News & Views

(Originally posted on the Code-and-Cocktails blog)

Recently I learned some Android programming by writing a simple app for a client. It was a great opportunity to learn the platform and how “easy” it is to write an app. I ran into one ‘gotcha’ that I thought might be valuable to others.

One feature that was needed was a swipeable carousel of YouTube videos. Google provides some widgets for showing YouTube videos on an Android device and YouTubePlayerFragment was a (almost) perfect fit for my needs1. Also ViewPager was just the thing for creating the swipeable list of items. It was easy enough to create a subclass of FragmentPageAdapter which knew the list of videos and created YouTubePlayerFragments as needed (actually a subclass whose job was to handle the initialization of the YouTubePlayerFragment).

While this was easy to code – it was not so easy to make it actually work.

Trying to play videos resulted in a cryptic message about the player not playing because it was not visible on the screen. The coordinates in the error message made it seem like the object was way to the left of the visible screen. That was the first clue. It was perplexing though since the player was quite obviously right there on the screen.

Some debugging gave me the second clue I needed. When I pressed play on the player on the screen, multiple players were firing events saying that they were playing. Multiple players?

Reading2 into the documentation of ViewPager some more told me that it will request multiple views from the ViewPageAdapter, so that other views are “ready to go”. But why did they all respond when I clicked on one of them?

More debugging did not solve the mystery but solidified my hypothesis: The YouTubePlayer and/or YouTubePlayerFragment has state shared between all their instances. That is the only explanation that would fit the observed behavior.

So I needed a way to ensure that only one YouTubePlayer was in play at a time. The ViewPager documentation says you can change the number of other pages that will be created. Changing that did not work for me – at least one other view was always created. That left me with ensuring that only one player was initialized.

I tried various event listeners but found that none of them fit the need. Sometimes I would get an event firing both on the active and the inactive viewer and it was not possible to tell the difference. Finally I found one thing that did seem consistent and usable: setUserVisibleHint. It was called on the fragment with a true value when that fragment was the one shown to the user and was called with false when it was not. So I made sure my fragment was not initialized until it got told that it was visible; and then released it when it was no longer visible.

1. Except for the supremely annoying fact that the YouTube player widgets DO NOT WORK on emulators. So I had to do all this work with a physical device tethered to my machine. Like a savage.

2. Reading is Fundamental