Support for visibility tracking in model groups

CHANGELOG.md

@@ -1,3 +1,6 @@
+#
+- Add support for `EpoxyModelGroup` in the `EpoxyVisibilityTracker` (#1091)
+
 # 4.2.0 (Nov 11, 2020)
 - Add notify model changed method (#1063)
 - Update to Kotlin 1.4.20-RC and remove dependency on kotlin-android-extensions

epoxy-adapter/src/main/java/com/airbnb/epoxy/EpoxyVisibilityTracker.kt

@@ -17,6 +17,11 @@ import java.util.HashMap
  * backed by an Epoxy controller. Once attached the events will be forwarded to the Epoxy model (or
  * to the Epoxy view when using annotations).
  *
+ * Note that support for visibility events on an [EpoxyModelGroup] is somewhat limited. Only model
+ * additions will receive visibility events. Models that are removed from the group will not receive
+ * events (e.g. [VisibilityState.INVISIBLE]) because the model group does not keep a reference,
+ * nor does it get notified of model removals.
+ *
  * @see OnVisibilityChanged
  *
  * @see OnVisibilityStateChanged
@@ -208,20 +213,77 @@ class EpoxyVisibilityTracker {
         // Preemptive check for child's parent validity to prevent `IllegalArgumentException` in
         // `getChildViewHolder`.
         val isParentValid = child.parent == null || child.parent === recyclerView
-        val holder = if (isParentValid) recyclerView.getChildViewHolder(child) else null
-        if (holder is EpoxyViewHolder) {
-            val changed = processVisibilityEvents(
+        val viewHolder = if (isParentValid) recyclerView.getChildViewHolder(child) else null
+        if (viewHolder is EpoxyViewHolder) {
+            val epoxyHolder = viewHolder.holder
+            processChild(recyclerView, child, detachEvent, eventOriginForDebug, viewHolder)
+            if (epoxyHolder is ModelGroupHolder) {
+                processModelGroupChildren(recyclerView, epoxyHolder, detachEvent, eventOriginForDebug)
+            }
+        }
+    }
+
+    /**
+     * Loop through the children of the model group and process visibility events on each one in
+     * relation to the model group's layout. This will attach or detach trackers to any nested
+     * [RecyclerView]s.
+     *
+     * @param epoxyHolder         the [ModelGroupHolder] with children to process
+     * @param detachEvent         true if the child was just detached
+     * @param eventOriginForDebug a debug strings used for logs
+     */
+    private fun processModelGroupChildren(
+        recyclerView: RecyclerView,
+        epoxyHolder: ModelGroupHolder,
+        detachEvent: Boolean,
+        eventOriginForDebug: String
+    ) {
+        // Iterate through models in the group and process each of them instead of the group
+        for (groupChildHolder in epoxyHolder.viewHolders) {
+            // Since the group is likely using a ViewGroup other than a RecyclerView, handle the
+            // potential of a nested RecyclerView. This cannot be done through the normal flow
+            // without recursively searching through the view children.
+            if (groupChildHolder.itemView is RecyclerView) {
+                if (detachEvent) {
+                    processChildRecyclerViewDetached(groupChildHolder.itemView)
+                } else {
+                    processChildRecyclerViewAttached(groupChildHolder.itemView)
+                }
+            }
+            processChild(
                 recyclerView,
-                holder,
+                groupChildHolder.itemView,
                 detachEvent,
-                eventOriginForDebug
+                eventOriginForDebug,
+                groupChildHolder
             )
-            if (changed) {
-                if (child is RecyclerView) {
-                    val tracker = nestedTrackers[child]
-                    tracker?.processChangeEvent("parent")
-                }
-            }
+        }
+    }
+
+    /**
+     * Process visibility events for a view and propagate to a nested tracker if the view is a
+     * [RecyclerView].
+     *
+     * @param child               the view to process for visibility event
+     * @param detachEvent         true if the child was just detached
+     * @param eventOriginForDebug a debug strings used for logs
+     * @param viewHolder          the view holder for the child view
+     */
+    private fun processChild(
+        recyclerView: RecyclerView,
+        child: View,
+        detachEvent: Boolean,
+        eventOriginForDebug: String,
+        viewHolder: EpoxyViewHolder
+    ) {
+        val changed = processVisibilityEvents(
+            recyclerView,
+            viewHolder,
+            detachEvent,
+            eventOriginForDebug
+        )
+        if (changed && child is RecyclerView) {
+            nestedTrackers[child]?.processChangeEvent("parent")
         }
     }
 

epoxy-integrationtest/src/main/res/layout/vertical_linear_group.xml

@@ -0,0 +1,5 @@
+<?xml version="1.0" encoding="utf-8"?>
+<LinearLayout xmlns:android="http://schemas.android.com/apk/res/android"
+    android:layout_width="match_parent"
+    android:layout_height="wrap_content"
+    android:orientation="vertical" />