onmyway133 onmyway133 - 3 months ago 16
iOS Question

Why is UIKeyboardWillShowNotification called every time another TextField is selected?

I have a project that contains a

UIScrollView
and many
UITextField
inside it.

For the first time I select a
UITextField
,
UIKeyboardWillShowNotification
is called, which is fine. But whenever I select new
UITextField
(THE KEYBOARD IS STILL THERE),
UIKeyboardWillShowNotification
is called again !!!, which is weird.

I also set a symbolic breakpoint for
[UIResponder resignFirstResponder]
and I see that it is hit before and after
UIKeyboardWillShowNotification
is called !!!

The other thing is that
UIKeyboardWillHideNotification
is only called when I hit the "Done" button on the keyboard

I'm sure to not call any
resignFirstResponder
,
becomeFirstResponder
,
endEditing
anywhere. (I mean not call wrongly)

What can cause this problem ?

Here is the stacktrace
enter image description here

Answer

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

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