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How to exit from a listener in Android

 原文  2015-01-09 15:59:14  0   2    java/ android/ android-activity

Question

I've Googled a lot but I still didn't find any solution for my problem. I'm new to Android so sorry for a stupid question. Here is my code: 

final Intent myIntent = new Intent(this.getBaseContext(), ShowActivity.class);
webView.setOnTouchListener(new SwipeListener(this)
{
    @Override
    public void onSwipeLeft()
    {
        articleID--;
        //System.out.println("SWIPE_LEFT");
        myIntent.putExtra("id", articleID);
        System.out.println(myIntent.toString());
        Swiped = true;
    }
    @Override
    public void onSwipeRight()
    {
        articleID++;
        //System.out.println("SWIPE_RIGHT");
        myIntent.putExtra("id", articleID);
        System.out.println(myIntent.toString());
        Swiped = true;
    }
});
if (Swiped == true)
    mContext.startActivity(myIntent);

So, the question is: why my new activity doesn't start? I mean it doesn't even get into if-condition. Seems like that because of listener it ignores any code below, doesn't it? How to exit from listener? Btw, i'm trying to start new activity on Swipe. Maybe I need to rework architecture? But i really don't want to =(

This is my SwipeListener class: 

public class SwipeListener implements View.OnTouchListener {
// Gesture Detector which has onTouchEvent action
// Gesture detector requires custom gesture listener (Which declared below)
private GestureDetector gestureDetector;
// Basic Construction
public SwipeListener(Context c) {
    gestureDetector = new GestureDetector(c, new GestureListener());
}
// onTouch action. Returns ANY (click and release) event thanks to onTouchEvent of gestureDetector
public boolean onTouch(final View view, final MotionEvent motionEvent) {
    return gestureDetector.onTouchEvent(motionEvent);
}

// GestureListener for GestureDetector declaration.
private final class GestureListener extends GestureDetector.SimpleOnGestureListener {

    private static final int SWIPE_MIN_DISTANCE = 100; // Swipe minimal distance
    private static final int SWIPE_MIN_VELOCITY = 100; // swipe minimal speed

    @Override
    public boolean onDown(MotionEvent e) {
        return true;
    }

    // Determines the fling velocity and then fires the appropriate swipe event accordingly
    // onFling MUST return false so don't touch it.
    @Override
    public boolean onFling(MotionEvent e1, MotionEvent e2, float velocityX, float velocityY) {
        try {
            float diffY = e2.getY() - e1.getY(); // Y(vertical) Difference between release and press coordinates
            float diffX = e2.getX() - e1.getX(); // X(horizontal) Difference between release and press coordinates
            // Determine if swipe was in horizontal or in vertical align
            // Horizontal (left-right)
            if (Math.abs(diffX) > Math.abs(diffY)) {
                // Check if speed and distance are big enough
                if (Math.abs(diffX) > SWIPE_MIN_DISTANCE && Math.abs(velocityX) > SWIPE_MIN_VELOCITY) {
                    if (diffX > 0) {
                        onSwipeRight();
                    } else {
                        onSwipeLeft();
                    }
                }
            // Vertical (up-down)
            } else {
                if (Math.abs(diffY) > SWIPE_MIN_DISTANCE && Math.abs(velocityY) > SWIPE_MIN_VELOCITY) {
                    if (diffY > 0) {
                        onSwipeDown();
                    } else {
                        onSwipeUp();
                    }
                }
            }
        } catch (Exception exception) {
            System.out.println("YOU SHALL NOT SWIPE");
        }
        return false;
    }
}

public void onSwipeRight() {
    // U need to override this
}

public void onSwipeLeft() {
    // and this
}

public void onSwipeUp() {
    // and this one too
}

public void onSwipeDown() {
    // U know what to do
}

}

Here is a log: 

