[ASDisplayNode] Implement accessibilityElementsHidden (#1859)

Most of this code comes from an old PR that @fruitcoder put up #795 2 years ago.

When creating our array of accessibilityElements, we need to respect the value of `accessibilityElementsHidden`. If the value of this property changes, we need to invalidate the cached accessibility elements (unless we are in the experiment that doesn’t cache `accessibilityElements`).

I created a simple test app and made sure this matched UIKit’s implementation. I also added a test case that changes the value of `accessibilityElementsHidden` and makes sure the proper accessibilityElements are returned.

Source/Details/_ASDisplayViewAccessiblity.mm

@@ -89,6 +89,7 @@ + (ASAccessibilityElement *)accessibilityElementWithContainer:(UIView *)containe
   accessibilityElement.accessibilityHint = node.accessibilityHint;
   accessibilityElement.accessibilityValue = node.accessibilityValue;
   accessibilityElement.accessibilityTraits = node.accessibilityTraits;
+  accessibilityElement.accessibilityElementsHidden = node.accessibilityElementsHidden;
   if (AS_AVAILABLE_IOS_TVOS(11, 11)) {
     accessibilityElement.accessibilityAttributedLabel = node.accessibilityAttributedLabel;
     accessibilityElement.accessibilityAttributedHint = node.accessibilityAttributedHint;
@@ -221,6 +222,11 @@ static BOOL recusivelyCheckSuperviewsForScrollView(UIView *view) {
     return recusivelyCheckSuperviewsForScrollView(view.superview);
 }
 
+/// returns YES if this node should be considered "hidden" from the screen reader.
+static BOOL nodeIsHiddenFromAcessibility(ASDisplayNode *node) {
+  return node.isHidden || node.alpha == 0.0 || node.accessibilityElementsHidden;
+}
+
 /// Collect all accessibliity elements for a given view and view node
 static void CollectAccessibilityElements(ASDisplayNode *node, NSMutableArray *elements)
 {
@@ -254,6 +260,10 @@ static void CollectAccessibilityElements(ASDisplayNode *node, NSMutableArray *el
     return;
   }
 
+  if (nodeIsHiddenFromAcessibility(node)) {
+    return;
+  }
+
   // see if one of the subnodes is modal. If it is, then we only need to collect accessibilityElements from that
   // node. If more than one subnode is modal, UIKit uses the last view in subviews as the modal view (it appears to
   // be based on the index in the subviews array, not the location on screen). Let's do the same.
@@ -270,7 +280,7 @@ static void CollectAccessibilityElements(ASDisplayNode *node, NSMutableArray *el
 
   for (ASDisplayNode *subnode in subnodes) {
     // If a node is hidden or has an alpha of 0.0 we should not include it
-    if (subnode.hidden || subnode.alpha == 0.0) {
+    if (nodeIsHiddenFromAcessibility(subnode)) {
       continue;
     }
 

Source/Private/ASDisplayNode+UIViewBridge.mm

@@ -1310,7 +1310,13 @@ - (BOOL)accessibilityElementsHidden
 - (void)setAccessibilityElementsHidden:(BOOL)accessibilityElementsHidden
 {
   _bridge_prologue_write;
+  BOOL oldHiddenValue = _getFromViewOnly(accessibilityElementsHidden);
   _setAccessibilityToViewAndProperty(_flags.accessibilityElementsHidden, accessibilityElementsHidden, accessibilityElementsHidden, accessibilityElementsHidden);
+
+  // if we made a change, we need to clear the view's accessibilityElements cache.
+  if (!ASActivateExperimentalFeature(ASExperimentalDoNotCacheAccessibilityElements) && self.isNodeLoaded && oldHiddenValue != accessibilityElementsHidden) {
+    [self invalidateAccessibilityElements];
+  }
 }
 
 - (BOOL)accessibilityViewIsModal

Tests/ASDisplayViewAccessibilityTests.mm

@@ -591,5 +591,42 @@ - (void)testMultipleSubnodesAreModal {
   XCTAssertTrue([elements containsObject:modalNode2.view]);
 }
 
+- (void)testAccessibilityElementsHidden {
+
+  UIWindow *window = [[UIWindow alloc] initWithFrame:CGRectMake(0, 0, 320, 568)];
+  ASDisplayNode *node = [[ASDisplayNode alloc] init];
+  node.automaticallyManagesSubnodes = YES;
+
+  ASViewController *vc = [[ASViewController alloc] initWithNode:node];
+  window.rootViewController = vc;
+  [window makeKeyAndVisible];
+  [window layoutIfNeeded];
+
+  ASTextNode *label1 = [[ASTextNode alloc] init];
+  label1.attributedText = [[NSAttributedString alloc] initWithString:@"on screen"];
+  label1.frame = CGRectMake(0, 0, 100, 20);
+
+  ASTextNode *label2 = [[ASTextNode alloc] init];
+  label2.attributedText = [[NSAttributedString alloc] initWithString:@"partially on screen y"];
+  label2.frame = CGRectMake(0, 20, 100, 20);
+
+  [node addSubnode:label1];
+  [node addSubnode:label2];
+
+  NSArray *elements = [node.view accessibilityElements];
+  XCTAssertTrue(elements.count == 2);
+  XCTAssertTrue([elements containsObject:label1.view]);
+  XCTAssertTrue([elements containsObject:label2.view]);
+
+  node.accessibilityElementsHidden = YES;
+  elements = [node.view accessibilityElements];
+  XCTAssertTrue(elements.count == 0);
+
+  node.accessibilityElementsHidden = NO;
+  elements = [node.view accessibilityElements];
+  XCTAssertTrue(elements.count == 2);
+  XCTAssertTrue([elements containsObject:label1.view]);
+  XCTAssertTrue([elements containsObject:label2.view]);
+}
 
 @end