UILongPressGestureRecognizer 没有调用它的目标方法

问题描述

这适用于 iOS 11 上的设备，但我的设备更新到 iOS 12 后它不再适用:

This worked with devices on iOS 11, but with my device updated to iOS 12 it no longer works:

//the viewcontroller is initiated with UIGestureRecognizerDelegate

let longPressGesture = UILongPressGestureRecognizer(target: self, action: #selector(longPress))

//in cellForRowAt:
longPressGesture.minimumPressDuration = 1.0
longPressGesture.delegate = self
longPressGesture.cancelsTouchesInView = false
cell.addGestureRecognizer(longPressGesture)

@objc func longPress(longPressGestureRecognizer: UILongPressGestureRecognizer) {
     //never called
}

我还尝试将手势识别器添加到 viewDidLoad 中的按钮，以确保它不是 tableview 的问题，并且 longPress 函数仍然没有被调用.

I also tried adding the gesture recognizer to a button in viewDidLoad to ensure it wasn't an issue with the tableview, and the longPress function is still not getting called.

推荐答案

//the viewcontroller is initiated with UIGestureRecognizerDelegate
let longPressGesture = UILongPressGestureRecognizer(target: self, action: #selector(longPress))

看起来您正试图使 longPressGesture 成为 UIViewController 的实例属性，同时为其初始化程序提供目标和操作.这是行不通的，因为在初始化时，目标 self 并不是实例.还没有实例；实例就是我们正在创建的对象！

It looks like you are trying to make longPressGesture an instance property of your UIViewController while giving it a target and action as part of its initializer. That's not going to work, because at the time it is initialized, the target, self, is not the instance. There is no instance yet; the instance is what we are in the middle of creating!

相反，将该行移到 cellForRowAt: 中，如下所示:

Instead, move that line into cellForRowAt:, like this:

//in cellForRowAt:
let longPressGesture = UILongPressGestureRecognizer(target: self, action: #selector(longPress))
longPressGesture.minimumPressDuration = 1.0
longPressGesture.delegate = self
longPressGesture.cancelsTouchesInView = false
cell.addGestureRecognizer(longPressGesture)

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