我的Android进阶之旅------>Android关于TextWatcher的初步了解

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);
			}
		}
	};

        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);
	}