Why is UIKeyboardWillShowNotification called every time another TextField is selected?
The problem is I set inputAccessoryView
for the UITextField
, and this cause UIKeyboardWillShowNotification
being called again when new UITextField
is selected
This article Working With Keyboard on iOS explains this well
Additional changes take place when we connect an external keyboard to the iPad. In this particular case, the notification behavior depends on the inputAccessoryView property of the control which was the reason for displaying the keyboard.
If inputAccessoryView is not present or its height is equal to 0 points, no keyboard notifications are sent. My guess is that this is because in this case, no visual changes take place in application. Otherwise, all notifications behave as expected – which means they are being sent as in the majority of cases when the keyboard is displayed or hidden in a normal (not undocked or split) state.
Whenever new UITextField
is selected, the OS needs to compute the frame for the keyboard again, and the following notifications are posted
UIKeyboardWillChangeFrameNotification
UIKeyboardWillShowNotification
UIKeyboardDidChangeFrameNotification
UIKeyboardDidShowNotification
The same applies for when the TextField loses its first responder status
Note that using the same View for inputAccessoryView
will cause UIKeyboardWillShowNotification
only called once
To workaround the problem, I used the following code to cancel the UIKeyboardWillShowNotification
callback if the keyboard's frame is not changing.
func keyboardWillShow(notification: NSNotification) {
let beginFrame = notification.userInfo![UIKeyboardFrameBeginUserInfoKey]!.CGRectValue()
let endFrame = notification.userInfo![UIKeyboardFrameEndUserInfoKey]!.CGRectValue()
// Return early if the keyboard's frame isn't changing.
guard CGRectEqualToRect(beginFrame, endFrame) == false else {
return
}
...
}
For Swift 3/4:
func keyboardWillShow(notification: Notification) {
let userInfo = notification.userInfo!
let beginFrameValue = (userInfo[UIKeyboardFrameBeginUserInfoKey] as? NSValue)!
let beginFrame = beginFrameValue.cgRectValue
let endFrameValue = (userInfo[UIKeyboardFrameEndUserInfoKey] as? NSValue)!
let endFrame = endFrameValue.cgRectValue
if beginFrame.equalTo(endFrame) {
return
}
// Do something with 'will show' event
...
}
In general I find that many things can cause spurious UIKeyboardWillShow
and UIKeyboardWillHide
notifications. My solution is to use a property to track whether the keyboard is already showing:
func keyboardShow(_ n:Notification) {
if self.keyboardShowing {
return
}
self.keyboardShowing = true
// ... other stuff
}
func keyboardHide(_ n:Notification) {
if !self.keyboardShowing {
return
}
self.keyboardShowing = false
// ... other stuff
}
Those guards block exactly the spurious notifications, and all is well after that. And the keyboardShowing
property can be useful for other reasons, so that it is something worth tracking anyway.