/* -*- Mode: C++; tab-width: 2; indent-tabs-mode: nil; c-basic-offset: 2 -*- */ /* vim:set ts=2 sw=2 sts=2 et cindent: */ /* This Source Code Form is subject to the terms of the Mozilla Public * License, v. 2.0. If a copy of the MPL was not distributed with this * file, You can obtain one at http://mozilla.org/MPL/2.0/. */ #ifndef MediaEventSource_h_ #define MediaEventSource_h_ #include <type_traits> #include <utility> #include "mozilla/AbstractThread.h" #include "mozilla/Atomics.h" #include "mozilla/Mutex.h" #include "mozilla/Tuple.h" #include "mozilla/Unused.h" #include "nsISupportsImpl.h" #include "nsTArray.h" #include "nsThreadUtils.h" namespace mozilla { /** * A thread-safe tool to communicate "revocation" across threads. It is used to * disconnect a listener from the event source to prevent future notifications * from coming. Revoke() can be called on any thread. However, it is recommended * to be called on the target thread to avoid race condition. * * RevocableToken is not exposed to the client code directly. * Use MediaEventListener below to do the job. */ class RevocableToken { NS_INLINE_DECL_THREADSAFE_REFCOUNTING(RevocableToken); public: RevocableToken() = default; void Revoke() { mRevoked = true; } bool IsRevoked() const { return mRevoked; } protected: // Virtual destructor is required since we might delete a Listener object // through its base type pointer. virtual ~RevocableToken() = default; private: Atomic<bool> mRevoked{false}; }; enum class ListenerPolicy : int8_t { // Allow at most one listener. Move will be used when possible // to pass the event data to save copy. Exclusive, // Allow multiple listeners. Event data will always be copied when passed // to the listeners. NonExclusive }; namespace detail { /** * Define how an event type is passed internally in MediaEventSource and to the * listeners. Specialized for the void type to pass a dummy bool instead. */ template <typename T> struct EventTypeTraits { typedef T ArgType; }; template <> struct EventTypeTraits<void> { typedef bool ArgType; }; /** * Test if a method function or lambda accepts one or more arguments. */ template <typename T> class TakeArgsHelper { template <typename C> static std::false_type test(void (C::*)(), int); template <typename C> static std::false_type test(void (C::*)() const, int); template <typename C> static std::false_type test(void (C::*)() volatile, int); template <typename C> static std::false_type test(void (C::*)() const volatile, int); template <typename F> static std::false_type test(F&&, decltype(std::declval<F>()(), 0)); static std::true_type test(...); public: typedef decltype(test(std::declval<T>(), 0)) type; }; template <typename T> struct TakeArgs : public TakeArgsHelper<T>::type {}; template <typename T> struct EventTarget; template <> struct EventTarget<nsIEventTarget> { static void Dispatch(nsIEventTarget* aTarget, already_AddRefed<nsIRunnable> aTask) { aTarget->Dispatch(std::move(aTask), NS_DISPATCH_NORMAL); } }; template <> struct EventTarget<AbstractThread> { static void Dispatch(AbstractThread* aTarget, already_AddRefed<nsIRunnable> aTask) { Unused << aTarget->Dispatch(std::move(aTask)); } }; /** * Encapsulate a raw pointer to be captured by a lambda without causing * static-analysis errors. */ template <typename T> class RawPtr { public: explicit RawPtr(T* aPtr) : mPtr(aPtr) {} T* get() const { return mPtr; } private: T* const mPtr; }; template <typename... As> class Listener : public RevocableToken { public: template <typename... Ts> void Dispatch(Ts&&... aEvents) { if (CanTakeArgs()) { DispatchTask(NewRunnableMethod<std::decay_t<Ts>&&...>( "detail::Listener::ApplyWithArgs", this, &Listener::ApplyWithArgs, std::forward<Ts>(aEvents)...)); } else { DispatchTask(NewRunnableMethod("detail::Listener::ApplyWithNoArgs", this, &Listener::ApplyWithNoArgs)); } } protected: virtual ~Listener() { MOZ_ASSERT(IsRevoked(), "Must disconnect the listener."); } private: virtual void DispatchTask(already_AddRefed<nsIRunnable> aTask) = 0; // True if the underlying listener function takes non-zero arguments. virtual bool CanTakeArgs() const = 0; // Pass the event data to the underlying listener function. Should be called // only when CanTakeArgs() returns true. virtual void ApplyWithArgs(As&&... aEvents) = 0; // Invoke the underlying listener function. Should be called only when // CanTakeArgs() returns false. virtual void ApplyWithNoArgs() = 0; }; /** * Store the registered target thread and function so it knows where and to * whom to send the event data. */ template <typename Target, typename Function, typename... As> class ListenerImpl : public Listener<As...