Android系统中长按事件的实现机制解析

在Android的触摸消息中，已经实现了三种监测，它们分别是

1）pre-pressed：对应的语义是用户轻触(tap)了屏幕

2）pressed:对应的语义是用户点击(press)了屏幕

3）long pressed：对应的语义是用户长按(long press)了屏幕

下图是触摸消息随时间变化的时间轴示意图：

其中，t0和t1定义在ViewConfiguration类中，标识了tap和longpress的超时时间，定义如下：

/**
 * Defines the duration in milliseconds we will wait to see if a touch event
 * is a tap or a scroll. If the user does not move within this interval, it is
 * considered to be a tap.
 */
private static final int TAP_TIMEOUT = 115; // t0

/**
 * Defines the duration in milliseconds before a press turns into
 * a long press
 */
private static final int LONG_PRESS_TIMEOUT = 500; // t1

代码中实现监测的地方在View类的OnTouchEvent函数中，当View监测到ACTION_DOWN事件时，首先发送一个延迟为t0的异步消息，代码如下：

case MotionEvent.ACTION_DOWN:
    if (mPendingCheckForTap == null) {
        mPendingCheckForTap = new CheckForTap();
    }
    mPrivateFlags |= PREPRESSED;
    mHasPerformedLongPress = false;
    postDelayed(mPendingCheckForTap, ViewConfiguration.getTapTimeout());
    break;

如果在t0时间内用户释放了屏幕，即ACTION_UP事件在t0时间内发生，则本次触摸对应的是pre-pressed处理代码，语义是"用户轻触(TAP)了一下屏幕"；如果用户在t1时间内释放了屏幕，那么本次操作是一个"press"操作；如果用户超过t1时间释放屏幕，则系统认为监测到了长按事件。其中处理"press"操作的代码在类CheckForTap类中，处理长按操作的代码在类CheckForLongPress类中。而处理pre-pressed的代码在ACTION_UP事件响应中，ACTION_UP事件响应中大部分代码用于处理触摸的状态变化，如下所示：

case MotionEvent.ACTION_UP:
    boolean prepressed = (mPrivateFlags & PREPRESSED) != 0; //获取prepressed状态
    if ((mPrivateFlags & PRESSED) != 0 || prepressed) { //如果是pressed状态或者是prepressed状态，才进行处理
        // 如果当前view不具有焦点，则需要先获取焦点，因为我们当前处理触摸模式
        boolean focusTaken = false;
        if (isFocusable() && isFocusableInTouchMode() && !isFocused()) {
            focusTaken = requestFocus(); // 请求获得焦点
        }

        if (!mHasPerformedLongPress) { // 是否处理过长按操作了，如果是，则直接返回
            // 进入该代码段，说明这是一个tap操作，首先移除长按回调操作
            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(); // 执行点击的处理函数
                }
            }
        }

        if (mUnsetPressedState == null) {
            mUnsetPressedState = new UnsetPressedState();
        }

        if (prepressed) {
            mPrivateFlags |= PRESSED;
            refreshDrawableState();
            // 发送重置触摸状态的异步延迟消息
            postDelayed(mUnsetPressedState,
                    ViewConfiguration.getPressedStateDuration());
        } else if (!post(mUnsetPressedState)) {
            // If the post failed, unpress right now
            mUnsetPressedState.run();
        }
        removeTapCallback(); // 移除tap的回调操作
    }
    break;

在上面的代码分析中，可以看出，整个监测过程涉及到两个Runnable对象和一个利用Handler发送异步延迟消息的函数，下面就来分析一下：

 

1）PostDelayed函数

该函数的主要工作是获取UI线程的Handler对象，然后调用Handler类的postDelayed函数将指定的Runnable对象放到消息队列中。

public boolean postDelayed(Runnable action, long delayMillis) {
    Handler handler;
    if (mAttachInfo != null) {
        handler = mAttachInfo.mHandler;
    } else {
        // Assume that post will succeed later
        ViewRoot.getRunQueue().postDelayed(action, delayMillis);
        return true;
    }

