apache / incubator-weex / daf618fb4a9669b4741236c328b97b030965084f / . / weex_core / Source / include / wtf / SharedTask.h

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#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 | |