> { // Strip CV and reference from Function. using FunctionStorage = std::decay_t<Function>; public: template <typename F> ListenerImpl(Target* aTarget, F&& aFunction) : mTarget(aTarget), mFunction(std::forward<F>(aFunction)) {} private: void DispatchTask(already_AddRefed<nsIRunnable> aTask) override { EventTarget<Target>::Dispatch(mTarget.get(), std::move(aTask)); } bool CanTakeArgs() const override { return TakeArgs<FunctionStorage>::value; } // |F| takes one or more arguments. template <typename F> std::enable_if_t<TakeArgs<F>::value, void> ApplyWithArgsImpl( const F& aFunc, As&&... aEvents) { aFunc(std::move(aEvents)...); } // |F| takes no arguments. template <typename F> std::enable_if_t<!TakeArgs<F>::value, void> ApplyWithArgsImpl( const F& aFunc, As&&... aEvents) { MOZ_CRASH("Call ApplyWithNoArgs instead."); } void ApplyWithArgs(As&&... aEvents) override { MOZ_RELEASE_ASSERT(TakeArgs<Function>::value); // Don't call the listener if it is disconnected. if (!RevocableToken::IsRevoked()) { ApplyWithArgsImpl(mFunction, std::move(aEvents)...); } } // |F| takes one or more arguments. template <typename F> std::enable_if_t<TakeArgs<F>::value, void> ApplyWithNoArgsImpl( const F& aFunc) { MOZ_CRASH("Call ApplyWithArgs instead."); } // |F| takes no arguments. template <typename F> std::enable_if_t<!TakeArgs<F>::value, void> ApplyWithNoArgsImpl( const F& aFunc) { aFunc(); } virtual void ApplyWithNoArgs() override { MOZ_RELEASE_ASSERT(!TakeArgs<Function>::value); // Don't call the listener if it is disconnected. if (!RevocableToken::IsRevoked()) { ApplyWithNoArgsImpl(mFunction); } } const RefPtr<Target> mTarget; FunctionStorage mFunction; }; /** * Return true if any type is a reference type. */ template <typename Head, typename... Tails> struct IsAnyReference { static const bool value = std::is_reference_v<Head> || IsAnyReference<Tails...>::value; }; template <typename T> struct IsAnyReference<T> { static const bool value = std::is_reference_v<T>; }; } // namespace detail template <ListenerPolicy, typename... Ts> class MediaEventSourceImpl; /** * Not thread-safe since this is not meant to be shared and therefore only * move constructor is provided. Used to hold the result of * MediaEventSource<T>::Connect() and call Disconnect() to disconnect the * listener from an event source. */ class MediaEventListener { template <ListenerPolicy, typename... Ts> friend class MediaEventSourceImpl; public: MediaEventListener() = default; MediaEventListener(MediaEventListener&& aOther) : mToken(std::move(aOther.mToken)) {} MediaEventListener& operator=(MediaEventListener&& aOther) { MOZ_ASSERT(!mToken, "Must disconnect the listener."); mToken = std::move(aOther.mToken); return *this; } ~MediaEventListener() { MOZ_ASSERT(!mToken, "Must disconnect the listener."); } void Disconnect() { mToken->Revoke(); mToken = nullptr; } void DisconnectIfExists() { if (mToken) { Disconnect(); } } private: // Avoid exposing RevocableToken directly to the client code so that // listeners can be disconnected in a controlled manner. explicit MediaEventListener(RevocableToken* aToken) : mToken(aToken) {} RefPtr<RevocableToken> mToken; }; /** * A generic and thread-safe class to implement the observer pattern. */ template <ListenerPolicy Lp, typename... Es> class MediaEventSourceImpl { static_assert(!detail::IsAnyReference<Es...>::value, "Ref-type not supported!"); template <typename T> using ArgType = typename detail::EventTypeTraits<T>::ArgType; typedef detail::Listener<ArgType<Es>...> Listener; template <typename Target, typename Func> using ListenerImpl = detail::ListenerImpl<Target, Func, ArgType<Es>...