Android中回调函数机制解析

2014/12/04 17:49
阅读数 16

提示:在阅读本文章之前,请确保您对Touch事件的分发机制有一定的了解,如果您不了解请参阅:Touch事件的分发机制

Android的学习过程中经常会听到或者见到“回调”这个词,那么什么是回调呢?所谓的回调函数就是:在A类中定义了一个方法,这个方法中用到了一个接口和该接口中的抽象方法,但是抽象方法没有具体的实现,需要B类去实现,B类实现该方法后,它本身不会去调用该方法,而是传递给A类,供A类去调用,这种机制就称为回调。

下面我们拿具体的Button的点击事件进行模拟分析:

首先,在View类中我们能找到setOnClickListener(OnClickListener l)方法:

public void setOnClickListener(OnClickListener l) {

     if (!isClickable()) {

         setClickable(true);

     }

     getListenerInfo().mOnClickListener = l;

}


 

可以看到,在该方法中将OnClickListener赋值给了mOnClickListener,那么我们继续向下找,会看到在performClick()方法中执行了我们实现的onClick()方法。

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;

}

由此我们可以清楚的看不到,在父类中我们要用到onClick()方法,但是父类却没有去实现该方法,而是定义了一个方法setOnClickListener(OnClickListener l),如果子类想要自己能够响应点击事件,则它就必须重写父类的该方法,实现OnClickListener接口和它的onClick()方法。在子类实现该接口和方法后,将其通过参数传递给父类,在父类中执行onClick()方法。

那么,为什么会在父类中执行到该方法呢,这就要说到Android中的另一个重要的机制——触摸事件的传递机制。

我们知道,只要我们的手指触摸到手机屏幕,就一定会执行dispatchTouchEvent(MotionEvent event)方法,接下来我们就看一下dispatchTouchEvent方法中都有哪些内容:

public boolean dispatchTouchEvent(MotionEvent event) {

        if (mInputEventConsistencyVerifier != null) {

            mInputEventConsistencyVerifier.onTouchEvent(event, 0);

        }

 

        if (onFilterTouchEventForSecurity(event)) {

            //noinspection SimplifiableIfStatement

            ListenerInfo li = mListenerInfo;

            if (li != null && li.mOnTouchListener != null 

                    && (mViewFlags & ENABLED_MASK) == ENABLED

                    && li.mOnTouchListener.onTouch(this, event)) {

                return true;

            }

 

            if (onTouchEvent(event)) {

                return true;

            }

        }

 

        if (mInputEventConsistencyVerifier != null) {

            mInputEventConsistencyVerifier.onUnhandledEvent(event, 0);

        }

        return false;

}


 

这里我们不细讲Touch事件的分发机制,因为网上有哥们已经讲的很清楚了。请参看篇首提供的链接。

我们看一下第17行,由于我们没有实现OnTouchListener接口,而onTouch()方法的默认返回值为false,所以第一个if语句中的代码不会被执行到,进入第二个if语句中,执行了onTouchEvent()方法。那么我们再来看一下该方法:

public boolean onTouchEvent(MotionEvent event) {

        final int viewFlags = mViewFlags;

 

        if ((viewFlags & ENABLED_MASK) == DISABLED) {

            if (event.getAction() == MotionEvent.ACTION_UP 

                     && (mPrivateFlags & PFLAG_PRESSED) != 0) {

                setPressed(false);

            }

            // A disabled view that is clickable still consumes the touch

            // events, it just doesn't respond to them.

            return (((viewFlags & CLICKABLE) == CLICKABLE ||

                    (viewFlags & LONG_CLICKABLE) == LONG_CLICKABLE));

        }

 

        if (mTouchDelegate != null) {

            if (mTouchDelegate.onTouchEvent(event)) {

                return true;

            }

        }

 

        if (((viewFlags & CLICKABLE) == CLICKABLE ||

                (viewFlags & LONG_CLICKABLE) == LONG_CLICKABLE)) {

            switch (event.getAction()) {

                case MotionEvent.ACTION_UP:

                    boolean prepressed = (mPrivateFlags & PFLAG_PREPRESSED) != 0;

                    if ((mPrivateFlags & PFLAG_PRESSED) != 0 || prepressed) {

                        // take focus if we don't have it already and we should in

                        // touch mode.

                        boolean focusTaken = false;

                        if (isFocusable() && isFocusableInTouchMode() && !isFocused()) {

                            focusTaken = requestFocus();

                        }

 

                        if (prepressed) {

                            // The button is being released before we actually

                            // showed it as pressed.  Make it show the pressed

                            // state now (before scheduling the click) to ensure

                            // the user sees it.

                            setPressed(true);

                       }

 

                        if (!mHasPerformedLongPress) {

                            // This is a tap, so remove the longpress check

                            removeLongPressCallback();

 

                            // Only perform take click actions if we were in the pressed state

                            if (!focusTaken) {

                                // Use a Runnable and post this rather than calling

                                // performClick directly. This lets other visual state

                                // of the view update before click actions start.

                                if (mPerformClick == null) {

                                    mPerformClick = new PerformClick();

                                }

                                if (!post(mPerformClick)) {

                                    performClick();

                                }

                            }

                        }

 

                        if (mUnsetPressedState == null) {

                            mUnsetPressedState = new UnsetPressedState();

                        }

 

                        if (prepressed) {

                            postDelayed(mUnsetPressedState,

                                    ViewConfiguration.getPressedStateDuration());

                        } else if (!post(mUnsetPressedState)) {

                            // If the post failed, unpress right now

                            mUnsetPressedState.run();

                        }

                        removeTapCallback();

                    }

                    break;

 

                case MotionEvent.ACTION_DOWN:

                    mHasPerformedLongPress = false;

 

                    if (performButtonActionOnTouchDown(event)) {

                        break;

                    }

 

                    // Walk up the hierarchy to determine if we're inside a scrolling container.

                    boolean isInScrollingContainer = isInScrollingContainer();

 

                    // For views inside a scrolling container, delay the pressed feedback for

                    // a short period in case this is a scroll.

                    if (isInScrollingContainer) {

                        mPrivateFlags |= PFLAG_PREPRESSED;

                        if (mPendingCheckForTap == null) {

                            mPendingCheckForTap = new CheckForTap();

                        }

                        postDelayed(mPendingCheckForTap, 

                                   ViewConfiguration.getTapTimeout());

                    } else {

                        // Not inside a scrolling container, so show the feedback right away

                        setPressed(true);

                        checkForLongClick(0);

                    }

                    break;

 

                case MotionEvent.ACTION_CANCEL:

                    setPressed(false);

                    removeTapCallback();

                    removeLongPressCallback();

                    break;

 

                case MotionEvent.ACTION_MOVE:

                    final int x = (int) event.getX();

                    final int y = (int) event.getY();

 

                    // Be lenient about moving outside of buttons

                    if (!pointInView(x, y, mTouchSlop)) {

                        // Outside button

                        removeTapCallback();

                        if ((mPrivateFlags & PFLAG_PRESSED) != 0) {

                            // Remove any future long press/tap checks

                            removeLongPressCallback();

 

                            setPressed(false);

                        }

                    }

                    break;

            }

            return true;

        }

 

        return false;

    }

代码太长,我们只看重点,在ACTION_UP这个case当中,我们找到了关键的代码(第109行):performClick()

至此,我们已经基本搞清楚了回调机制的整个过程。

 

原创文章,可以自由传播,但转载请注明出处。

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