ConcurrentModificationException with ViewTreeObserver onPreDrawListener

Question

I have a HashMap that gets created in my method

final HashMap<Long, Rect> listViewItemBounds = new HashMap<Long, Rect>();

that saves all the bounds of visible views in a ListView

for (int i = 0; i < getChildCount(); ++i) {
    View child = getChildAt(i);
    if(child != null){
        int position = firstVisiblePosition+i;
        long itemID = adapter.getItemId(position);
        Rect startRect = new Rect(child.getLeft(), child.getTop(), child.getRight(),child.getBottom());
        listViewItemBounds.put(itemID, startRect);
        listViewItemDrawables.put(itemID, getBitmapDrawableFromView(child));
    }

}

then I start an OnPreDrawListener on the ListView

final ViewTreeObserver observer = getViewTreeObserver();
    observer.addOnPreDrawListener(new ViewTreeObserver.OnPreDrawListener() {
    public boolean onPreDraw() {
        .
        .  
        .
    }
)};

and in the onPreDrawListener I loop through he hashmap to get the bounds of the saved views

for (Long itemId: listViewItemBounds.keySet()) { //points here
    .
    .
    .
    istViewItemBounds.remove(itemId);
}

the problem comes when I change/update the adapter where all this gets called again and onPreDraw is currently looping through the hashmap while it is being updated again due to the adapter change.

So how can I prevent the concurrency here so I dont get ConcurrentModificationException?

Stacktrace:

08-31 19:09:41.398  29127-29127/? E/AndroidRuntime﹕ FATAL EXCEPTION: main
    java.util.ConcurrentModificationException
            at java.util.HashMap$HashIterator.nextEntry(HashMap.java:792)
            at java.util.HashMap$KeyIterator.next(HashMap.java:819)
            at com.tyczj.myapp.views.HeaderGridView$3.onPreDraw(HeaderGridView.java:511)
            at android.view.ViewTreeObserver.dispatchOnPreDraw(ViewTreeObserver.java:680)
            at android.view.ViewRootImpl.performTraversals(ViewRootImpl.java:1842)
            at android.view.ViewRootImpl.doTraversal(ViewRootImpl.java:989)
            at android.view.ViewRootImpl$TraversalRunnable.run(ViewRootImpl.java:4351)
            at android.view.Choreographer$CallbackRecord.run(Choreographer.java:749)
            at android.view.Choreographer.doCallbacks(Choreographer.java:562)
            at android.view.Choreographer.doFrame(Choreographer.java:532)
            at android.view.Choreographer$FrameDisplayEventReceiver.run(Choreographer.java:735)
            at android.os.Handler.handleCallback(Handler.java:725)
            at android.os.Handler.dispatchMessage(Handler.java:92)
            at android.os.Looper.loop(Looper.java:137)
            at android.app.ActivityThread.main(ActivityThread.java:5041)
            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:793)
            at com.android.internal.os.ZygoteInit.main(ZygoteInit.java:560)
            at dalvik.system.NativeStart.main(Native Method)

Edit:

Iterator<Long> it = listViewItemBounds.keySet().iterator();
    while(it.hasNext()){
        long itemId = it.next(); //error
    }

score 2 · Accepted Answer · answered Sep 01 '14 at 00:03

2

You can never remove entries from a hashMap while iterating over its keySet unless you do it using an Iterator and use its remove() method. This is where the ConcurrentModificationException comes from. It's nothing to do with threads.

answered Sep 01 '14 at 00:03

Paul Boddington

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You are correct, i wasn't. Mea culpa for not seeing the remove() call. – SME_Dev Sep 01 '14 at 00:04
No worries. My first attempt at answering the question wasn't exactly clear - I was trying to type code on a mobile with limited success. – Paul Boddington Sep 01 '14 at 00:12
ok see my edit, I changed to use `Iterator` but now I get the error on `it.next()` did I miss something? – tyczj Sep 01 '14 at 00:15
oh sorry I forgot to save it its there, yes same error – tyczj Sep 01 '14 at 00:22
The usual way is to iterate over Map.Entry objects. See this answer. http://stackoverflow.com/q/1884889/3973077 – Paul Boddington Sep 01 '14 at 00:28

ConcurrentModificationException with ViewTreeObserver onPreDrawListener

1 Answers1