>; template <typename Method> using TakeArgs = detail::TakeArgs<Method>; void PruneListeners() { mListeners.RemoveElementsBy( [](const auto& listener) { return listener->IsRevoked(); }); } template <typename Target, typename Function> MediaEventListener ConnectInternal(Target* aTarget, Function&& aFunction) { MutexAutoLock lock(mMutex); PruneListeners(); MOZ_ASSERT(Lp == ListenerPolicy::NonExclusive || mListeners.IsEmpty()); auto l = mListeners.AppendElement(); *l = new ListenerImpl<Target, Function>(aTarget, std::forward<Function>(aFunction)); return MediaEventListener(*l); } // |Method| takes one or more arguments. template <typename Target, typename This, typename Method> std::enable_if_t<TakeArgs<Method>::value, MediaEventListener> ConnectInternal( Target* aTarget, This* aThis, Method aMethod) { detail::RawPtr<This> thiz(aThis); return ConnectInternal(aTarget, [=](ArgType<Es>&&... aEvents) { (thiz.get()->*aMethod)(std::move(aEvents)...); }); } // |Method| takes no arguments. Don't bother passing the event data. template <typename Target, typename This, typename Method> std::enable_if_t<!TakeArgs<Method>::value, MediaEventListener> ConnectInternal(Target* aTarget, This* aThis, Method aMethod) { detail::RawPtr<This> thiz(aThis); return ConnectInternal(aTarget, [=]() { (thiz.get()->*aMethod)(); }); } public: /** * Register a function to receive notifications from the event source. * * @param aTarget The target thread on which the function will run. * @param aFunction A function to be called on the target thread. The function * parameter must be convertible from |EventType|. * @return An object used to disconnect from the event source. */ template <typename Function> MediaEventListener Connect(AbstractThread* aTarget, Function&& aFunction) { return ConnectInternal(aTarget, std::forward<Function>(aFunction)); } template <typename Function> MediaEventListener Connect(nsIEventTarget* aTarget, Function&& aFunction) { return ConnectInternal(aTarget, std::forward<Function>(aFunction)); } /** * As above. * * Note we deliberately keep a weak reference to |aThis| in order not to * change its lifetime. This is because notifications are dispatched * asynchronously and removing a listener doesn't always break the reference * cycle for the pending event could still hold a reference to |aThis|. * * The caller must call MediaEventListener::Disconnect() to avoid dangling * pointers. */ template <typename This, typename Method> MediaEventListener Connect(AbstractThread* aTarget, This* aThis, Method aMethod) { return ConnectInternal(aTarget, aThis, aMethod); } template <typename This, typename Method> MediaEventListener Connect(nsIEventTarget* aTarget, This* aThis, Method aMethod) { return ConnectInternal(aTarget, aThis, aMethod); } protected: MediaEventSourceImpl() : mMutex("MediaEventSourceImpl::mMutex") {} template <typename... Ts> void NotifyInternal(Ts&&... aEvents) { MutexAutoLock lock(mMutex); int32_t last = static_cast<int32_t>(mListeners.Length()) - 1; for (int32_t i = last; i >= 0; --i) { auto&& l = mListeners[i]; // Remove disconnected listeners. // It is not optimal but is simple and works well. if (l->IsRevoked()) { mListeners.RemoveElementAt(i); continue; } l->Dispatch(std::forward<Ts>(aEvents)...); } } private: Mutex mMutex; nsTArray<RefPtr<Listener>> mListeners; }; template <typename... Es> using MediaEventSource = MediaEventSourceImpl<ListenerPolicy::NonExclusive, Es...>; template <typename... Es> using MediaEventSourceExc = MediaEventSourceImpl<ListenerPolicy::Exclusive, Es...>; /** * A class to separate the interface of event subject (MediaEventSource) * and event publisher. Mostly used as a member variable to publish events * to the listeners. */ template <typename... Es> class MediaEventProducer : public MediaEventSource<Es...> { public: template <typename... Ts> void Notify(Ts&&... aEvents) { // Pass lvalues to prevent move in NonExclusive mode. this->NotifyInternal(aEvents...); } }; /** * Specialization for void type. A dummy bool is passed to NotifyInternal * since there is no way to pass a void value. */ template <> class MediaEventProducer<void> : public MediaEventSource<void> { public: void Notify() { this->NotifyInternal(true /* dummy */); } }; /** * A producer allowing at most one listener. */ template <typename... Es> class MediaEventProducerExc : public MediaEventSourceExc<Es...> { public: template <typename... Ts> void Notify(Ts&&... aEvents) { this->NotifyInternal(std::forward<Ts>(aEvents)...); } }; } // namespace mozilla #endif // MediaEventSource_h_