Описание тега textwatcher
TextWatcher is an interface in the Android SDK that can be attached to an Editable object to see when that Editable's text changes.
The android.text.TextWatcher Java interface is implemented by objects attached to an editable widget on 
android. It exposes three methods, each of this methods will be called at a certain moment of the text being modified (for example: after the text is changed). Care must be taken when using the TextWatcher's methods to avoid getting into an infinite loop or to incorrectly modify the text.