Guard against orphaned semantic objects from referencing dead accessibility bridge on iOS (#13857)

* Guard against orphaned semantic objects trying to reference a dead bridge on iOS.

* Switched back to a function instead of a macro for checking the bridge.

* Fixed some formatting issues.

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

@@ -47,11 +47,25 @@ class AccessibilityBridge;
 
 /**
  * The accessibility bridge that this semantics object is attached to. This
- * object may use the bridge to access contextual application information. A weak pointer is used
- * because the platform view owns the accessibility bridge.
+ * object may use the bridge to access contextual application information. A weak
+ * pointer is used because the platform view owns the accessibility bridge.
+ * If you are referencing this property from an iOS callback, be sure to
+ * use `isAccessibilityBridgeActive` to protect against the case where this
+ * node may be orphaned.
  */
 @property(nonatomic, readonly) fml::WeakPtr<flutter::AccessibilityBridge> bridge;
 
+/**
+ * Due to the fact that VoiceOver may hold onto SemanticObjects even after it shuts down,
+ * there can be situations where the AccessibilityBridge is shutdown, but the SemanticObject
+ * will still be alive. If VoiceOver is turned on again, it may try to access this orphaned
+ * SemanticObject. Methods that are called from the accessiblity framework should use
+ * this to guard against this case by just returning early if its bridge has been shutdown.
+ *
+ * See https://github.com/flutter/flutter/issues/43795 for more information.
+ */
+- (BOOL)isAccessibilityBridgeAlive;
+
 /**
  * The semantics node used to produce this semantics object.
  */

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

@@ -140,6 +140,10 @@ flutter::SemanticsAction GetSemanticsActionForScrollDirection(
 
 #pragma mark - Semantic object methods
 
+- (BOOL)isAccessibilityBridgeAlive {
+  return [self bridge].get() != nil;
+}
+
 - (void)setSemanticsNode:(const flutter::SemanticsNode*)node {
   _node = *node;
 }
@@ -169,6 +173,9 @@ flutter::SemanticsAction GetSemanticsActionForScrollDirection(
 #pragma mark - UIAccessibility overrides
 
 - (BOOL)isAccessibilityElement {
+  if (![self isAccessibilityBridgeAlive])
+    return false;
+
   // Note: hit detection will only apply to elements that report
   // -isAccessibilityElement of YES. The framework will continue scanning the
   // entire element tree looking for such a hit.
@@ -238,24 +245,35 @@ flutter::SemanticsAction GetSemanticsActionForScrollDirection(
 }
 
 - (NSString*)accessibilityLabel {
+  if (![self isAccessibilityBridgeAlive])
+    return nil;
+
   if ([self node].label.empty())
     return nil;
   return @([self node].label.data());
 }
 
 - (NSString*)accessibilityHint {
+  if (![self isAccessibilityBridgeAlive])
+    return nil;
+
   if ([self node].hint.empty())
     return nil;
   return @([self node].hint.data());
 }
 
 - (NSString*)accessibilityValue {
+  if (![self isAccessibilityBridgeAlive])
+    return nil;
   if ([self node].value.empty())
     return nil;
   return @([self node].value.data());
 }
 
 - (CGRect)accessibilityFrame {
+  if (![self isAccessibilityBridgeAlive])
+    return CGRectMake(0, 0, 0, 0);
+
   if ([self node].HasFlag(flutter::SemanticsFlags::kIsHidden)) {
     return [super accessibilityFrame];
   }
@@ -308,6 +326,8 @@ flutter::SemanticsAction GetSemanticsActionForScrollDirection(
 #pragma mark - UIAccessibilityAction overrides
 
 - (BOOL)accessibilityActivate {
+  if (![self isAccessibilityBridgeAlive])
+    return NO;
   if (![self node].HasAction(flutter::SemanticsAction::kTap))
     return NO;
   [self bridge] -> DispatchSemanticsAction([self uid], flutter::SemanticsAction::kTap);
@@ -315,6 +335,8 @@ flutter::SemanticsAction GetSemanticsActionForScrollDirection(
 }
 
 - (void)accessibilityIncrement {
+  if (![self isAccessibilityBridgeAlive])
+    return;
   if ([self node].HasAction(flutter::SemanticsAction::kIncrease)) {
     [self node].value = [self node].increasedValue;
     [self bridge] -> DispatchSemanticsAction([self uid], flutter::SemanticsAction::kIncrease);
@@ -322,6 +344,8 @@ flutter::SemanticsAction GetSemanticsActionForScrollDirection(
 }
 
