UIBarButtonItem的accessibilityElementDidBecomeFocused回调未触发问题

问题描述 #

需要监听UIBarButtonItem获取/失去无障碍焦点的事件，实现了UIAccessibilityFocus非正式协议的方法，但回调始终未触发。

尝试的实现方案 #

方案1：Swift扩展UIBarButtonItem #

extension UIBarButtonItem {
    override open func accessibilityElementDidBecomeFocused() {
        if self.accessibilityElementIsFocused() {
            print("My element has become focused.")
        }
    }

    override open func accessibilityElementDidLoseFocus() {
        if self.accessibilityElementIsFocused() {
            print("My element has lost focus.")
        }
    }

    override open func accessibilityElementIsFocused() -> Bool {
        return self.accessibilityIdentifier == "hamburger"
    }
}

在Objective-C的ViewController中导入Swift桥接文件：

#import "Sample-Swift.h"

方案2：子类化UIBarButtonItem（HamburgerButton） #

.h文件

#import <UIKit/UIKit.h>
#import <UIKit/UIAccessibility.h>

NS_ASSUME_NONNULL_BEGIN

@interface HamburgerButton : UIBarButtonItem

@end

NS_ASSUME_NONNULL_END

.m文件

@implementation HamburgerButton

- (BOOL)isAccessibilityElement
{
    return YES;
}

- (void)accessibilityElementDidBecomeFocused {
    if ([self accessibilityElementIsFocused]) {
        NSLog(@"My element has become focused.");
    }    
}

- (void)accessibilityElementDidLoseFocus {
    if ([self accessibilityElementIsFocused]) {
        NSLog(@"My element has lost focus.");
    }
}

- (BOOL)accessibilityElementIsFocused {
    return [self.accessibilityIdentifier isEqualToString:@"hamburger"];
}
@end

ViewController中使用子类实例

HamburgerButton *leftButton = [[HamburgerButton alloc]
                               initWithTitle:@"Hamburger"
                               style:UIBarButtonItemStylePlain
                               target:self
                               action:@selector(flipView:)];
leftButton.accessibilityIdentifier=@"Hamburger";
leftButton.tag = 88;
leftButton.isAccessibilityElement = YES;

HamburgerButton *rightButton = [[HamburgerButton alloc]
                               initWithTitle:@"Chat"
                               style:UIBarButtonItemStylePlain
                               target:self
                               action:@selector(flipView:)];
rightButton.accessibilityIdentifier=@"Chat";
rightButton.tag = 89;
rightButton.isAccessibilityElement = YES;

self.navigationItem.leftBarButtonItem = leftButton;
self.navigationItem.rightBarButtonItem = rightButton;

实际测试中，焦点确实在两个BarButton间切换，但accessibilityElementDidBecomeFocused和accessibilityElementDidLoseFocus始终未触发。

后续尝试与疑问 #

更新1：通过通知监听，但无法区分具体按钮 #

注册无障碍焦点通知：

[[NSNotificationCenter defaultCenter]addObserver:self selector:@selector(hamburgerGotFocus:) name:UIAccessibilityElementFocusedNotification object:nil];

回调方法：

-(void)hamburgerGotFocus:(NSNotification *) notification{
    NSLog(@"Focus:%@",notification);
    UIView *receiver = notification.userInfo[@"UIAccessibilityFocusedElementKey"];
    if(receiver!=nil){
        NSString *strElement = [[NSString alloc]initWithFormat:@"%@",notification.userInfo[@"UIAccessibilityFocusedElementKey"]];
        if([strElement containsString:@"UIButtonBarButton"]){
            // 无法判断具体是哪个BarButtonItem
        }
    }
}

通知日志示例：

2022-10-12 18:57:03.992859+0530 Sample[32427:1579550] Focus:NSConcreteNotification 0x280ac9980 {name = UIAccessibilityElementFocusedNotification; userInfo = {
    UIAccessibilityAssistiveTechnologyKey = UIAccessibilityNotificationVoiceOverIdentifier;
    UIAccessibilityFocusedElementKey = "<_UIButtonBarButton: 0x10690fce0; frame = (0 0; 49 44); tintColor = UIExtendedSRGBColorSpace 0 1 0 1; gestureRecognizers = <NSArray: 0x2804262e0>; layer = <CALayer: 0x280afa9e0>>";
}}

更新2：UILabel测试正常，怀疑兼容性问题 #

对UILabel使用分类和子类化的方式实现UIAccessibilityFocus协议方法，均能正常触发焦点回调。由此怀疑UIBarButtonItem是否与UIAccessibilityFocus非正式协议存在兼容性问题。

参考教程内容（翻译） #

iOS无障碍开发：聚焦元素 #

• 当VoiceOver等辅助技术聚焦到某个元素时，系统会调用accessibilityElementDidBecomeFocused方法；当元素失去焦点时，会调用accessibilityElementDidLoseFocus方法。
• 这两个方法属于UIAccessibilityFocus非正式协议，主要针对继承自UIView的元素，用于在焦点变化时执行自定义逻辑。

内容的提问来源于stack exchange，提问作者Durai Amuthan.H