Support modifying/overriding value returned by accessibilityElements

Sometimes we want to override `accessibilityElements` so that we can
skip over elements unnecessary for interacting with the app using VoiceOver
or limit the interaction to child viewcontrollers presented, etc.

For this purpose, this commit introduces `accessibilityElementsBlock`
that receives the default `accessibilityElements` as input and modifies the
elements returned. If not set, `accessibilityElements` should still return
the elements found from the node hierarchy.

Source/ASDisplayNode.h

@@ -785,6 +785,12 @@ AS_EXTERN NSInteger const ASDefaultDrawingPriority;
 
 @end
 
+/**
+ * Accessibility elements modification block. Used to modify/override the elements returned by
+ * `accessibilityElements` for a node.
+ */
+typedef NSArray *_Nonnull(^ASDisplayNodeAccessibilityElementsBlock)(NSArray * _Nonnull);
+
 @interface ASDisplayNode (UIViewBridgeAccessibility)
 
 // Accessibility support
@@ -805,13 +811,17 @@ AS_EXTERN NSInteger const ASDefaultDrawingPriority;
 @property           BOOL shouldGroupAccessibilityChildren;
 @property           UIAccessibilityNavigationStyle accessibilityNavigationStyle;
 @property (nullable, copy)   NSArray *accessibilityCustomActions API_AVAILABLE(ios(8.0),tvos(9.0));
+
 #if TARGET_OS_TV
 @property (nullable, copy) 	NSArray *accessibilityHeaderElements;
 #endif
 
 // Accessibility identification support
 @property (nullable, copy)   NSString *accessibilityIdentifier;
 
+/// Block used for modifying/overriding the a11y elements returned by `accessibilityElements`.
+@property (nullable, copy)   ASDisplayNodeAccessibilityElementsBlock accessibilityElementsBlock;
+
 @end
 
 @interface ASDisplayNode (ASLayoutElement) <ASLayoutElement>

Source/ASDisplayNode.mm

@@ -3546,6 +3546,18 @@ - (BOOL)isAccessibilityContainer
   return _isAccessibilityContainer;
 }
 
+- (void)setAccessibilityElementsBlock:(ASDisplayNodeAccessibilityElementsBlock)accessibilityElementsBlock
+{
+  MutexLocker l(__instanceLock__);
+  _accessibilityElementsBlock = accessibilityElementsBlock;
+}
+
+- (ASDisplayNodeAccessibilityElementsBlock)accessibilityElementsBlock
+{
+  MutexLocker l(__instanceLock__);
+  return _accessibilityElementsBlock;
+}
+
 - (NSString *)defaultAccessibilityLabel
 {
   return nil;

Source/Details/_ASDisplayViewAccessiblity.mm

@@ -287,6 +287,12 @@ - (NSArray *)accessibilityElements
   NSMutableArray *accessibilityElements = [[NSMutableArray alloc] init];
   CollectAccessibilityElementsForView(self.view, accessibilityElements);
   SortAccessibilityElements(accessibilityElements);
+
+  if (_accessibilityElementsBlock) {
+    NSArray *accessibilityElementsCopy = [accessibilityElements copy];
+    accessibilityElements = _accessibilityElementsBlock(accessibilityElementsCopy);
+  }
+
   return accessibilityElements;
 }
 

Source/Private/ASDisplayNodeInternal.h

@@ -243,6 +243,7 @@ static constexpr CACornerMask kASCACornerAllCorners =
   CGPoint _accessibilityActivationPoint;
   UIBezierPath *_accessibilityPath;
   BOOL _isAccessibilityContainer;
+  ASDisplayNodeAccessibilityElementsBlock _accessibilityElementsBlock;
 
 
   // Safe Area support

Tests/ASDisplayViewAccessibilityTests.mm

@@ -50,6 +50,40 @@ - (void)testAccessibilityElementsAccessors
   XCTAssertEqual([node.view indexOfAccessibilityElement:node.view.accessibilityElements.firstObject], 0);*/
 }
 
+- (void)testAccessibilityElementsBlock
+{
+  // Setup nodes
+  ASDisplayNode *node = nil;
+  ASDisplayNode *innerNode1 = nil;
+  ASDisplayNode *innerNode2 = nil;
+  node = [[ASDisplayNode alloc] init];
+  innerNode1 = [[ASDisplayNode alloc] init];
+  innerNode2 = [[ASDisplayNode alloc] init];
+
+  innerNode1.accessibilityLabel = @"hello";
+  innerNode1.isAccessibilityElement = YES;
+  innerNode2.accessibilityLabel = @"world";
+  innerNode2.isAccessibilityElement = YES;
+
+  // Attach the subnodes to the parent node, then check that all the subnodes added are returned as accessibilityElements
+  [node addSubnode:innerNode1];
+  [node addSubnode:innerNode2];
+
+  XCTAssertTrue(node.view.accessibilityElements.count == 2);
+  XCTAssertEqualObjects([node.view.accessibilityElements[0] accessibilityLabel], [innerNode1 accessibilityLabel]);
+  XCTAssertEqualObjects([node.view.accessibilityElements[1] accessibilityLabel], [innerNode2 accessibilityLabel]);
+
+  // Override accessibilityElements, check that only the specified element is returned as accessibilityElements
+  node.accessibilityElementsBlock = ^NSArray * _Nonnull(NSArray *_Nonnull elements) {
+    return @[innerNode2];
+  };
+
+  XCTAssertTrue(node.view.accessibilityElements.count == 1);
+  XCTAssertEqualObjects([node.view.accessibilityElements.firstObject accessibilityLabel],
+                        [innerNode2 accessibilityLabel],
+                        @"Parent node accessibilityElements override broken");
+}
+
 - (void)testThatSubnodeAccessibilityLabelAggregationWorks
 {
   // Setup nodes