Fabric: MountingCoordinator::waitForTransaction(...)

Summary:
`MountingCoordinator::waitForTransaction()` allows waiting for a new coming transaction synchronously, as efficient as waiting for a mutex.
This feature can be used to implement more high-level synchronous rendering APIs.

Reviewed By: mdvacca

Differential Revision: D17629425

fbshipit-source-id: acd91b941e4d6d43bc4518f332a1604e14506be9

ReactCommon/fabric/mounting/MountingCoordinator.cpp

@@ -11,6 +11,8 @@
 #include <glog/logging.h>
 #endif
 
+#include <condition_variable>
+
 #include <react/mounting/Differentiator.h>
 #include <react/mounting/ShadowViewMutation.h>
 #include <react/utils/TimeUtils.h>
@@ -31,24 +33,35 @@ SurfaceId MountingCoordinator::getSurfaceId() const {
 }
 
 void MountingCoordinator::push(ShadowTreeRevision &&revision) const {
- std::lock_guard<std::mutex> lock(mutex_);
-
- assert(revision.getNumber() > baseRevision_.getNumber());
- assert(
- !lastRevision_.has_value() ||
- revision.getNumber() != lastRevision_->getNumber());
-
- if (!lastRevision_.has_value() ||
- lastRevision_->getNumber() < revision.getNumber()) {
- lastRevision_ = std::move(revision);
+ {
+ std::lock_guard<std::mutex> lock(mutex_);
+
+ assert(revision.getNumber() > baseRevision_.getNumber());
+ assert(
+ !lastRevision_.has_value() ||
+ revision.getNumber() != lastRevision_->getNumber());
+
+ if (!lastRevision_.has_value() ||
+ lastRevision_->getNumber() < revision.getNumber()) {
+ lastRevision_ = std::move(revision);
+ }
  }
+
+ signal_.notify_all();
 }
 
 void MountingCoordinator::revoke() const {
  std::lock_guard<std::mutex> lock(mutex_);
  lastRevision_.reset();
 }
 
+bool MountingCoordinator::waitForTransaction(
+ std::chrono::duration<double> timeout) const {
+ std::unique_lock<std::mutex> lock(mutex_);
+ return signal_.wait_for(
+ lock, timeout, [this]() { return lastRevision_.has_value(); });
+}
+
 better::optional<MountingTransaction> MountingCoordinator::pullTransaction()
  const {
  std::lock_guard<std::mutex> lock(mutex_);

ReactCommon/fabric/mounting/MountingCoordinator.h

@@ -8,6 +8,7 @@
 #pragma once
 
 #include <better/optional.h>
+#include <chrono>
 
 #include <react/mounting/MountingTransaction.h>
 #include <react/mounting/ShadowTreeRevision.h>
@@ -48,10 +49,21 @@ class MountingCoordinator final {
  * mount.
  * The method is thread-safe and can be called from any thread.
  * However, a consumer should always call it on the same thread (e.g. on the
- * main thread) or ensure sequentiality of mount transaction separately.
+ * main thread) or ensure sequentiality of mount transactions separately.
  */
  better::optional<MountingTransaction> pullTransaction() const;
 
+ /*
+ * Blocks the current thread until a new mounting transaction is available or
+ * after the specified `timeout` duration.
+ * Returns `false` if a timeout occurred before a new transaction available.
+ * Call `pullTransaction` right after the method to retrieve the transaction.
+ * Similarly to `pullTransaction` this method is thread-safe but the consumer
+ * should call it on the same thread (e.g. on the main thread) or ensure
+ * sequentiality of mount transactions separately.
+ */
+ bool waitForTransaction(std::chrono::duration<double> timeout) const;
+
  private:
  friend class ShadowTree;
 
@@ -76,6 +88,7 @@ class MountingCoordinator final {
  mutable ShadowTreeRevision baseRevision_;
  mutable better::optional<ShadowTreeRevision> lastRevision_{};
  mutable MountingTransaction::Number number_{0};
+ mutable std::condition_variable signal_;
 
 #ifdef RN_SHADOW_TREE_INTROSPECTION
  mutable StubViewTree stubViewTree_; // Protected by `mutex_`.