以前用過android.text.TextWatcher來監聽文字發生變化，但沒有仔細去想它，今天興致來了就發個瘋來玩玩吧！

有點擔心自己理解錯，所以還是先把英文API解釋給大家看看

1、什麼情況下使用了？

When an object of a type is attached to an Editable, its methods will be called when the text is changed.

2、下面是它的三個抽象方法

/**
*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 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 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 onTextChanged(CharSequence s, int start, int before, int count)

/**
*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(遞迴). (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.
*/
public void afterTextChanged(Editable s)
 

我輸入結果：a		ab		about		abou
得到的結果：
s:是改變前的結果，start是改變前的位置，count是減少時改變的個數，after是增加時改變的個數
beforeTextChanged(CharSequence s, int start, int count,int after)  
        //null		0		0		1
	//a		1		0		1
	//ab		2		0		3
	//about		4		1		0
s:是改變後的結果，start是改變前的位置，before是減少時改變的個數，count是增加時改變的個數
onTextChanged(CharSequence s, int start, int before,int count)
//a		0		0		1
	//ab		1		0		1
	//about	2		0		3
	//abou	4		1		0

afterTextChanged(Editable s)
	//a
	//ab
	//about
	//abou
 

首先是實現他的介面 implements TextWatcher

這是他的監聽事件addTextChangedListener(this)