java.util.ConcurrentModificationException in View.setVisibility

Question

I'm implementing drag'n'drop for views. When drag is started, I set visibility of the view to INVISIBLE, then, if the drag was interrupted - back to VISIBLE:

public boolean onTouch(View v, MotionEvent event) {
    if (event.getAction() == MotionEvent.ACTION_DOWN) {
        // Skipped some code
        boolean dragStarted = v.startDrag(data, shadowBuilder, v, 0);

        if (dragStarted) {
            v.setVisibility(View.INVISIBLE)
        }
    }
}

And:

if (event.getAction() == DragEvent.ACTION_DRAG_ENDED) {
    View droppedView = (View) event.getLocalState();
    droppedView.setVisibility(View.VISIBLE);
}

And when "Drag ended" event is called, I'm getting exception:

E/AndroidRuntime(7118): FATAL EXCEPTION: main 
E/AndroidRuntime(7118): java.util.ConcurrentModificationException 
E/AndroidRuntime(7118):     at java.util.HashMap$HashIterator.nextEntry(HashMap.java:792)
E/AndroidRuntime(7118):     at java.util.HashMap$KeyIterator.next(HashMap.java:819) 
E/AndroidRuntime(7118):     at android.view.ViewGroup.dispatchDragEvent(ViewGroup.java:1046)
E/AndroidRuntime(7118):     at android.view.ViewGroup.dispatchDragEvent(ViewGroup.java:1048)
E/AndroidRuntime(7118):     at android.view.ViewGroup.dispatchDragEvent(ViewGroup.java:1048)
E/AndroidRuntime(7118):     at android.view.ViewGroup.dispatchDragEvent(ViewGroup.java:1048)
E/AndroidRuntime(7118):     at android.view.ViewRootImpl.handleDragEvent(ViewRootImpl.java:3471)
E/AndroidRuntime(7118):     at android.view.ViewRootImpl.handleMessage(ViewRootImpl.java:2620)
E/AndroidRuntime(7118):     at android.os.Handler.dispatchMessage(Handler.java:99) 
E/AndroidRuntime(7118):     at android.os.Looper.loop(Looper.java:137) 
E/AndroidRuntime(7118):     at android.app.ActivityThread.main(ActivityThread.java:4575)
E/AndroidRuntime(7118):     at java.lang.reflect.Method.invokeNative(Native Method) 
E/AndroidRuntime(7118):     at java.lang.reflect.Method.invoke(Method.java:511)
E/AndroidRuntime(7118):     at com.android.internal.os.ZygoteInit$MethodAndArgsCaller.run(ZygoteInit.java:789)
E/AndroidRuntime(7118):     at com.android.internal.os.ZygoteInit.main(ZygoteInit.java:556)
E/AndroidRuntime(7118):     at dalvik.system.NativeStart.main(NativeMethod)

Why and how to fix it?

just an idea , but have you tried the API demos ? they have a nice drag&drop example there. just play with it and change the visibility exactly when you wish to. since i don't have a device with API11+ , i cannot check if whatever i do helps (the emulator is not enough as it sometimes gives false results) — android developer
– android developer, Commented Jun 15, 2012 at 11:14
about why, this exception occurs when you're inserting or deleting some element of an iterable, when you're iterating on that. but it seems to bee a bug in android. — Kayhan Asghari
– Kayhan Asghari, Commented Jun 15, 2012 at 14:10
try droppedView.post(new Runnable() {public void run() {droppedView.setVisibility(View.VISIBLE);}}); you will need to make droppedView final — Sherif elKhatib
– Sherif elKhatib, Commented Jun 18, 2012 at 12:08
I've written some explanations why the exception happens stackoverflow.com/questions/9899382/… — Roman Mazur
– Roman Mazur, Commented Jul 26, 2012 at 12:04

Indrek Kõue · Accepted Answer · 2015-10-30 11:13:08Z

49

+50

You can try this

if (event.getAction() == DragEvent.ACTION_DRAG_ENDED) {
    final View droppedView = (View) event.getLocalState();
    droppedView.post(new Runnable(){
        @Override
        public void run() {
            droppedView.setVisibility(View.VISIBLE);
        }
    });
}

Looks like Android itself trying to access View state at the same time as you end dragging.

EDIT

More precise explanation. By setting setVisibility(), you're including or excluding View from Android internal collection of views, that should respond to drag events. This collection is used during dispatch of drag events, and therefore by trying to setVisibility (in other words, trying to modify listeners of drag events) you're causing ConcurrentModificationException

edited Oct 30, 2015 at 11:13

Indrek Kõue

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answered Jun 19, 2012 at 17:40

Dmitry Zaytsev

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Comments

Jhoan River · Accepted Answer · 2019-05-15 10:46:12Z

0

The best way is :

    view.post(new Runnable() {
  public void run() {
    view.setVisibility(View.VISIBLE);
  }
});

if we use:

 if (event.getAction() == DragEvent.ACTION_DRAG_ENDED) {
    final View droppedView = (View) event.getLocalState();
    droppedView.post(new Runnable(){
        @Override
        public void run() {
            droppedView.setVisibility(View.VISIBLE);
        }
    });
}

you will still get force close Null.pointer. ...

answered May 15, 2019 at 10:46

Jhoan River

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Comments

Community · Accepted Answer · 2017-05-23 12:26:06Z

-1

Maybe this can help. here in the given link says: instead of DragEvent.ACTION_DRAG_ENDED use DragEvent.ACTION_DROP.

edited May 23, 2017 at 12:26

CommunityBot

11 silver badge

answered Jun 15, 2012 at 14:15

Kayhan Asghari

2,8641 gold badge30 silver badges47 bronze badges

1 Comment

artem Over a year ago

I need to process the cancellation of the drag'n'drop, when the view is not dropped.

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