Android 回调机制

Android 回调机制

在android开发中经常遇到各种各样的回调方法，然而，到底什么是回调呢？
现在，以android中View的onClick()回调方法为例，讲解一下。

OnClickListener 接口，当view被点击后回调该接口

 /**     * Interface definition for a callback to be invoked when a view is clicked.     */    public interface OnClickListener {        /**         * Called when a view has been clicked.         *         * @param v The view that was clicked.         */        void onClick(View v);    }

在Activity中实现回调接口

public class MainActivity extends Activity implements OnClickListener {    Button btn_location;    @Override    protected void onCreate(Bundle savedInstanceState) {        super.onCreate(savedInstanceState);        setContentView(R.layout.activity_main);        btn_location = (Button) findViewById(R.id.btn_location);        btn_location.setOnClickListener(new View.OnClickListener() {            @Override            public void onClick(View v) {                //TODO action            }        });    }}

View 类 当View被点击后，注册一个回调接口，用于调用。

public class View implements Drawable.Callback, KeyEvent.Callback,        AccessibilityEventSource {    /**         * Listener used to dispatch click events.         * This field should be made private, so it is hidden from the SDK.         * {@hide}         */        public OnClickListener mOnClickListener;    /**     * Register a callback to be invoked when this view is clicked. If this view is not     * clickable, it becomes clickable.     *     * @param l The callback that will run     *     * @see #setClickable(boolean)     */    public void setOnClickListener(OnClickListener l) {        if (!isClickable()) {            setClickable(true);        }        getListenerInfo().mOnClickListener = l;    }     /**     * Call this view's OnClickListener, if it is defined.  Performs all normal     * actions associated with clicking: reporting accessibility event, playing     * a sound, etc.     *     * @return True there was an assigned OnClickListener that was called, false     *         otherwise is returned.     */    public boolean performClick() {        sendAccessibilityEvent(AccessibilityEvent.TYPE_VIEW_CLICKED);        ListenerInfo li = mListenerInfo;        if (li != null && li.mOnClickListener != null) {            playSoundEffect(SoundEffectConstants.CLICK);            li.mOnClickListener.onClick(this);            return true;        }        return false;    }}