Swift：通过NSNotificationCenter的键盘观察器不起作用

Question

我正在尝试在我的iOS 8 Swift应用程序中实现一个简单的键盘观察器，但它真的不起作用。这是我目前使用的代码：

override func viewDidAppear(animated: Bool) {
    NSNotificationCenter().addObserver(self, selector: Selector(keyboardWillAppear()), name: UIKeyboardWillShowNotification, object: nil)
    NSNotificationCenter().addObserver(self, selector: Selector(keyboardWillHide()), name: UIKeyboardWillHideNotification, object: nil)
}

override func viewDidDisappear(animated: Bool) {
    NSNotificationCenter().removeObserver(self)
}

func keyboardWillAppear() {
    logoHeightConstraint.constant = 128.0
}

func keyboardWillHide() {
    logoHeightConstraint.constant = 256.0
}

奇怪的是，在启动应用程序后，这两个响应键盘的函数都会被调用一次。当我进入或离开文本框时什么也没有发生。我做错了什么？顺便问一下：更改约束是改变图像大小的最佳解决方案吗？

非常感谢您的帮助！

- stoeffn

---

我可能错了，但我认为 NSNotificationCenter() 每次调用时都会实例化一个新的 NSNotificationCenter。你尝试过使用 NSNotificationCenter.defaultCenter() 吗？ - Patrick Goley

Answer 1

//keyboard observers 
NSNotificationCenter.defaultCenter().addObserver(self, selector: Selector("keyboardWillAppear"), name: UIKeyboardWillShowNotification, object: nil)
NSNotificationCenter.defaultCenter().addObserver(self, selector: Selector("keyboardWillHide"), name: UIKeyboardWillHideNotification, object: nil)




func keyboardWillAppear() {
    println("Keyboard appeared")
}

func keyboardWillHide() {
    println("Keyboard hidden")
}

- JSA986

---

很抱歉要说这个不起作用，因为它是Objective C选择器。#selector(keyboardWillAppear) #selector(keyboardWIllHide) 对我有用。请参考这篇文章：https://dev59.com/9mAg5IYBdhLWcg3wBXER - Werner Kratochwil

当然它能工作，而且确实工作，因此它是最高评分的答案。但是当它用Swift编写时，请说明你的声明“它是Objective C选择器”。 - JSA986

使用给定的代码在XCode中会收到警告：应该使用#selector而不是显式构造'Selector'。此外，println不再有效（我会收到错误），应该替换为print。 - Werner Kratochwil

那是因为在回答这个问题的时候（2年前），针对当时Swift的当前版本，答案是正确的。 - JSA986

Answer 2

NSNotificationCenter() 的调用会每次实例化一个新的 NSNotificationCenter。尝试使用 NSNotificationCenter.defaultCenter() 替代。

- Patrick Goley

Answer 3

使用Swift 3及以上版本的代码。添加了获取键盘高度的代码

func addObservers() {
    //keyboard observers
    NotificationCenter.default.addObserver(self, selector: #selector(keyboardDidAppear(notification:)), name: NSNotification.Name.UIKeyboardDidShow, object: nil)
    NotificationCenter.default.addObserver(self, selector: #selector(keyboardWillHide(notification:)), name: NSNotification.Name.UIKeyboardDidHide, object: nil)
}

@objc func keyboardDidAppear(notification: NSNotification) {
    print("Keyboard appeared")
    let keyboardSize:CGSize = (notification.userInfo![UIKeyboardFrameEndUserInfoKey] as! NSValue).cgRectValue.size
    print("Keyboard size: \(keyboardSize)")

    let height = min(keyboardSize.height, keyboardSize.width)
    let width = max(keyboardSize.height, keyboardSize.width)
    print(height)
    print(width)
}

@objc func keyboardWillHide(notification: NSNotification) {
    print("Keyboard hidden")
}

- Van

Answer 4

Swift 4.0，闭包：

NotificationCenter.default.addObserver(forName: .UIKeyboardDidShow, object: nil, queue: nil, using: { notification in
   // do stuff
})

- schirrmacher

Answer 5

我更喜欢使用UIKeyboardWillChangeFrameNotification，因为它会在大小发生变化时通知您，例如当建议被隐藏/显示时

//keyboard observers 
NSNotificationCenter.defaultCenter().addObserver(self, selector: #selector(keyboardWillChange), name: UIKeyboardWillChangeFrameNotification, object: nil)
NSNotificationCenter.defaultCenter().addObserver(self, selector: #selector(keyboardWillHide), name: UIKeyboardWillHideNotification, object: nil)

func keyboardWillChange(notification:NSNotification)
{
    print("Keyboard size changed")
}

func keyboardWillHide(notification:NSNotification)
{
    print("Keyboard hidden")
}

- Werner Kratochwil