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+/*
+ * Copyright (C) 2015 Apple Inc. All rights reserved.
+ *
+ * Redistribution and use in source and binary forms, with or without
+ * modification, are permitted provided that the following conditions
+ * are met:
+ * 1. Redistributions of source code must retain the above copyright
+ *    notice, this list of conditions and the following disclaimer.
+ * 2. Redistributions in binary form must reproduce the above copyright
+ *    notice, this list of conditions and the following disclaimer in the
+ *    documentation and/or other materials provided with the distribution.
+ *
+ * THIS SOFTWARE IS PROVIDED BY APPLE INC. ``AS IS'' AND ANY
+ * EXPRESS OR IMPLIED WARRANTIES, INCLUDING, BUT NOT LIMITED TO, THE
+ * IMPLIED WARRANTIES OF MERCHANTABILITY AND FITNESS FOR A PARTICULAR
+ * PURPOSE ARE DISCLAIMED.  IN NO EVENT SHALL APPLE INC. OR
+ * CONTRIBUTORS BE LIABLE FOR ANY DIRECT, INDIRECT, INCIDENTAL, SPECIAL,
+ * EXEMPLARY, OR CONSEQUENTIAL DAMAGES (INCLUDING, BUT NOT LIMITED TO,
+ * PROCUREMENT OF SUBSTITUTE GOODS OR SERVICES; LOSS OF USE, DATA, OR
+ * PROFITS; OR BUSINESS INTERRUPTION) HOWEVER CAUSED AND ON ANY THEORY
+ * OF LIABILITY, WHETHER IN CONTRACT, STRICT LIABILITY, OR TORT
+ * (INCLUDING NEGLIGENCE OR OTHERWISE) ARISING IN ANY WAY OUT OF THE USE
+ * OF THIS SOFTWARE, EVEN IF ADVISED OF THE POSSIBILITY OF SUCH DAMAGE. 
+ */
+
+#ifndef SharedTask_h
+#define SharedTask_h
+
+#include <wtf/Ref.h>
+#include <wtf/ThreadSafeRefCounted.h>
+
+namespace WTF {
+
+// SharedTask is a replacement for std::function for cases where:
+//
+// - You'd like to avoid the cost of copying, and would prefer to have reference semantics rather
+//   than value semantics.
+// - You want to use FastMalloc rather than system malloc. Note that std::function may avoid malloc
+//   entirely in some cases, but that's hard to guarantee.
+// - You intend to share the task with other threads and so want thread-safe reference counting.
+//
+// Here's an example of how SharedTask can be better than std::function. If you do:
+//
+// std::function<int(double)> a = b;
+//
+// Then "a" will get its own copy of all captured by-value variables. The act of copying may
+// require calls to system malloc, and it may be linear time in the total size of captured
+// variables. On the other hand, if you do:
+//
+// RefPtr<SharedTask<int(double)> a = b;
+//
+// Then "a" will point to the same task as b, and the only work involved is the CAS to increase the
+// reference count.
+//
+// Also, SharedTask allows for more flexibility when sharing state between everyone who runs the
+// task. With std::function, you can only share state using by-reference captured variables.
+// SharedTask supports this since, like std::function, it can be built from a lambda (see
+// createSharedTask(), below). But SharedTask also allows you to create your own subclass and put
+// state in member fields. This can be more natural if you want fine-grained control over what
+// state is shared between instances of the task.
+template<typename FunctionType> class SharedTask;
+template<typename ResultType, typename... ArgumentTypes>
+class SharedTask<ResultType (ArgumentTypes...)> : public ThreadSafeRefCounted<SharedTask<ResultType (ArgumentTypes...)>> {
+public:
+    SharedTask() { }
+    virtual ~SharedTask() { }
+
+    virtual ResultType run(ArgumentTypes...) = 0;
+};
+
+// This is a utility class that allows you to create a SharedTask subclass using a lambda. Usually,
+// you don't want to use this class directly. Use createSharedTask() instead.
+template<typename FunctionType, typename Functor> class SharedTaskFunctor;
+template<typename ResultType, typename... ArgumentTypes, typename Functor>
+class SharedTaskFunctor<ResultType (ArgumentTypes...), Functor> : public SharedTask<ResultType (ArgumentTypes...)> {
+public:
+    SharedTaskFunctor(const Functor& functor)
+        : m_functor(functor)
+    {
+    }
+
+    SharedTaskFunctor(Functor&& functor)
+        : m_functor(WTFMove(functor))
+    {
+    }
+
+private:
+    ResultType run(ArgumentTypes... arguments) override
+    {
+        return m_functor(arguments...);
+    }
+
+    Functor m_functor;
+};
+
+// Create a SharedTask from a functor, such as a lambda. You can use this like so:
+//
+// RefPtr<SharedTask<void()>> task = createSharedTask<void()>(
+//     [=] () {
+//         do things;
+//     });
+//
+// Note that if you use the [&] capture list, then you're probably doing it wrong. That's because
+// [&] will lead to pointers to the stack (the only exception is if you do something like &x where
+// x is a reference to the heap - but in that case, it's better to use [=, &x] to be explicit). You
+// probably don't want pointers to the stack if you will have tasks running on other threads.
+// Probably the best way to be sure that you're not making a horrible mistake is to always use
+// explicit capture lists. In many cases, [this] is sufficient.
+//
+// On the other hand, if you use something like ParallelHelperClient::runTaskInParallel() (or its
+// helper, runFunctionInParallel(), which does createSharedTask() for you), then it can be OK to
+// use [&], since the stack frame will remain live for the entire duration of the task's lifetime.
+template<typename FunctionType, typename Functor>
+Ref<SharedTask<FunctionType>> createSharedTask(const Functor& functor)
+{
+    return adoptRef(*new SharedTaskFunctor<FunctionType, Functor>(functor));
+}
+template<typename FunctionType, typename Functor>
+Ref<SharedTask<FunctionType>> createSharedTask(Functor&& functor)
+{
+    return adoptRef(*new SharedTaskFunctor<FunctionType, Functor>(WTFMove(functor)));
+}
+
+} // namespace WTF
+
+using WTF::createSharedTask;
+using WTF::SharedTask;
+using WTF::SharedTaskFunctor;
+
+#endif // SharedTask_h
+