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/* -*- 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_