android.text.TextWatcher

/**
* When an object of a type is attached to an Editable, its methods will
* be called when the text is changed.
*/
public interface TextWatcher extends NoCopySpan {
/**
* This method is called to notify you that, within <code>s</code>,
* the <code>count</code> characters beginning at <code>start</code>
* are about to be replaced by new text with length <code>after</code>.
* It is an error to attempt to make changes to <code>s</code> from
* this callback.
*/
public void beforeTextChanged(CharSequence s, int start,
int count, int after);
/**
* This method is called to notify you that, within <code>s</code>,
* the <code>count</code> characters beginning at <code>start</code>
* have just replaced old text that had length <code>before</code>.
* It is an error to attempt to make changes to <code>s</code> from
* this callback.
*/
public void onTextChanged(CharSequence s, int start, int before, int count);

/**
* This method is called to notify you that, somewhere within
* <code>s</code>, the text has been changed.
* It is legitimate to make further changes to <code>s</code> 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.
* (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 {@link Spannable#setSpan} in {@link #onTextChanged}
* to mark your place and then look up from here where the span
* ended up.
*/
public void afterTextChanged(Editable s);
}

说明：

public void beforeTextChanged(CharSequence s, int start, int count, int after)

s改变前的text字符，start当前起始位置，count被替换字符的长度（删除时为0），after新替代字符的长度

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

s改变后的text字符，start替换前的起始位置（同beforeTextChanged），before被替换字符的长度，count新替代字符的长度

public void afterTextChanged(Editable s)

s改变后的text字符