TextWatcher

  对于一些需求，如非法字符限制（例如不允许输入#号，如果输入了#给出错误提示），做成动态判断更方便一些，而且容易扩展；

     在Android里使用TextWatcher接口可以很方便的对EditText进行监听；TextWatcher中有3个函数需要重载：

    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);

     从函数名就可以知道其意思，每当敲击键盘编辑框的文字改变时，上面的三个函数都会执行，beforeTextChanged可以给出变化之前的内容，onTextChanged和afterTextChanged给出追加上新的字符之后的文本；

class MyWatcher implements TextWatcher{
       /**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.
        * 在s中，从start处开始的count个字符将要被长度为after的文本替代
        * s 为变化前的内容；
        * start 为开始变化位置的索引，从0开始计数；
        * count 为将要发生变化的字符数
        * after 为用来替换旧文本的长度，比如s由1变为12，after为1，由12变为1，after为0；
        */
		@Override
		public void beforeTextChanged(CharSequence s, int start, int count,
				int after) {
			// TODO Auto-generated method stub
			Log.i("test1", "before:s,"+s+",start,"+start+",count,"+count+",after,"+after);
			
		}
       /**
        * 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
        *  在s中，从start处开始的count个字符刚刚替换了原来长度为before的文本
        *  s 为变化后的内容；
        *  start 为开始变化位置的索引，从0开始计数；
        *  before 为被取代的老文本的长度，比如s由1变为12，before为0，由12变为1，before为1；
        *  count 为将要发生变化的字符数
        */
		@Override
		public void onTextChanged(CharSequence s, int start, int before,
				int count) {
			// TODO Auto-generated method stub
			Log.i("test1", "ing:s,"+s+",start,"+start+",before,"+before+",count,"+count);
			
		}
       /**
        * This method is called to notify you that, somewhere within s, the text has been changed.
        */
		@Override
		public void afterTextChanged(Editable s) {
			// TODO Auto-generated method stub
			Log.i("test1", "after,s,"+s);
			
		}
}

     注册监听：

EditText mEditor = (EditText)findViewById(R.id.editor_input);
mEditor.addTextChangedListener(mTextWatcher);