Clears RenderTree from MountState on unmount all

Reviewed By: pasqualeanatriello

Differential Revision: D46150681

fbshipit-source-id: b187c1057d6c4d9da6d896eff5736f068455bb75

litho-rendercore/src/main/java/com/facebook/rendercore/MountState.java

@@ -30,6 +30,7 @@
 import android.view.View;
 import androidx.annotation.Nullable;
 import androidx.collection.LongSparseArray;
+import androidx.core.util.Preconditions;
 import com.facebook.rendercore.debug.DebugEvent;
 import com.facebook.rendercore.extensions.ExtensionState;
 import com.facebook.rendercore.extensions.MountExtension;
@@ -53,7 +54,7 @@ public class MountState implements MountDelegateTarget {
 
   private boolean mIsMounting;
   private boolean mNeedsRemount;
-  private RenderTree mRenderTree;
+  private @Nullable RenderTree mRenderTree;
   private @Nullable MountDelegate mMountDelegate;
   private @Nullable UnmountDelegateExtension mUnmountDelegateExtension;
 
@@ -145,6 +146,8 @@ public void mount(RenderTree renderTree) {
         return;
       }
 
+      Preconditions.checkNotNull(mRenderTree);
+
       final boolean isTracing = mTracer.isTracing();
       if (isTracing) {
         mTracer.beginSection("MountState.mount");
@@ -356,6 +359,11 @@ public void unmountAllItems() {
       }
 
       mNeedsRemount = true;
+
+      if (RenderCoreConfig.shouldClearRenderTreeOnUnmountAll) {
+        mRenderTree = null;
+      }
+
     } finally {
       if (RenderCoreConfig.shouldSetInLayoutDuringUnmountAll) {
         mRootHost.unsetInLayout();

litho-rendercore/src/main/java/com/facebook/rendercore/RenderCoreConfig.java

@@ -33,4 +33,7 @@ public class RenderCoreConfig {
    * so that layouts are not request when removing content from the hierarchy.
    */
   public static boolean shouldSetInLayoutDuringUnmountAll = false;
+
+  /** Enabling this will clear the render tree from the MountState when unmount all is invoked. */
+  public static boolean shouldClearRenderTreeOnUnmountAll = false;
 }