01-09 19:21:36.564    7758-7758/com.weel.ur W/System.err﹕ java.lang.NullPointerException: Attempt to invoke virtual method 'int android.content.Intent.getFlags()' on a null object reference
01-09 19:21:36.565    7758-7758/com.weel.ur W/System.err﹕ at android.app.ContextImpl.startActivity(ContextImpl.java:1231)
01-09 19:21:36.565    7758-7758/com.weel.ur W/System.err﹕ at android.app.ContextImpl.startActivity(ContextImpl.java:1219)
01-09 19:21:36.565    7758-7758/com.weel.ur W/System.err﹕ at com.weel.ur.ShowActivity.startAct(ShowActivity.java:755)
01-09 19:21:36.565    7758-7758/com.weel.ur W/System.err﹕ at com.weel.ur.ShowActivity$2.onSwipeLeft(ShowActivity.java:347)
01-09 19:21:36.565    7758-7758/com.weel.ur W/System.err﹕ at com.weel.ur.calligraphy.SwipeListener$GestureListener.onFling(SwipeListener.java:51)
01-09 19:21:36.565    7758-7758/com.weel.ur W/System.err﹕ at android.view.GestureDetector.onTouchEvent(GestureDetector.java:610)
01-09 19:21:36.565    7758-7758/com.weel.ur W/System.err﹕ at com.weel.ur.calligraphy.SwipeListener.onTouch(SwipeListener.java:22)
01-09 19:21:36.565    7758-7758/com.weel.ur W/System.err﹕ at android.view.View.dispatchTouchEvent(View.java:8382)
01-09 19:21:36.565    7758-7758/com.weel.ur W/System.err﹕ at android.view.ViewGroup.dispatchTransformedTouchEvent(ViewGroup.java:2424)
01-09 19:21:36.565    7758-7758/com.weel.ur W/System.err﹕ at android.view.ViewGroup.dispatchTouchEvent(ViewGroup.java:2158)
01-09 19:21:36.565    7758-7758/com.weel.ur W/System.err﹕ at android.view.ViewGroup.dispatchTransformedTouchEvent(ViewGroup.java:2430)
01-09 19:21:36.565    7758-7758/com.weel.ur W/System.err﹕ at android.view.ViewGroup.dispatchTouchEvent(ViewGroup.java:2172)
01-09 19:21:36.565    7758-7758/com.weel.ur W/System.err﹕ at android.view.ViewGroup.dispatchTransformedTouchEvent(ViewGroup.java:2430)
01-09 19:21:36.565    7758-7758/com.weel.ur W/System.err﹕ at android.view.ViewGroup.dispatchTouchEvent(ViewGroup.java:2172)
01-09 19:21:36.565    7758-7758/com.weel.ur W/System.err﹕ at android.view.ViewGroup.dispatchTransformedTouchEvent(ViewGroup.java:2430)
01-09 19:21:36.565    7758-7758/com.weel.ur W/System.err﹕ at android.view.ViewGroup.dispatchTouchEvent(ViewGroup.java:2172)
01-09 19:21:36.565    7758-7758/com.weel.ur W/System.err﹕ at android.view.ViewGroup.dispatchTransformedTouchEvent(ViewGroup.java:2430)
01-09 19:21:36.565    7758-7758/com.weel.ur W/System.err﹕ at android.view.ViewGroup.dispatchTouchEvent(ViewGroup.java:2172)
01-09 19:21:36.565    7758-7758/com.weel.ur W/System.err﹕ at android.view.ViewGroup.dispatchTransformedTouchEvent(ViewGroup.java:2430)
01-09 19:21:36.565    7758-7758/com.weel.ur W/System.err﹕ at android.view.ViewGroup.dispatchTouchEvent(ViewGroup.java:2172)
01-09 19:21:36.565    7758-7758/com.weel.ur W/System.err﹕ at com.android.internal.policy.impl.PhoneWindow$DecorView.superDispatchTouchEvent(PhoneWindow.java:2314)
01-09 19:21:36.565    7758-7758/com.weel.ur W/System.err﹕ at com.android.internal.policy.impl.PhoneWindow.superDispatchTouchEvent(PhoneWindow.java:1692)
01-09 19:21:36.565    7758-7758/com.weel.ur W/System.err﹕ at android.app.Activity.dispatchTouchEvent(Activity.java:2739)
01-09 19:21:36.565    7758-7758/com.weel.ur W/System.err﹕ at android.support.v7.app.ActionBarActivityDelegateICS$WindowCallbackWrapper.dispatchTouchEvent(ActionBarActivityDelegateICS.java:268)
01-09 19:21:36.565    7758-7758/com.weel.ur W/System.err﹕ at com.android.internal.policy.impl.PhoneWindow$DecorView.dispatchTouchEvent(PhoneWindow.java:2275)
