EventListener isn't called in correct thread's looper

[Khang-NT]

Issue description

The document of method Player#addListener said:

Register a listener to receive events from the player. The listener's methods will be called on the thread that was used to construct the player. However, if the thread used to construct the player does not have a Looper, then the listener will be called on the main thread.

I did construct new SimpleExoPlayer instance within my own Looper (which is not Looper.mainLooper()). But when I call prepare() media source on main thread, ExoPlayer callback to all listeners within main thread instead of using my looper to dispatch the event.

I can see the method ExoPlayerImpl#updatePlaybackInfo invoke listener's methods within caller thread, regardless the thread was used to construct the player.

Reproduction steps

• Create HandlerThread and then create ExoPlayer object within HandlerThread#onLooperPrepared
• Add a listener: Player#addListener
• Switch to main thread and call Player#prepare

Link to test content

Can test with demo app.

Version of ExoPlayer being used

2.8.0

[tonihei]

That's more or less working as intended. ExoPlayer also requires you to call all methods on the thread the player was created on (or the main thread if it doesn't have a looper). See also the points under "threading model" here: http://google.github.io/ExoPlayer/doc/reference/com/google/android/exoplayer2/ExoPlayer.html.

The documentation is not as clear as it should be, and based on the discussion in #4278, we also just changed it to spell out the requirements more explicitly.

[ojw28]

Threading requirements were clarified in 2813c06. We'll also update the developer guide.

In addition, we're planning a change that will allow you to specify a Looper when instantiating the player, which will add a little more flexibility. Follow #4278 for updates.