GestureDetector.onTouchEvent中的NullPointerException

Question

我在onTouch中使用GestureDetector进行了一项活动.在我的布局中,我还有一个实现onClickListener的视图.在Android ICS上,我在处理TouchEvent时遇到NullPointerException.这个错误的原因是什么？我看过几篇帖子说onClick和onTouch不能很好地结合在一起,但是当看到这个错误时,没有真正的原因解释或问题的真正解决方案.

这是代码:

public class FlipCardActivity extends Activity implements
    View.OnClickListener, View.OnTouchListener {

    protected GestureDetector gestureDetector;

    protected class TouchSwipeListener extends GestureDetector.
        SimpleOnGestureListener {       
        @Override
        public boolean onDown(MotionEvent e) {
            return false;
        }

        @Override
        public boolean onFling(MotionEvent e1, MotionEvent e2, float velocityX,
            float velocityY) {
            // Calculate dx, gradient, velocity, etc
            // Check for and discard unacceptable swipes
            if (Math.abs(gradient) > SWIPE_MAX_GRADIENT || Math.abs(distance) < 
                SWIPE_MIN_DISTANCE || Math.abs(velocity) < SWIPE_MIN_VELOCITY)
                return false;

            // Determine whether it's a left or a right swipe
            if (dx < 0)
                activity.showNext();
            else
                activity.showPrevious();
            return true;
        }
    }

    @Override
    protected void onCreate(Bundle savedInstance) {
        super.onCreate(savedInstance);
        gestureDetector = new GestureDetector(this, new TouchSwipeListener(this));
        setContentView(R.layout.main);
    }

    @Override
    public boolean onTouch(View v, MotionEvent ev) {
        return gestureDetector.onTouchEvent(ev);
    }
}

堆栈跟踪是:

java.lang.NullPointerException
at android.view.GestureDetector.onTouchEvent(GestureDetector.java:587)
at com.fivepumpkins.common.FlipCardActivity.onTouch(FlipCardActivity.java:602)
at android.view.View.dispatchTouchEvent(View.java:5536)
at android.view.ViewGroup.dispatchTransformedTouchEvent(ViewGroup.java:1957)
at android.view.ViewGroup.dispatchTouchEvent(ViewGroup.java:1726)
at android.view.ViewGroup.dispatchTransformedTouchEvent(ViewGroup.java:1957)
at android.view.ViewGroup.dispatchTouchEvent(ViewGroup.java:1726)
at android.view.ViewGroup.dispatchTransformedTouchEvent(ViewGroup.java:1957)
at android.view.ViewGroup.dispatchTouchEvent(ViewGroup.java:1726)
at android.view.ViewGroup.dispatchTransformedTouchEvent(ViewGroup.java:1957)
at android.view.ViewGroup.dispatchTouchEvent(ViewGroup.java:1726)
at android.view.ViewGroup.dispatchTransformedTouchEvent(ViewGroup.java:1957)
at android.view.ViewGroup.dispatchTouchEvent(ViewGroup.java:1726)
at android.view.ViewGroup.dispatchTransformedTouchEvent(ViewGroup.java:1957)
at android.view.ViewGroup.dispatchTouchEvent(ViewGroup.java:1726)
at com.android.internal.policy.impl.PhoneWindow$DecorView.superDispatchTouchEvent(PhoneWindow.java:1912)
at com.android.internal.policy.impl.PhoneWindow.superDispatchTouchEvent(PhoneWindow.java:1371)
at android.app.Activity.dispatchTouchEvent(Activity.java:2364)
at com.android.internal.policy.impl.PhoneWindow$DecorView.dispatchTouchEvent(PhoneWindow.java:1860)
at android.view.View.dispatchPointerEvent(View.java:5721)
at android.view.ViewRootImpl.deliverPointerEvent(ViewRootImpl.java:2890)
at android.view.ViewRootImpl.handleMessage(ViewRootImpl.java:2466)
at android.view.ViewRootImpl.processInputEvents(ViewRootImpl.java:845)
at android.view.ViewRootImpl.handleMessage(ViewRootImpl.java:2475)
at android.os.Handler.dispatchMessage(Handler.java:99)
at android.os.Looper.loop(Looper.java:137)
at android.app.ActivityThread.main(ActivityThread.java:4575)
at java.lang.reflect.Method.invokeNative(Native Method)
at java.lang.reflect.Method.invoke(Method.java:511)
at com.android.internal.os.ZygoteInit$MethodAndArgsCaller.run(ZygoteInit.java:789)
at com.android.internal.os.ZygoteInit.main(ZygoteInit.java:556)
at dalvik.system.NativeStart.main(Native Method)

看看ICS Source,我可以提取出错误的行:GestureDetector.java:587.onTouchEvent()在这种情况下,它发生在方法内部case MotionEvent.ACTION_UP:.以下是摘录:

// Hold the event we obtained above - listeners may have changed the original.
586            mPreviousUpEvent = currentUpEvent;
587            mVelocityTracker.recycle();
588            mVelocityTracker = null;
589            mIsDoubleTapping = false;
590            mHandler.removeMessages(SHOW_PRESS);
591            mHandler.removeMessages(LONG_PRESS);
592            break;

NullPointerException第587行中的A 表示mVelocityTracker为空.这是该类的私有VelocityTracker属性SimpleOnGestureListener.为什么此变量在此时为空？

Answer 1

它确实可能与点击侦听器有关。GestureDetector 访问 mVelocityTracker 并将其在 MotionEvent.ACTION_UP 和 MotionEvent.ACTION_CANCEL 中设置为 null。点击监听器在View类的onTouchEvent方法的MotionEvent.ACTION_UP中执行。可能存在一种关系（我现在没有时间找到）与 GestureDetector 在单击时执行的触发器 MotionEvent.ACTION_UP 或 MotionEvent.ACTION_CANCEL，在它之前被其中之一设置为 null 之后。

尝试删除点击侦听器，看看它是否仍然发生。如果是，您可能可以更好地处理触摸监听器中的所有内容。