recycler: make fast scroller respond to changes

Make the fast scroller respond to changes in the RecyclerView data.
This again prepares it for use in the library with it's sorting and
item configuration.

app/src/main/java/org/oxycblt/auxio/songs/FastScrollView.kt

@@ -49,6 +49,31 @@ class FastScrollView @JvmOverloads constructor(
 
     private var mRecycler: RecyclerView? = null
     private var mGetItem: ((Int) -> Char)? = null
+    private val mObserver = object : RecyclerView.AdapterDataObserver() {
+        override fun onChanged() = postIndicatorUpdate()
+
+        override fun onItemRangeChanged(
+            positionStart: Int,
+            itemCount: Int,
+            payload: Any?
+        ) = postIndicatorUpdate()
+
+        override fun onItemRangeInserted(
+            positionStart: Int,
+            itemCount: Int
+        ) = onChanged()
+
+        override fun onItemRangeMoved(
+            fromPosition: Int,
+            toPosition: Int,
+            itemCount: Int
+        ) = onChanged()
+
+        override fun onItemRangeRemoved(
+            positionStart: Int,
+            itemCount: Int
+        ) = onChanged()
+    }
 
     // --- INDICATORS ---
 
@@ -93,6 +118,8 @@ class FastScrollView @JvmOverloads constructor(
         mRecycler = recycler
         mGetItem = getItem
 
+        recycler.adapter?.registerAdapterDataObserver(mObserver)
+
         postIndicatorUpdate()
     }