Fabric: Clarifying life-time concerns of ShadowTree::delegate_

Summary:
This diff makes it clear from the code that ShadowTree delegate must be around for the whole life-time of a ShadowTree instance and there is no way to revoke/reset the delegate. This makes reasoning about the lifetime much simpler.

We didn't call `setDelegate(nullptr)` before, so nothing really changes here.

Changelog: [Internal] Fabric-specific internal change.

Reviewed By: mdvacca

Differential Revision: D18542502

fbshipit-source-id: f57ee21e0bb533fb82cb6f8ba7723e40ffb25a38

ReactCommon/fabric/mounting/ShadowTree.cpp

@@ -91,10 +91,11 @@ static void updateMountedFlag(
 
 ShadowTree::ShadowTree(
     SurfaceId surfaceId,
-    const LayoutConstraints &layoutConstraints,
-    const LayoutContext &layoutContext,
-    const RootComponentDescriptor &rootComponentDescriptor)
-    : surfaceId_(surfaceId) {
+    LayoutConstraints const &layoutConstraints,
+    LayoutContext const &layoutContext,
+    RootComponentDescriptor const &rootComponentDescriptor,
+    ShadowTreeDelegate const &delegate)
+    : surfaceId_(surfaceId), delegate_(delegate) {
   const auto noopEventEmitter = std::make_shared<const ViewEventEmitter>(
       nullptr, -1, std::shared_ptr<const EventDispatcher>());
 
@@ -197,9 +198,7 @@ bool ShadowTree::tryCommit(ShadowTreeCommitTransaction transaction) const {
   mountingCoordinator_->push(
       ShadowTreeRevision{newRootShadowNode, revisionNumber, telemetry});
 
-  if (delegate_) {
-    delegate_->shadowTreeDidFinishTransaction(*this, mountingCoordinator_);
-  }
+  delegate_.shadowTreeDidFinishTransaction(*this, mountingCoordinator_);
 
   return true;
 }
@@ -244,15 +243,5 @@ void ShadowTree::emitLayoutEvents(
   }
 }
 
-#pragma mark - Delegate
-
-void ShadowTree::setDelegate(ShadowTreeDelegate const *delegate) {
-  delegate_ = delegate;
-}
-
-ShadowTreeDelegate const *ShadowTree::getDelegate() const {
-  return delegate_;
-}
-
 } // namespace react
 } // namespace facebook

ReactCommon/fabric/mounting/ShadowTree.h

@@ -38,7 +38,8 @@ class ShadowTree final {
       SurfaceId surfaceId,
       LayoutConstraints const &layoutConstraints,
       LayoutContext const &layoutContext,
-      RootComponentDescriptor const &rootComponentDescriptor);
+      RootComponentDescriptor const &rootComponentDescriptor,
+      ShadowTreeDelegate const &delegate);
 
   ~ShadowTree();
 
@@ -65,16 +66,6 @@ class ShadowTree final {
    */
   void commitEmptyTree() const;
 
-#pragma mark - Delegate
-
-  /*
-   * Sets and gets the delegate.
-   * The delegate is stored as a raw pointer, so the owner must null
-   * the pointer before being destroyed.
-   */
-  void setDelegate(ShadowTreeDelegate const *delegate);
-  ShadowTreeDelegate const *getDelegate() const;
-
  private:
   RootShadowNode::Unshared cloneRootShadowNode(
       RootShadowNode::Shared const &oldRootShadowNode,
@@ -85,12 +76,12 @@ class ShadowTree final {
       std::vector<LayoutableShadowNode const *> &affectedLayoutableNodes) const;
 
   SurfaceId const surfaceId_;
+  ShadowTreeDelegate const &delegate_;
   mutable better::shared_mutex commitMutex_;
   mutable RootShadowNode::Shared
       rootShadowNode_; // Protected by `commitMutex_`.
   mutable ShadowTreeRevision::Number revisionNumber_{
       0}; // Protected by `commitMutex_`.
-  ShadowTreeDelegate const *delegate_;
   MountingCoordinator::Shared mountingCoordinator_;
 };
 

ReactCommon/fabric/uimanager/Scheduler.cpp

@@ -134,8 +134,11 @@ void Scheduler::startSurface(
   SystraceSection s("Scheduler::startSurface");
 
   auto shadowTree = std::make_unique<ShadowTree>(
-      surfaceId, layoutConstraints, layoutContext, *rootComponentDescriptor_);
-  shadowTree->setDelegate(uiManager_.get());
+      surfaceId,
+      layoutConstraints,
+      layoutContext,
+      *rootComponentDescriptor_,
+      *uiManager_);
 
   auto uiManager = uiManager_;