[Snackbar] Adds elementToFocusOnDismiss to MDCSnackbar (#9148)

As part of maintaining same functionality across our iterations of the Snackbar, this code was ported from the older iteration to allow users to choose focus on different views after Snackbar is dismissed.

components/Snackbar/src/MDCSnackbarMessage.h

@@ -170,6 +170,15 @@ extern NSString *__nonnull const MDCSnackbarMessageBoldAttributeName;
  */
 @property(nonatomic) BOOL focusOnShow;
 
+/**
+ Element to focus on snackbar message dismiss. Focuses the first element on screen
+ after dismiss by default. The focus will change to the element only if the focus is on the snackbar
+ message.
+
+ Defaults to nil.
+ */
+@property(nonatomic, weak, nullable) UIView *elementToFocusOnDismiss;
+
 /**
  A block that is invoked when the corresponding @c MDCSnackbarMessageView of the @c
  MDCSnackbarMessage instance will be presented. Use this to customize @c MDCSnackbarMessageView

components/Snackbar/src/MDCSnackbarMessage.m

@@ -71,6 +71,7 @@ - (instancetype)copyWithZone:(__unused NSZone *)zone {
 #pragma clang diagnostic pop
   copy.enableRippleBehavior = self.enableRippleBehavior;
   copy.focusOnShow = self.focusOnShow;
+  copy.elementToFocusOnDismiss = self.elementToFocusOnDismiss;
 
   // Unfortunately there's not really a concept of 'copying' a block (in the same way you would copy
   // a string, for example). A block's pointer is immutable once it is created and copied to the

components/Snackbar/src/MDCSnackbarMessageView.m

@@ -40,6 +40,17 @@
                          alpha:(a)];
 }
 
+/** Test whether any of the accessibility elements of a view is focused */
+static BOOL UIViewHasFocusedAccessibilityElement(UIView *view) {
+  for (NSInteger i = 0; i < [view accessibilityElementCount]; i++) {
+    id accessibilityElement = [view accessibilityElementAtIndex:i];
+    if ([accessibilityElement accessibilityElementIsFocused]) {
+      return YES;
+    }
+  }
+  return NO;
+};
+
 /**
  The thickness of the Snackbar border.
  */
@@ -421,6 +432,16 @@ - (void)dismissWithAction:(MDCSnackbarMessageAction *)action userInitiated:(BOOL
   if (self.dismissalHandler) {
     self.dismissalHandler(userInitiated, action);
 
+    // Change focus only if the focus is on this view.
+    if (self.message.elementToFocusOnDismiss) {
+      BOOL hasVoiceOverFocus =
+          UIAccessibilityIsVoiceOverRunning() && UIViewHasFocusedAccessibilityElement(self);
+      if (hasVoiceOverFocus) {
+        UIAccessibilityPostNotification(UIAccessibilityScreenChangedNotification,
+                                        self.message.elementToFocusOnDismiss);
+      }
+    }
+
     // In case our dismissal handler has a reference to us, release the block.
     self.dismissalHandler = nil;
   }