01-09 19:21:36.565    7758-7758/com.weel.ur W/System.err﹕ at android.view.View.dispatchPointerEvent(View.java:8578)
01-09 19:21:36.566    7758-7758/com.weel.ur W/System.err﹕ at android.view.ViewRootImpl$ViewPostImeInputStage.processPointerEvent(ViewRootImpl.java:4021)
01-09 19:21:36.566    7758-7758/com.weel.ur W/System.err﹕ at android.view.ViewRootImpl$ViewPostImeInputStage.onProcess(ViewRootImpl.java:3887)
01-09 19:21:36.566    7758-7758/com.weel.ur W/System.err﹕ at android.view.ViewRootImpl$InputStage.deliver(ViewRootImpl.java:3449)
01-09 19:21:36.566    7758-7758/com.weel.ur W/System.err﹕ at android.view.ViewRootImpl$InputStage.onDeliverToNext(ViewRootImpl.java:3502)
01-09 19:21:36.566    7758-7758/com.weel.ur W/System.err﹕ at android.view.ViewRootImpl$InputStage.forward(ViewRootImpl.java:3468)
01-09 19:21:36.566    7758-7758/com.weel.ur W/System.err﹕ at android.view.ViewRootImpl$AsyncInputStage.forward(ViewRootImpl.java:3578)
01-09 19:21:36.566    7758-7758/com.weel.ur W/System.err﹕ at android.view.ViewRootImpl$InputStage.apply(ViewRootImpl.java:3476)
01-09 19:21:36.566    7758-7758/com.weel.ur W/System.err﹕ at android.view.ViewRootImpl$AsyncInputStage.apply(ViewRootImpl.java:3635)
01-09 19:21:36.566    7758-7758/com.weel.ur W/System.err﹕ at android.view.ViewRootImpl$InputStage.deliver(ViewRootImpl.java:3449)
01-09 19:21:36.566    7758-7758/com.weel.ur W/System.err﹕ at android.view.ViewRootImpl$InputStage.onDeliverToNext(ViewRootImpl.java:3502)
01-09 19:21:36.566    7758-7758/com.weel.ur W/System.err﹕ at android.view.ViewRootImpl$InputStage.forward(ViewRootImpl.java:3468)
01-09 19:21:36.566    7758-7758/com.weel.ur W/System.err﹕ at android.view.ViewRootImpl$InputStage.apply(ViewRootImpl.java:3476)
01-09 19:21:36.566    7758-7758/com.weel.ur W/System.err﹕ at android.view.ViewRootImpl$InputStage.deliver(ViewRootImpl.java:3449)
01-09 19:21:36.566    7758-7758/com.weel.ur W/System.err﹕ at android.view.ViewRootImpl.deliverInputEvent(ViewRootImpl.java:5701)
01-09 19:21:36.566    7758-7758/com.weel.ur W/System.err﹕ at android.view.ViewRootImpl.doProcessInputEvents(ViewRootImpl.java:5675)
01-09 19:21:36.566    7758-7758/com.weel.ur W/System.err﹕ at android.view.ViewRootImpl.enqueueInputEvent(ViewRootImpl.java:5646)
01-09 19:21:36.566    7758-7758/com.weel.ur W/System.err﹕ at android.view.ViewRootImpl$WindowInputEventReceiver.onInputEvent(ViewRootImpl.java:5791)
01-09 19:21:36.566    7758-7758/com.weel.ur W/System.err﹕ at android.view.InputEventReceiver.dispatchInputEvent(InputEventReceiver.java:185)
01-09 19:21:36.566    7758-7758/com.weel.ur W/System.err﹕ at android.os.MessageQueue.nativePollOnce(Native Method)
01-09 19:21:36.566    7758-7758/com.weel.ur W/System.err﹕ at android.os.MessageQueue.next(MessageQueue.java:143)
01-09 19:21:36.566    7758-7758/com.weel.ur W/System.err﹕ at android.os.Looper.loop(Looper.java:122)
01-09 19:21:36.566    7758-7758/com.weel.ur W/System.err﹕ at android.app.ActivityThread.main(ActivityThread.java:5221)
01-09 19:21:36.566    7758-7758/com.weel.ur W/System.err﹕ at java.lang.reflect.Method.invoke(Native Method)
01-09 19:21:36.566    7758-7758/com.weel.ur W/System.err﹕ at java.lang.reflect.Method.invoke(Method.java:372)
01-09 19:21:36.567    7758-7758/com.weel.ur W/System.err﹕ at com.android.internal.os.ZygoteInit$MethodAndArgsCaller.run(ZygoteInit.java:899)
01-09 19:21:36.567    7758-7758/com.weel.ur W/System.err﹕ at com.android.internal.os.ZygoteInit.main(ZygoteInit.java:694)

