Android 回調機制

Android 回調機制

在android開發中經常遇到各種各樣的回調方法，然而，到底什麼是回調呢？
現在，以android中View的onClick()回調方法爲例，講解一下。

OnClickListener 接口，當view被點擊後回調該接口

 /**
     * 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中實現回調接口

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 類 當View被點擊後，註冊一個回調接口，用於調用。

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;
    }
}