Add artist to kotlin version of uamp

uamp-artist-kotlin.diff

diff --git a/app/src/main/java/com/example/android/uamp/MediaItemAdapter.kt b/app/src/main/java/com/example/android/uamp/MediaItemAdapter.kt
index d220374..a855a81 100644
--- a/app/src/main/java/com/example/android/uamp/MediaItemAdapter.kt
+++ b/app/src/main/java/com/example/android/uamp/MediaItemAdapter.kt
@@ -71,11 +71,10 @@ class MediaItemAdapter(private val itemClickedListener: (MediaItemData) -> Unit
             holder.playbackState.setImageResource(mediaItem.playbackRes)
 
             Glide.with(holder.albumArt)
-                    .load(mediaItem.albumArtUri)
-                    .into(holder.albumArt)
+                .load(mediaItem.albumArtUri ?: R.drawable.default_art)
+                .into(holder.albumArt)
         }
     }
-
     override fun onBindViewHolder(holder: MediaViewHolder, position: Int) {
         onBindViewHolder(holder, position, mutableListOf())
     }
diff --git a/app/src/main/java/com/example/android/uamp/MediaItemData.kt b/app/src/main/java/com/example/android/uamp/MediaItemData.kt
index a058d9e..83a3000 100644
--- a/app/src/main/java/com/example/android/uamp/MediaItemData.kt
+++ b/app/src/main/java/com/example/android/uamp/MediaItemData.kt
@@ -34,8 +34,8 @@ import com.example.android.uamp.viewmodels.MediaItemFragmentViewModel
 data class MediaItemData(
         val mediaId: String,
         val title: String,
-        val subtitle: String,
-        val albumArtUri: Uri,
+        val subtitle: String?,
+        val albumArtUri: Uri?,
         val browsable: Boolean,
         var playbackRes: Int) {
 
diff --git a/app/src/main/java/com/example/android/uamp/viewmodels/MediaItemFragmentViewModel.kt b/app/src/main/java/com/example/android/uamp/viewmodels/MediaItemFragmentViewModel.kt
index 7088bb8..6b94cc0 100644
--- a/app/src/main/java/com/example/android/uamp/viewmodels/MediaItemFragmentViewModel.kt
+++ b/app/src/main/java/com/example/android/uamp/viewmodels/MediaItemFragmentViewModel.kt
@@ -58,8 +58,8 @@ class MediaItemFragmentViewModel(private val mediaId: String,
             val itemsList = children.map { child ->
                 MediaItemData(child.mediaId!!,
                         child.description.title.toString(),
-                        child.description.subtitle.toString(),
-                        child.description.iconUri!!,
+                        child.description.subtitle?.toString() ?: "",
+                        child.description.iconUri,
                         child.isBrowsable,
                         getResourceForMediaId(child.mediaId!!))
             }
diff --git a/media/src/main/java/com/example/android/uamp/media/library/BrowseTree.kt b/media/src/main/java/com/example/android/uamp/media/library/BrowseTree.kt
index c389450..65add39 100644
--- a/media/src/main/java/com/example/android/uamp/media/library/BrowseTree.kt
+++ b/media/src/main/java/com/example/android/uamp/media/library/BrowseTree.kt
@@ -63,9 +63,31 @@ class BrowseTree(musicSource: MusicSource) {
      */
     init {
         musicSource.forEach { mediaItem ->
-            val albumMediaId = mediaItem.album.urlEncoded
-            val albumChildren = mediaIdToChildren[albumMediaId] ?: buildAlbumRoot(mediaItem)
+            val albumMediaId = "ALBUM:${mediaItem.album.urlEncoded}"
+            val artistMediaId = "ARTIST:${mediaItem.artist.urlEncoded}"
+            val albumChildren = mediaIdToChildren[albumMediaId] ?: buildSongContainer(mediaItem) { builder ->
+                builder.apply {
+                    id = albumMediaId
+                    title = mediaItem.album
+                    artist = mediaItem.artist
+                    albumArt = mediaItem.albumArt
+                    albumArtUri = mediaItem.albumArtUri?.toString()
+                    flag = MediaItem.FLAG_BROWSABLE
+                }
+                builder.build()
+            }
+            val artistChildren = mediaIdToChildren[artistMediaId] ?: buildSongContainer(mediaItem) { builder ->
+                builder.apply {
+                    id = artistMediaId
+                    title = mediaItem.artist
+                    flag = MediaItem.FLAG_BROWSABLE
+                    //TODO: fill in artist-specific fields for artwork etc.
+                }
+                builder.build()
+            }
+
             albumChildren += mediaItem
+            artistChildren += mediaItem
         }
     }
 
@@ -81,15 +103,8 @@ class BrowseTree(musicSource: MusicSource) {
      * marking the item as [MediaItem.FLAG_BROWSABLE], since it will have child
      * node(s) AKA at least 1 song.
      */
-    private fun buildAlbumRoot(mediaItem: MediaMetadataCompat): MutableList<MediaMetadataCompat> {
-        val albumMetadata = MediaMetadataCompat.Builder().apply {
-            id = mediaItem.album.urlEncoded
-            title = mediaItem.album
-            artist = mediaItem.artist
-            albumArt = mediaItem.albumArt
-            albumArtUri = mediaItem.albumArtUri?.toString()
-            flag = MediaItem.FLAG_BROWSABLE
-        }.build()
+    private fun buildSongContainer(mediaItem: MediaMetadataCompat, buildAction: (MediaMetadataCompat.Builder) -> MediaMetadataCompat): MutableList<MediaMetadataCompat> {
+        val albumMetadata = buildAction.invoke(MediaMetadataCompat.Builder())
 
         // Ensure the root node exists and add this album to the list.
         val rootList = mediaIdToChildren[UAMP_BROWSABLE_ROOT] ?: mutableListOf()