    return handler.postDelayed(action, delayMillis);
}

2）CheckForTap类

该类实现了Runnable接口，在run函数中设置触摸标识，并刷新Drawable的状态，同时用于发送一个检测长按事件的异步延迟消息，代码如下：

private final class CheckForTap implements Runnable {
    public void run() {
        // 进入该函数，说明已经过了ViewConfiguration.getTapTimeout()时间，
        // 即pre-pressed状态结束，宣告触摸进入pressed状态
        mPrivateFlags &= ~PREPRESSED;
        mPrivateFlags |= PRESSED;
        refreshDrawableState(); // 刷新控件的背景Drawable
        // 如果长按检测没有被去使能，则发送一个检测长按事件的异步延迟消息
        if ((mViewFlags & LONG_CLICKABLE) == LONG_CLICKABLE) {
            postCheckForLongClick(ViewConfiguration.getTapTimeout());
        }
    }
}

private void postCheckForLongClick(int delayOffset) {
    mHasPerformedLongPress = false;

    // 实例化CheckForLongPress对象
    if (mPendingCheckForLongPress == null) {
        mPendingCheckForLongPress = new CheckForLongPress();
    }
    mPendingCheckForLongPress.rememberWindowAttachCount();
    // 调用PostDelayed函数发送长按事件的异步延迟消息
    postDelayed(mPendingCheckForLongPress,
            ViewConfiguration.getLongPressTimeout() - delayOffset);
}

3）CheckForLongPress类

该类定义了长按操作发生时的响应处理，同样实现了Runnable接口

public boolean postDelayed(Runnable action, long delayMillis) {
    Handler handler;
    if (mAttachInfo != null) {
        handler = mAttachInfo.mHandler;
    } else {
        // Assume that post will succeed later
        ViewRoot.getRunQueue().postDelayed(action, delayMillis);
        return true;
    }

    return handler.postDelayed(action, delayMillis);
}

2）CheckForTap类

该类实现了Runnable接口，在run函数中设置触摸标识，并刷新Drawable的状态，同时用于发送一个检测长按事件的异步延迟消息，代码如下：

private final class CheckForTap implements Runnable {
    public void run() {
        // 进入该函数，说明已经过了ViewConfiguration.getTapTimeout()时间，
        // 即pre-pressed状态结束，宣告触摸进入pressed状态
        mPrivateFlags &= ~PREPRESSED;
        mPrivateFlags |= PRESSED;
        refreshDrawableState(); // 刷新控件的背景Drawable
        // 如果长按检测没有被去使能，则发送一个检测长按事件的异步延迟消息
        if ((mViewFlags & LONG_CLICKABLE) == LONG_CLICKABLE) {
            postCheckForLongClick(ViewConfiguration.getTapTimeout());
        }
    }
}

private void postCheckForLongClick(int delayOffset) {
    mHasPerformedLongPress = false;

    // 实例化CheckForLongPress对象
    if (mPendingCheckForLongPress == null) {
        mPendingCheckForLongPress = new CheckForLongPress();
    }
    mPendingCheckForLongPress.rememberWindowAttachCount();
    // 调用PostDelayed函数发送长按事件的异步延迟消息
    postDelayed(mPendingCheckForLongPress,
            ViewConfiguration.getLongPressTimeout() - delayOffset);
}

3）CheckForLongPress类

该类定义了长按操作发生时的响应处理，同样实现了Runnable接口

class CheckForLongPress implements Runnable {

    private int mOriginalWindowAttachCount;

    public void run() {
        // 进入该函数，说明检测到了长按操作
        if (isPressed() && (mParent != null)
                && mOriginalWindowAttachCount == mWindowAttachCount) {
            if (performLongClick()) {
                mHasPerformedLongPress = true;
            }
        }
    }

    public void rememberWindowAttachCount() {
        mOriginalWindowAttachCount = mWindowAttachCount;
    }
}

public boolean performLongClick() {
    sendAccessibilityEvent(AccessibilityEvent.TYPE_VIEW_LONG_CLICKED);

    boolean handled = false;
    if (mOnLongClickListener != null) {
        // 回调用户实现的长按操作监听函数（OnLongClickListener）
        handled = mOnLongClickListener.onLongClick(View.this);
    }
    if (!handled) {
        // 如果OnLongClickListener的onLongClick返回false
        // 则需要继续处理该长按事件，这里是显示上下文菜单
        handled = showContextMenu();
    }
    if (handled) {
        // 长按操作事件被处理了，此时应该给用户触觉上的反馈
        performHapticFeedback(HapticFeedbackConstants.LONG_PRESS);
    }
    return handled;
}