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.