android - Nested Fragments within ViewPager have zero height and width -

i have activity single fragment inside of it. inside fragment have viewpager pages fragments. makes paged fragments children of fragment inside activity. this:

    activity        |        |     fragment        |    (viewpager)     |       |     |       | fragment  fragment 

every often, seemingly @ random, fragments inside viewpager not display. looked @ view hierarchy in hierarchy viewer , noticed when happens child fragments (at bottom of picture) are in layout, root view of children has zero height , 0 width (as every view in hierarchy tree there down leaf views). viewpager have height , width, fragments pages not.

if rotate device @ point, child fragments show up!

i'm @ loss why happening. adapter i'm using viewpager requires fragmentmanager, , i'm using 1 fragment inside activity calling getchildfragmentmanager(). feel i'm doing i'm supposed do, not sure next on one.

update:

interestingly enough, if put code attaches adapter viewpager in runnable , post handler issue 100% reproducible. i.e.

class myfragment extends fragment {    @override    public void onactivitycreated(bundle b) {     super.onactivitycreated(b);      runpager = new runnable() {        @override        public void run() {         viewpager.setadapter(adapter);       }      };      handler.post(runpager);   } } 

i figured out. if else runs this, make sure aren't manipulating ui components on background thread or weird things happen. in case doing bitmap manipulation on background thread rxjava, forgot line:

.observeon(androidschedulers.mainthread()); 

so when put bitmap in view in subscriber, on background thread. easy mistake make.