 - (void)accessibilityDecrement {
+  if (![self isAccessibilityBridgeAlive])
+    return;
   if ([self node].HasAction(flutter::SemanticsAction::kDecrease)) {
     [self node].value = [self node].decreasedValue;
     [self bridge] -> DispatchSemanticsAction([self uid], flutter::SemanticsAction::kDecrease);
@@ -329,6 +353,8 @@ flutter::SemanticsAction GetSemanticsActionForScrollDirection(
 }
 
 - (BOOL)accessibilityScroll:(UIAccessibilityScrollDirection)direction {
+  if (![self isAccessibilityBridgeAlive])
+    return NO;
   flutter::SemanticsAction action = GetSemanticsActionForScrollDirection(direction);
   if (![self node].HasAction(action))
     return NO;
@@ -337,6 +363,8 @@ flutter::SemanticsAction GetSemanticsActionForScrollDirection(
 }
 
 - (BOOL)accessibilityPerformEscape {
+  if (![self isAccessibilityBridgeAlive])
+    return NO;
   if (![self node].HasAction(flutter::SemanticsAction::kDismiss))
     return NO;
   [self bridge] -> DispatchSemanticsAction([self uid], flutter::SemanticsAction::kDismiss);
@@ -346,6 +374,8 @@ flutter::SemanticsAction GetSemanticsActionForScrollDirection(
 #pragma mark UIAccessibilityFocus overrides
 
 - (void)accessibilityElementDidBecomeFocused {
+  if (![self isAccessibilityBridgeAlive])
+    return;
   if ([self node].HasFlag(flutter::SemanticsFlags::kIsHidden)) {
     [self bridge] -> DispatchSemanticsAction([self uid], flutter::SemanticsAction::kShowOnScreen);
   }
@@ -356,6 +386,8 @@ flutter::SemanticsAction GetSemanticsActionForScrollDirection(
 }
 
 - (void)accessibilityElementDidLoseFocus {
+  if (![self isAccessibilityBridgeAlive])
+    return;
   if ([self node].HasAction(flutter::SemanticsAction::kDidLoseAccessibilityFocus)) {
     [self bridge] -> DispatchSemanticsAction([self uid],
                                              flutter::SemanticsAction::kDidLoseAccessibilityFocus);

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

@@ -219,24 +219,35 @@
 }
 
 - (void)accessibilityElementDidBecomeFocused {
+  if (![self isAccessibilityBridgeAlive])
+    return;
   [[self textInputSurrogate] accessibilityElementDidBecomeFocused];
   [super accessibilityElementDidBecomeFocused];
 }
 
 - (void)accessibilityElementDidLoseFocus {
+  if (![self isAccessibilityBridgeAlive])
+    return;
   [[self textInputSurrogate] accessibilityElementDidLoseFocus];
   [super accessibilityElementDidLoseFocus];
 }
 
 - (BOOL)accessibilityElementIsFocused {
+  if (![self isAccessibilityBridgeAlive])
+    return false;
   return [self node].HasFlag(flutter::SemanticsFlags::kIsFocused);
 }
 
 - (BOOL)accessibilityActivate {
+  if (![self isAccessibilityBridgeAlive])
+    return false;
   return [[self textInputSurrogate] accessibilityActivate];
 }
 
 - (NSString*)accessibilityLabel {
+  if (![self isAccessibilityBridgeAlive])
+    return nil;
+
   NSString* label = [super accessibilityLabel];
   if (label != nil)
     return label;
@@ -244,6 +255,8 @@
 }
 
 - (NSString*)accessibilityHint {
+  if (![self isAccessibilityBridgeAlive])
+    return nil;
   NSString* hint = [super accessibilityHint];
   if (hint != nil)
     return hint;
@@ -251,6 +264,8 @@
 }
 
 - (NSString*)accessibilityValue {
+  if (![self isAccessibilityBridgeAlive])
+    return nil;
   NSString* value = [super accessibilityValue];
   if (value != nil)
     return value;
@@ -258,6 +273,8 @@
 }
 
 - (UIAccessibilityTraits)accessibilityTraits {
+  if (![self isAccessibilityBridgeAlive])
+    return 0;
   // Adding UIAccessibilityTraitKeyboardKey to the trait list so that iOS treats it like
   // a keyboard entry control, thus adding support for text editing features, such as
   // pinch to select text, and up/down fling to move cursor.