android中textwatcher倒计时的问题

我想在程序中设置一个textwatcher，在打字时能倒数计时，从250倒数到0。我设置android:maxLength="250"。但是这个方法不能倒数到0.
这是我用的代码：

 et_Notes = (EditText) dialog_list.findViewById(R.id.EditText_Notes);
 tvC = (TextView)dialog_list.findViewById(R.id.TextViewCounter);

    final TextWatcher mTextEditorWatcher = new TextWatcher() {
        public void beforeTextChanged(CharSequence s, int start, int count, int after) {
        }

        public void onTextChanged(CharSequence s, int start, int before, int count) {

           tvC.setText(String.valueOf(s.length()));
        }

        public void afterTextChanged(Editable s) {

        }
    };

    et_Notes.addTextChangedListener(mTextEditorWatcher);

请求大家的帮忙，谢谢！

 et_Notes = (EditText) dialog_list.findViewById(R.id.EditText_Notes);

tvC = (TextView)dialog_list.findViewById(R.id.TextViewCounter);

final TextWatcher mTextEditorWatcher = new TextWatcher() {
    public void beforeTextChanged(CharSequence s, int start, int count, int after) {
    }

    public void onTextChanged(CharSequence s, int start, int before, int count) {

    }

    public void afterTextChanged(Editable s) {
       tvC.setText(String.valueOf(250 - s.getText().length()));

    }
};

et_Notes.addTextChangedListener(mTextEditorWatcher);