uamp Kotlin Version

MainActivity.kt

class MainActivity : AppCompatActivity() {
    private lateinit var viewModel: MainActivityViewModel

    override fun onCreate(savedInstanceState: Bundle?) {
        super.onCreate(savedInstanceState)
        setContentView(R.layout.activity_main)

        // Since UAMP is a music player, the volume controls should adjust the music volume while
        // in the app.
        volumeControlStream = AudioManager.STREAM_MUSIC

        viewModel = ViewModelProviders
                .of(this, InjectorUtils.provideMainActivityViewModel(this))
                .get(MainActivityViewModel::class.java)

        /**
         * Observe changes to the [MainActivityViewModel.rootMediaId]. When the app starts,
         * and the UI connects to [MusicService], this will be updated and the app will show
         * the initial list of media items.
         */
        viewModel.rootMediaId.observe(this,
                Observer<String> { rootMediaId ->
                    if (rootMediaId != null) {
                        navigateToMediaItem(rootMediaId)
                    }
                })

        /**
         * Observe [MainActivityViewModel.navigateToMediaItem] for [Event]s indicating
         * the user has requested to browse to a different [MediaItemData].
         */
        viewModel.navigateToMediaItem.observe(this, Observer {
            it?.getContentIfNotHandled()?.let { mediaId ->
                navigateToMediaItem(mediaId)
            }
        })
    }

    private fun navigateToMediaItem(mediaId: String) {
        var fragment: MediaItemFragment? = getBrowseFragment(mediaId)

        if (fragment == null) {
            fragment = MediaItemFragment.newInstance(mediaId)

            supportFragmentManager.beginTransaction()
                    .apply {
                        replace(R.id.browseFragment, fragment, mediaId)

                        // If this is not the top level media (root), we add it to the fragment
                        // back stack, so that actionbar toggle and Back will work appropriately:
                        if (!isRootId(mediaId)) {
                            addToBackStack(null)
                        }
                    }
                    .commit()
        }
    }

    private fun isRootId(mediaId: String) = mediaId == viewModel.rootMediaId.value

    private fun getBrowseFragment(mediaId: String): MediaItemFragment? {
        return supportFragmentManager.findFragmentByTag(mediaId) as MediaItemFragment?
    }
}