When an object of a type is attached to an Editable, its methods will be called when the text is changed.
接口TextWatcher定义了三个抽象方法：
public void beforeTextChanged(CharSequence s, int start, int count, int after)
This method is called to notify you that, within s, the count characters beginning at start are about to be replaced by new text with length after. It is an error to attempt to make changes to s from this callback.
public void onTextChanged(CharSequence s, int start, int before, int count)
This method is called to notify you that, within s, the count characters beginning at start have just replaced old text that had length before. It is an error to attempt to make changes to s from this callback.
public void afterTextChanged(Editable s)
This method is called to notify you that, somewhere within s, the text has been changed. It is legitimate(合理) to make further changes to s from this callback, but be careful not to get yourself into an infinite loop, because any changes you make will cause this method to be called again recursively(递归). (You are not told where the change took place because other afterTextChanged() methods may already have made other changes and invalidated the offsets. But if you need to know here, you can use Spannable.setSpan in onTextChanged to mark your place and then look up from here where the span ended up.