Fabric: Introducing ShadowTree::CommitMode

Summary:
CommitMode allows customizing the side-effects of commit operations. In `Suspended` mode, the results of commit operations will not be passed down to a MountingCoordinator which will result in skipping the mounting phase completely.
This is one of the core elements of the Pre-rendering infrastructure.

Changelog: [Internal] Fabric-specific internal change.

Differential Revision: D24290773

fbshipit-source-id: c10ec20d13f3131fc632352ef22f4465c9dfb3c2

ReactCommon/react/renderer/mounting/ShadowTree.cpp

@@ -22,6 +22,7 @@ namespace facebook {
 namespace react {
 
 using CommitStatus = ShadowTree::CommitStatus;
+using CommitMode = ShadowTree::CommitMode;
 
 /*
  * Generates (possibly) a new tree where all nodes with non-obsolete `State`
@@ -257,6 +258,29 @@ Tag ShadowTree::getSurfaceId() const {
   return surfaceId_;
 }
 
+void ShadowTree::setCommitMode(CommitMode commitMode) const {
+  auto revision = ShadowTreeRevision{};
+
+  {
+    std::unique_lock<better::shared_mutex> lock(commitMutex_);
+    if (commitMode_ == commitMode) {
+      return;
+    }
+
+    commitMode_ = commitMode;
+    revision = currentRevision_;
+  }
+
+  if (commitMode == CommitMode::Normal) {
+    mount(revision);
+  }
+}
+
+CommitMode ShadowTree::getCommitMode() const {
+  std::shared_lock<better::shared_mutex> lock(commitMutex_);
+  return commitMode_;
+}
+
 MountingCoordinator::Shared ShadowTree::getMountingCoordinator() const {
   return mountingCoordinator_;
 }
@@ -290,12 +314,14 @@ CommitStatus ShadowTree::tryCommit(
   auto telemetry = TransactionTelemetry{};
   telemetry.willCommit();
 
+  CommitMode commitMode;
   auto oldRevision = ShadowTreeRevision{};
   auto newRevision = ShadowTreeRevision{};
 
   {
     // Reading `currentRevision_` in shared manner.
     std::shared_lock<better::shared_mutex> lock(commitMutex_);
+    commitMode = commitMode_;
     oldRevision = currentRevision_;
   }
 
@@ -369,9 +395,9 @@ CommitStatus ShadowTree::tryCommit(
 
   emitLayoutEvents(affectedLayoutableNodes);
 
-  mountingCoordinator_->push(newRevision);
-
-  notifyDelegatesOfUpdates();
+  if (commitMode == CommitMode::Normal) {
+    mount(newRevision);
+  }
 
   return CommitStatus::Succeeded;
 }
@@ -381,6 +407,11 @@ ShadowTreeRevision ShadowTree::getCurrentRevision() const {
   return currentRevision_;
 }
 
+void ShadowTree::mount(ShadowTreeRevision const &revision) const {
+  mountingCoordinator_->push(revision);
+  delegate_.shadowTreeDidFinishTransaction(*this, mountingCoordinator_);
+}
+
 void ShadowTree::commitEmptyTree() const {
   commit(
       [](RootShadowNode const &oldRootShadowNode) -> RootShadowNode::Unshared {

ReactCommon/react/renderer/mounting/ShadowTree.h

@@ -31,12 +31,28 @@ using ShadowTreeCommitTransaction = std::function<RootShadowNode::Unshared(
  */
 class ShadowTree final {
  public:
+  /*
+   * Represents a result of a `commit` operation.
+   */
   enum class CommitStatus {
     Succeeded,
     Failed,
     Cancelled,
   };
 
+  /*
+   * Represents commits' side-effects propagation mode.
+   */
+  enum class CommitMode {
+    // Commits' side-effects are observable via `MountingCoordinator`.
+    // The rendering pipeline fully works end-to-end.
+    Normal,
+
+    // Commits' side-effects are *not* observable via `MountingCoordinator`.
+    // The mounting phase is skipped in the rendering pipeline.
+    Suspended,
+  };
+
   struct CommitOptions {
     bool enableStateReconciliation{false};
     // Lambda called inside `tryCommit`. If false is returned, commit is
@@ -62,6 +78,14 @@ class ShadowTree final {
    */
   SurfaceId getSurfaceId() const;
 
+  /*
+   * Sets and gets the commit mode.
+   * Changing commit mode from `Suspended` to `Normal` will flush all suspended
+   * changes to `MountingCoordinator`.
+   */
+  void setCommitMode(CommitMode commitMode) const;
+  CommitMode getCommitMode() const;
+
   /*
    * Performs commit calling `transaction` function with a `oldRootShadowNode`
    * and expecting a `newRootShadowNode` as a return value.
@@ -99,12 +123,16 @@ class ShadowTree final {
   MountingCoordinator::Shared getMountingCoordinator() const;
 
  private:
+  void mount(ShadowTreeRevision const &revision) const;
+
   void emitLayoutEvents(
       std::vector<LayoutableShadowNode const *> &affectedLayoutableNodes) const;
 
   SurfaceId const surfaceId_;
   ShadowTreeDelegate const &delegate_;
   mutable better::shared_mutex commitMutex_;
+  mutable CommitMode commitMode_{
+      CommitMode::Normal}; // Protected by `commitMutex_`.
   mutable ShadowTreeRevision currentRevision_; // Protected by `commitMutex_`.
   MountingCoordinator::Shared mountingCoordinator_;
 };