首先来看一下TextWatcher的源代码

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 <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);

}

下面通过通过一个小实例来学习TextWatcher的相关用法

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);

			}

		}

	};

再注册这个监听

        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);

	}

看运行效果