Do not list artists without albums.

As of now, Otter cannot list albumless tracks (for example, tracks in
compilation, appearing under the original artist, but not part of an
album published by this artist). This created a lot of "empty" artists
(with no albums) in the Artists section of the app.

This may be rolled back if we are some day able to list "orphan"
tracks.

app/src/main/java/com/github/apognu/otter/adapters/ArtistsAdapter.kt

@@ -15,13 +15,31 @@ import jp.wasabeef.picasso.transformations.RoundedCornersTransformation
 import kotlinx.android.synthetic.main.row_artist.view.*
 
 class ArtistsAdapter(val context: Context?, private val listener: OnArtistClickListener) : FunkwhaleAdapter<Artist, ArtistsAdapter.ViewHolder>() {
+  private var active: List<Artist> = mutableListOf()
+
   interface OnArtistClickListener {
     fun onClick(holder: View?, artist: Artist)
   }
 
-  override fun getItemCount() = data.size
+  init {
+    registerAdapterDataObserver(object : RecyclerView.AdapterDataObserver() {
+      override fun onChanged() {
+        active = data.filter { it.albums?.isNotEmpty() ?: false }
+
+        super.onChanged()
+      }
+
+      override fun onItemRangeInserted(positionStart: Int, itemCount: Int) {
+        active = data.filter { it.albums?.isNotEmpty() ?: false }
+
+        super.onItemRangeInserted(positionStart, itemCount)
+      }
+    })
+  }
+
+  override fun getItemCount() = active.size
 
-  override fun getItemId(position: Int) = data[position].id.toLong()
+  override fun getItemId(position: Int) = active[position].id.toLong()
 
   override fun onCreateViewHolder(parent: ViewGroup, viewType: Int): ViewHolder {
     val view = LayoutInflater.from(context).inflate(R.layout.row_artist, parent, false)
@@ -32,7 +50,7 @@ class ArtistsAdapter(val context: Context?, private val listener: OnArtistClickL
   }
 
   override fun onBindViewHolder(holder: ViewHolder, position: Int) {
-    val artist = data[position]
+    val artist = active[position]
 
     artist.albums?.let { albums ->
       if (albums.isNotEmpty()) {
@@ -59,7 +77,7 @@ class ArtistsAdapter(val context: Context?, private val listener: OnArtistClickL
     val albums = view.albums
 
     override fun onClick(view: View?) {
-      listener.onClick(view, data[layoutPosition])
+      listener.onClick(view, active[layoutPosition])
     }
   }
 }