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

Question

I'm trying to implement a simple keyboard observer in my iOS 8 Swift app but it really doesn't work. This is the code im currently using:

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
}

Strangely both functions to react to the keyboard are called once after starting the app. Nothing happens when I enter or leave a textfield. What am I doing wrong? And by the way: Is altering a constraint the best solution for changing the size of an image?

I really appreciate your help!

Answer 1

Calling NSNotificationCenter() is instantiating a new NSNotificationCenter each time you call it. Try using NSNotificationCenter.defaultCenter() instead.

Answer 2

//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")
}

Answer 3

I prefer to use UIKeyboardWillChangeFrameNotification, because you get informed if the size changes, if eg when suggestions are hidden/shown

//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")
}

Answer 4

Swift 4.0, closures 😍:

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

Answer 5

Swift 3 and above code. Added code for getting height for keyboard

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")
}