为 UIControl 实现线程安全的 Block 事件扩展：原理与实践 | Notes

一、背景与痛点

在 iOS 开发中，UIControl 的 Target-Action 机制是处理用户交互事件的标准方式。然而当面临以下场景时，传统方案会暴露明显缺陷：

线程安全问题：在后台线程修改事件监听可能导致 UI 操作不同步
代码冗余：需为每个事件单独创建 selector 方法
维护成本：难以批量移除特定类型的事件监听
内存风险：容易因循环引用导致内存泄漏

代码如下：

import UIKit

// MARK: - 线程安全关联对象键
private struct AssociatedKeys {
    @MainActor
    static var blockKey: UInt8 = 0
}

// MARK: - Block 事件包装类
final class ControlBlockTarget: NSObject {
    var block: ((UIControl) -> Void)?
    var events: UIControl.Event
    
    init(block: ((UIControl) -> Void)?, events: UIControl.Event) {
        self.block = block
        self.events = events
        super.init()
    }
    
    @objc func invoke(sender: UIControl) {
        block?(sender)
    }
}

// MARK: - 线程安全扩展
extension UIControl {
    // MARK: - Public Methods
    
    /// 添加 Block 事件（线程安全）
    public func sb_addBlock(for events: UIControl.Event, block: @escaping (UIControl) -> Void) {
        DispatchQueue.main.async { [weak self] in
            guard let self else { return }
            
            let target = ControlBlockTarget(block: block, events: events)
            self.addTarget(target, action: #selector(ControlBlockTarget.invoke(sender:)), for: events)
            self.blockTargets.append(target)
        }
    }
    
    /// 设置唯一 Block（覆盖原有同类型事件）
    public func sb_setBlock(for events: UIControl.Event, block: @escaping (UIControl) -> Void) {
        DispatchQueue.main.async { [weak self] in
            guard let self else { return }
            
            // 原子化移除指定事件的所有处理
            self.sb_removeAllBlocks(for: events)
            
            // 添加新处理
            let target = ControlBlockTarget(block: block, events: events)
            self.addTarget(target, action: #selector(ControlBlockTarget.invoke(sender:)), for: events)
            self.blockTargets.append(target)
        }
    }
    
    /// 移除指定事件的所有 Block
    public func sb_removeAllBlocks(for events: UIControl.Event) {
        DispatchQueue.main.async { [weak self] in
            guard let self else { return }
            
            var remainingTargets: [ControlBlockTarget] = []
            
            for target in self.blockTargets {
                if target.events == events {
                    // 完全匹配时移除
                    self.removeTarget(target, action: nil, for: target.events)
                } else if target.events.contains(events) {
                    // 处理事件交集的情况
                    let newEvents = target.events.subtracting(events)
                    if !newEvents.isEmpty {
                        self.removeTarget(target, action: nil, for: target.events)
                        target.events = newEvents
                        self.addTarget(target, action: #selector(ControlBlockTarget.invoke(sender:)), for: newEvents)
                        remainingTargets.append(target)
                    } else {
                        self.removeTarget(target, action: nil, for: target.events)
                    }
                } else {
                    remainingTargets.append(target)
                }
            }
            
            self.blockTargets = remainingTargets
        }
    }
    
    /// 原子化移除所有 Block
    public func sb_removeAllBlocks() {
        sb_removeAllBlocks(for: .allEvents)
    }
    
    // MARK: - Private Properties
    
    private var blockTargets: [ControlBlockTarget] {
        get {
            objc_getAssociatedObject(self, &AssociatedKeys.blockKey) as? [ControlBlockTarget] ?? []

        }
        set {
            objc_setAssociatedObject(
                self,
                &AssociatedKeys.blockKey,
                newValue,
                .OBJC_ASSOCIATION_RETAIN_NONATOMIC
            )
        }
    }
}

使用示例

let button = UIButton()
button.sb_addBlock(for: .touchUpInside) { [weak self] _ in
    self?.handleLogin()
}

// 替换现有事件
button.sb_setBlock(for: .touchDragExit) { _ in
    print("拖动取消操作")
}

// 批量移除事件
view.sb_removeAllBlocks(for: .allTouchEvents)

注意事项

不可嵌套调用：避免在 Block 内执行 sb_removeAllBlocks
优先级控制：与系统 addTarget 方法混用时注意执行顺序
性能监控：在列表滚动等高频场景建议使用节流包装器
调试技巧：通过 po blockTargets 在 LLDB 查看当前事件绑定状态