首先来看一下TextWatcher的源码

package android.text;/** * 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 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 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.     * (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);}

以下通过通过一个小实例来学习TextWatcher的相关使用方法

实现该接口

TextWatcher mTextWatcher = new TextWatcher() {		private CharSequence temp;		private int editStart;		private int editEnd;		@Override		public void beforeTextChanged(CharSequence s, int arg1, int arg2,				int arg3) {			temp = s;		}		@Override		public void onTextChanged(CharSequence s, int arg1, int arg2, int arg3) {			mDetailView.setText(s);		}		@Override		public void afterTextChanged(Editable s) {			editStart = mEditText.getSelectionStart();			editEnd = mEditText.getSelectionEnd();			if (temp.length() > 11) {				Toast.makeText(MainActivity.this, "你输入的字数已经超过了限制！",						Toast.LENGTH_SHORT).show();				s.delete(editStart - 1, editEnd);				int tempSelection = editStart;				mEditText.setText(s);				mEditText.setSelection(tempSelection);			}		}	};

再注冊这个监听

TextView mDetailView;	EditText mEditText;	@Override	protected void onCreate(Bundle savedInstanceState) {		super.onCreate(savedInstanceState);		setContentView(R.layout.activity_main);		mDetailView = (TextView) findViewById(R.id.mDetailView);		mEditText = (EditText) findViewById(R.id.mEditText);		mEditText.addTextChangedListener(mTextWatcher);	}

看执行效果

 

 

 

                           

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