Fix for a11y crash on iOS (#12990)

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

@@ -32,7 +32,7 @@ class AccessibilityBridge;
 /**
  * A node in the iOS semantics tree.
  */
-@interface SemanticsObject : NSObject
+@interface SemanticsObject : UIAccessibilityElement
 
 /**
  * The globally unique identifier for this node.
@@ -126,7 +126,7 @@ class AccessibilityBridge;
  * * `SemanticsObject` for the other type of semantics objects.
  * * `FlutterSemanticsObject` for default implementation of `SemanticsObject`.
  */
-@interface FlutterPlatformViewSemanticsContainer : NSObject
+@interface FlutterPlatformViewSemanticsContainer : UIAccessibilityElement
 
 /**
  * The position inside an accessibility container.

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

@@ -81,7 +81,7 @@ flutter::SemanticsAction GetSemanticsActionForScrollDirection(
  * there for structure and they don't provide any semantic information to VoiceOver (they return
  * NO for isAccessibilityElement).
  */
-@interface SemanticsObjectContainer : NSObject
+@interface SemanticsObjectContainer : UIAccessibilityElement
 - (instancetype)init __attribute__((unavailable("Use initWithSemanticsObject instead")));
 - (instancetype)initWithSemanticsObject:(SemanticsObject*)semanticsObject
                                  bridge:(fml::WeakPtr<flutter::AccessibilityBridge>)bridge
@@ -109,7 +109,11 @@ flutter::SemanticsAction GetSemanticsActionForScrollDirection(
 - (instancetype)initWithBridge:(fml::WeakPtr<flutter::AccessibilityBridge>)bridge uid:(int32_t)uid {
   FML_DCHECK(bridge) << "bridge must be set";
   FML_DCHECK(uid >= kRootNodeId);
-  self = [super init];
+  // Initialize with the UIView as the container.
+  // The UIView will not necessarily be accessibility parent for this object.
+  // The bridge informs the OS of the actual structure via
+  // `accessibilityContainer` and `accessibilityElementAtIndex`.
+  self = [super initWithAccessibilityContainer:bridge->view()];
 
   if (self) {
     _bridge = bridge;
@@ -429,7 +433,11 @@ flutter::SemanticsAction GetSemanticsActionForScrollDirection(
 
 - (instancetype)initWithSemanticsObject:(SemanticsObject*)object {
   FML_CHECK(object);
-  if (self = [super init]) {
+  // Initialize with the UIView as the container.
+  // The UIView will not necessarily be accessibility parent for this object.
+  // The bridge informs the OS of the actual structure via
+  // `accessibilityContainer` and `accessibilityElementAtIndex`.
+  if (self = [super initWithAccessibilityContainer:object.bridge->view()]) {
     _semanticsObject = object;
     flutter::FlutterPlatformViewsController* controller =
         object.bridge->GetPlatformViewsController();
@@ -476,7 +484,11 @@ flutter::SemanticsAction GetSemanticsActionForScrollDirection(
 - (instancetype)initWithSemanticsObject:(SemanticsObject*)semanticsObject
                                  bridge:(fml::WeakPtr<flutter::AccessibilityBridge>)bridge {
   FML_DCHECK(semanticsObject) << "semanticsObject must be set";
-  self = [super init];
+  // Initialize with the UIView as the container.
+  // The UIView will not necessarily be accessibility parent for this object.
+  // The bridge informs the OS of the actual structure via
+  // `accessibilityContainer` and `accessibilityElementAtIndex`.
+  self = [super initWithAccessibilityContainer:bridge->view()];
 
   if (self) {
     _semanticsObject = semanticsObject;