How to use OpenGL with Timer

Question

The following code works when I load all bitmaps beforehand (with testing data)

for (bitmap in bitmaps){
  feedInOpenGL(bitmap)  
}

But when I try to create the bitmap using a timer,

timer!!.scheduleAtFixedRate(object : TimerTask() {
            override fun run() {
                if (!recording) return
                val bitmap = getNextBitmap()
                feedInOpenGL(bitmap)
            }
        }, 0, frameDuration)

I cannot stop the MediaMuxer anymore. When I try to stop it, it get

D/AndroidRuntime: Shutting down VM
E/AndroidRuntime: FATAL EXCEPTION: main
    Process: com.tolotra.screenrecordexample, PID: 31248
    java.lang.IllegalStateException: Failed to stop the muxer
        at android.media.MediaMuxer.nativeStop(Native Method)
        at android.media.MediaMuxer.stop(MediaMuxer.java:454)
        at com.tolotra.screen_recorder.VideoBuilder._cleanUp(VideoBuilder.kt:292)
        at com.tolotra.screen_recorder.VideoBuilder.finish(VideoBuilder.kt:95)
        at com.tolotra.screen_recorder.ScreenRecorder$stop$1.run(ScreenRecorder.kt:156)
        at android.os.Handler.handleCallback(Handler.java:873)
        at android.os.Handler.dispatchMessage(Handler.java:99)
        at android.os.Looper.loop(Looper.java:193)
        at android.app.ActivityThread.main(ActivityThread.java:6734)
        at java.lang.reflect.Method.invoke(Native Method)
        at com.android.internal.os.RuntimeInit$MethodAndArgsCaller.run(RuntimeInit.java:493)
        at com.android.internal.os.ZygoteInit.main(ZygoteInit.java:858)

After digging, I read somewhere

OpenGL always operates on a context that is made "current" and the context holds data in thread-local storage. This means it's thread-specific. Therefore you always need to be aware from which thread you're calling the OpenGL functions.

So I suspect that the problem is that my timer is the issue because it creates a new thread every time.

If my suspicion is true, how to I make this work with a timer ?

Answer 1

The solution is to Run the OpenGL/MediaMuxer/MediaEncoder setup/start/stop and timer callback on the same thread. This is achieved using single threaded executor

tpe = Executors.newSingleThreadExecutor()
        tpe?.submit(Runnable {
            this.recording = true;
            _startRecorderWorker();
        })

And in the timer

 timer!!.scheduleAtFixedRate(object : TimerTask() {
            override fun run() {
                if (!recording) return;
                tpe?.submit(Runnable {
                    loop()
                })

            }
        }, 0, frameDuration)