Keyboard event for editbox

I want to show the warning label which fades in and out (tween) after MaxLength is reached and the user keeps on clicking at the keyboard key. I tried with TextChanged but it stops triggering after MaxLength is reached.
I even tried this :

addEventListener(Input.EventType.KEY_DOWN ,this.onKeyDown.bind(this));

But this only works perfectly well for desktop not for mobile.

And if you process the length of the string manually? For example, the maximum string length should be 100 characters. Set maxLength to -1 (or 105, just in case you never know). When the event is triggered, if the string length exceeds 100 characters, display a warning, and do not display any further characters.