Implement RuntimeScheduler::cancelTask

Summary:
Changelog: [internal]

Implement RuntimeScheduler::cancelTask and expose it through JSI.

JavaScript implementation:
https://github.com/facebook/react/blob/master/packages/scheduler/src/forks/SchedulerNoDOM.js#L384-L397

Reviewed By: mdvacca

Differential Revision: D27648071

fbshipit-source-id: 91ae92b336017596fb658831824e971c7e047cd2

Author: sammy-SC

ReactCommon/react/renderer/runtimescheduler/BUCK

@@ -50,5 +50,6 @@ rn_xplat_cxx_library(
     deps = [
         react_native_xplat_target("runtimeexecutor:runtimeexecutor"),
         react_native_xplat_target("react/renderer/debug:debug"),
+        react_native_xplat_target("better:better"),
     ],
 )

ReactCommon/react/renderer/runtimescheduler/RuntimeScheduler.cpp

@@ -22,4 +22,8 @@ void RuntimeScheduler::scheduleTask(std::shared_ptr<Task> const &task) {
   });
 }
 
+void RuntimeScheduler::cancelTask(const std::shared_ptr<Task> &task) {
+  task->cancel();
+}
+
 } // namespace facebook::react

ReactCommon/react/renderer/runtimescheduler/RuntimeScheduler.h

@@ -20,6 +20,8 @@ class RuntimeScheduler final {
 
   void scheduleTask(std::shared_ptr<Task> const &task);
 
+  void cancelTask(std::shared_ptr<Task> const &task);
+
  private:
   mutable std::priority_queue<
       std::shared_ptr<Task>,

ReactCommon/react/renderer/runtimescheduler/RuntimeSchedulerBinding.cpp

@@ -70,6 +70,20 @@ jsi::Value RuntimeSchedulerBinding::get(
         });
   }
 
+  if (propertyName == "unstable_cancelCallback") {
+    return jsi::Function::createFromHostFunction(
+        runtime,
+        name,
+        1,
+        [this](
+            jsi::Runtime &runtime,
+            jsi::Value const &,
+            jsi::Value const *arguments,
+            size_t) noexcept -> jsi::Value {
+          runtimeScheduler_.cancelTask(taskFromValue(runtime, arguments[0]));
+          return jsi::Value::undefined();
+        });
+  }
   if (propertyName == "unstable_ImmediatePriority") {
     return jsi::Value(runtime, serialize(SchedulerPriority::ImmediatePriority));
   }
@@ -91,6 +105,7 @@ jsi::Value RuntimeSchedulerBinding::get(
     return jsi::Value(runtime, serialize(SchedulerPriority::IdlePriority));
   }
 
+  react_native_assert(false && "undefined property");
   return jsi::Value::undefined();
 }
 

ReactCommon/react/renderer/runtimescheduler/Task.cpp

@@ -16,8 +16,18 @@ SchedulerPriority Task::getPriority() const {
   return priority_;
 }
 
+void Task::cancel() {
+  // Null out the callback to indicate the task has been canceled. (Can't
+  // remove from the priority_queue because you can't remove arbitrary nodes
+  // from an array based heap, only the first one.)
+  callback_.reset();
+}
+
 void Task::operator()(jsi::Runtime &runtime) const {
-  callback_.call(runtime, {});
+  if (callback_) {
+    // Cancelled task doesn't have a callback.
+    callback_.value().call(runtime, {});
+  }
 }
 
 } // namespace facebook::react

ReactCommon/react/renderer/runtimescheduler/Task.h

@@ -7,6 +7,7 @@
 
 #pragma once
 
+#include <better/optional.h>
 #include <jsi/jsi.h>
 #include <react/renderer/runtimescheduler/SchedulerPriority.h>
 
@@ -16,12 +17,14 @@ class Task final {
  public:
   Task(SchedulerPriority priority, jsi::Function callback);
 
-  SchedulerPriority priority_;
-  jsi::Function callback_;
-
   SchedulerPriority getPriority() const;
+  void cancel();
 
   void operator()(jsi::Runtime &runtime) const;
+
+ private:
+  SchedulerPriority priority_;
+  better::optional<jsi::Function> callback_;
 };
 
 class TaskPriorityComparer {

ReactCommon/react/renderer/runtimescheduler/primitives.h

@@ -26,4 +26,14 @@ inline static jsi::Value valueFromTask(
       runtime, std::make_shared<TaskWrapper>(task));
 }
 
+inline static std::shared_ptr<Task> taskFromValue(
+    jsi::Runtime &runtime,
+    jsi::Value const &value) {
+  if (value.isNull()) {
+    return nullptr;
+  }
+
+  return value.getObject(runtime).getHostObject<TaskWrapper>(runtime)->task;
+}
+
 } // namespace facebook::react