
-@property(nonatomic, assign, readwrite) SemanticsObject* parent;
+@property(nonatomic, weak, readwrite) SemanticsObject* parent;
 
 @end
 
@@ -261,15 +255,6 @@ CGRect ConvertRectToGlobal(SemanticsObject* reference, CGRect local_rect) {
   BOOL _inDealloc;
 }
 
-#pragma mark - Override base class designated initializers
-
-// Method declared as unavailable in the interface
-- (instancetype)init {
-  [self release];
-  [super doesNotRecognizeSelector:_cmd];
-  return nil;
-}
-
 #pragma mark - Designated initializers
 
 - (instancetype)initWithBridge:(fml::WeakPtr<flutter::AccessibilityBridgeIos>)bridge
@@ -293,17 +278,19 @@ CGRect ConvertRectToGlobal(SemanticsObject* reference, CGRect local_rect) {
 }
 
 - (void)dealloc {
+  // Set parent and children parents to nil explicitly in dealloc.
+  // -[UIAccessibilityElement dealloc] has in the past called into -accessibilityContainer
+  // and self.children. There have also been crashes related to iOS
+  // accessing methods during dealloc, and there's a lag before the tree changes.
+  // See https://github.com/flutter/engine/pull/4602 and
+  // https://github.com/flutter/engine/pull/27786.
   for (SemanticsObject* child in _children) {
     child.parent = nil;
   }
   [_children removeAllObjects];
-  [_children release];
-  [_childrenInHitTestOrder release];
 
   _parent = nil;
-  [_container release];
   _inDealloc = YES;
-  [super dealloc];
 }
 
 #pragma mark - Semantic object property accesser
@@ -312,7 +299,6 @@ CGRect ConvertRectToGlobal(SemanticsObject* reference, CGRect local_rect) {
   for (SemanticsObject* child in _children) {
     child.parent = nil;
   }
-  [_children release];
   _children = [children mutableCopy];
   for (SemanticsObject* child in _children) {
     child.parent = self;
@@ -323,7 +309,6 @@ CGRect ConvertRectToGlobal(SemanticsObject* reference, CGRect local_rect) {
   for (SemanticsObject* child in _childrenInHitTestOrder) {
     child.parent = nil;
   }
-  [_childrenInHitTestOrder release];
   _childrenInHitTestOrder = [childrenInHitTestOrder copy];
   for (SemanticsObject* child in _childrenInHitTestOrder) {
     child.parent = self;
@@ -416,7 +401,7 @@ CGRect ConvertRectToGlobal(SemanticsObject* reference, CGRect local_rect) {
                                          withAttributes:
                                              (const flutter::StringAttributes&)attributes {
   NSMutableAttributedString* attributedString =
-      [[[NSMutableAttributedString alloc] initWithString:string] autorelease];
+      [[NSMutableAttributedString alloc] initWithString:string];
   for (const auto& attribute : attributes) {
     NSRange range = NSMakeRange(attribute->start, attribute->end - attribute->start);
     switch (attribute->type) {
@@ -686,9 +671,8 @@ CGRect ConvertRectToGlobal(SemanticsObject* reference, CGRect local_rect) {
 
   if ([self hasChildren] || self.uid == kRootNodeId) {
     if (self.container == nil) {
-      self.container =
-          [[[SemanticsObjectContainer alloc] initWithSemanticsObject:self
-                                                              bridge:self.bridge] autorelease];
+      self.container = [[SemanticsObjectContainer alloc] initWithSemanticsObject:self
+                                                                          bridge:self.bridge];
     }
     return self.container;
   }
@@ -787,17 +771,7 @@ CGRect ConvertRectToGlobal(SemanticsObject* reference, CGRect local_rect) {
 
 @end
 
-@implementation FlutterSemanticsObject {
-}
-
-#pragma mark - Override base class designated initializers
-
-// Method declared as unavailable in the interface
-- (instancetype)init {
-  [self release];
-  [super doesNotRecognizeSelector:_cmd];
-  return nil;
-}
+@implementation FlutterSemanticsObject
 
 #pragma mark - Designated initializers
 
@@ -853,7 +827,7 @@ CGRect ConvertRectToGlobal(SemanticsObject* reference, CGRect local_rect) {
 @end
 
 @interface FlutterPlatformViewSemanticsContainer ()
-@property(nonatomic, assign) UIView* platformView;
+@property(nonatomic, weak) UIView* platformView;
 @end
 
 @implementation FlutterPlatformViewSemanticsContainer
@@ -868,11 +842,6 @@ CGRect ConvertRectToGlobal(SemanticsObject* reference, CGRect local_rect) {
   return self;
 }
 
-- (void)dealloc {
-  _platformView = nil;
-  [super dealloc];
-}
-
 - (id)nativeAccessibility {
   return self.platformView;
 }
@@ -885,13 +854,6 @@ CGRect ConvertRectToGlobal(SemanticsObject* reference, CGRect local_rect) {
 
 #pragma mark - initializers
 
-// Method declared as unavailable in the interface
-- (instancetype)init {
-  [self release];
-  [super doesNotRecognizeSelector:_cmd];
-  return nil;
-}
-
 - (instancetype)initWithSemanticsObject:(SemanticsObject*)semanticsObject
                                  bridge:(fml::WeakPtr<flutter::AccessibilityBridgeIos>)bridge {
   FML_DCHECK(semanticsObject) << "semanticsObject must be set";