2 answers

solution1
 1 ACCPTED  2015-01-09 16:07:58

You must have set default value for Swiped to false. Because you are starting activity outside of the listener its is called even before you get a chance to touch the screen. When onTouchListener is called value of Swiped is changed but you are not starting the activity. 

    webView.setOnTouchListener(new SwipeListener(this)
    {
        @Override
        public void onSwipeLeft()
        {
            articleID--;
            //System.out.println("SWIPE_LEFT");
            Intent myIntent = new Intent(ShowActivity.this, ShowActivity.class);
            myIntent.putExtra("id", articleID);
            System.out.println(myIntent.toString());
            ShowActivity.this.startActivity(myIntent);
        }
        @Override
        public void onSwipeRight()
        {
            articleID++;
            //System.out.println("SWIPE_RIGHT");
            Intent myIntent = new Intent(ShowActivity.this, ShowActivity.class);
            myIntent.putExtra("id", articleID);
            System.out.println(myIntent.toString());
            Swiped = true;
            ShowActivity.this.startActivity(myIntent);
        }
    });

Also, you might want to add a condition before articleID-- and articleID++ . This will help you when you reach to first and last article in the queue. If so Swiped should be set to false or you can reset the articleID

solution2
 0  2015-01-09 16:28:45

You have confused the flow of your program. Setting a listener does not pause the program, and ending an in-listener method does not return to the method that created the listener. When you set a listener, you only set the listener , without running it. And then your program moves on.

This is what your program is dong:

  1. It sets the SwipeListener for webView.
  2. It asks if (Swiped == true) . But Swiped is false because listener is never automatically called upon setting, so activity is not started.
  3. It moves on.

And when a swipe happens, your program runs only what was defined in the listener . mContext.startActivity(myIntent); doesn't exist either in onSwipeLeft() or in onSwipeRight() methods. They just putExtra , then println , and then set Swipe to true. And then it moves on with user interface which has just called one of your onSwipe methods. You must insert mContext.startActivity(myIntent); in both of your listener methods.

You should change your code to this: 

webView.setOnTouchListener(new SwipeListener(this)
{
    @Override
    public void onSwipeLeft()
    {
        articleID--;
        //System.out.println("SWIPE_LEFT");
        myIntent.putExtra("id", articleID);
        System.out.println(myIntent.toString());
        mContext.startActivity(myIntent);
    }
    @Override
    public void onSwipeRight()
    {
        articleID++;
        //System.out.println("SWIPE_RIGHT");
        myIntent.putExtra("id", articleID);
        System.out.println(myIntent.toString());
        mContext.startActivity(myIntent);
    }
});

UPDATE:

On the new problem, it seems like your myIntent turns out to be null somehow. It might be because of code-changing during attempts to fix it. Anyway, this update will probably not fail you. Make sure you do exactly as I describe. 

webView.setOnTouchListener(new SwipeListener(this)
{
    @Override
    public void onSwipeLeft()
    {
        Intent myIntent = new Intent(mContext.getBaseContext(), ShowActivity.class);
        articleID--;
        myIntent.putExtra("id", articleID);
        System.out.println(myIntent.toString());
        mContext.startActivity(myIntent);
    }
    @Override
    public void onSwipeRight()
    {
        Intent myIntent = new Intent(mContext.getBaseContext(), ShowActivity.class);
        articleID++;
        myIntent.putExtra("id", articleID);
        System.out.println(myIntent.toString());
        mContext.startActivity(myIntent);
    }
});

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