使用 closures 代替 Gesture Recognizers 选择器

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原文 Adding Gesture Recognizers with Closures Instead of Selectors

添加UITapGestureRecognizer或任何 recognizer/target action 的最糟糕的部分是仅针对选择器参数实现新功能。今天，我想分享一个巧妙的技巧，让你添加不带选择器的手势识别器。

假设我们在View Controller中有一个UIImageView，我们想向其中添加一个UITapGestureRecognizer，以便在点击它时打印出一条语句.

通常，我们会创建一个 UITapGestureRecognizer 的实例，并将其目标设置为视图控制器及其选择器，因为我们会快速将它们组合在一起(myImageViewTapped(sender：UITapGestureRecognizer))。

这可能会有点多余，并且可能导致要添加交互性的每个子视图的函数的代码混乱。

我以为我可以快速扩展一下，以便为我的图像视图添加敲击手势识别器，但是然后我必须为每个识别器创建一个新功能，对吗？错误！利用关联对象的功能，我们实际上可以将闭包存储为扩展中的计算属性！

import UIKit

extension UIView {
  // In order to create computed properties for extensions, we need a key to 
  // store and access the stored property
  fileprivate struct AssociatedObjectKeys {
    static var tapGestureRecognizer = "MediaViewerAssociatedObjectKey_mediaViewer"
  }
  
  fileprivate typealias Action = (() -> Void)?
  
  // Set our computed property type to a closure
  fileprivate var tapGestureRecognizerAction: Action? {
    set {
      if let newValue = newValue {
        // Computed properties get stored as associated objects
        objc_setAssociatedObject(self, &AssociatedObjectKeys.tapGestureRecognizer, newValue, objc_AssociationPolicy.OBJC_ASSOCIATION_RETAIN)
      }
    }
    get {
      let tapGestureRecognizerActionInstance = objc_getAssociatedObject(self, &AssociatedObjectKeys.tapGestureRecognizer) as? Action
      return tapGestureRecognizerActionInstance
    }
  }
  
  // This is the meat of the sauce, here we create the tap gesture recognizer and
  // store the closure the user passed to us in the associated object we declared above
  public func addTapGestureRecognizer(action: (() -> Void)?) {
    self.isUserInteractionEnabled = true
    self.tapGestureRecognizerAction = action
    let tapGestureRecognizer = UITapGestureRecognizer(target: self, action: #selector(handleTapGesture))
    self.addGestureRecognizer(tapGestureRecognizer)
  }
  
  // Every time the user taps on the UIImageView, this function gets called,
  // which triggers the closure we stored
  @objc fileprivate func handleTapGesture(sender: UITapGestureRecognizer) {
    if let action = self.tapGestureRecognizerAction {
      action?()
    } else {
      print("no action")
    }
  }
}

现在，每当我们要将UITapGestureRecognizer添加到UIView或UIView子类（如UIImageView）时，都可以这样做，而无需为选择器创建关联的功能！这是一个例子：

sampleImageView.addTapGestureRecognizer {
    print("image tapped")
}

没有UITapGestureRecognizers实例，没有targets，没有selectors，没有不必要的functions！