gecko-dev/dom/media/MediaEventSource.h

/* -*- 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 "mozilla/AbstractThread.h"
#include "mozilla/Atomics.h"
#include "mozilla/Mutex.h"
#include "mozilla/TypeTraits.h"
#include "mozilla/UniquePtr.h"

#include "nsISupportsImpl.h"
#include "nsTArray.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() : mRevoked(false) {}

  void Revoke() {
    mRevoked = true;
  }

  bool IsRevoked() const {
    return mRevoked;
  }

private:
  ~RevocableToken() {}
  Atomic<bool> mRevoked;
};

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 TakeArgs {
  template <typename C> static FalseType test(void(C::*)(), int);
  template <typename C> static FalseType test(void(C::*)() const, int);
  template <typename C> static FalseType test(void(C::*)() volatile, int);
  template <typename C> static FalseType test(void(C::*)() const volatile, int);
  template <typename F> static FalseType test(F&&, decltype(DeclVal<F>()(), 0));
  static TrueType test(...);
public:
  typedef decltype(test(DeclVal<T>(), 0)) Type;
};

template <typename T> struct EventTarget;

template <>
struct EventTarget<nsIEventTarget> {
  static void
  Dispatch(nsIEventTarget* aTarget, already_AddRefed<nsIRunnable>&& aTask) {
    aTarget->Dispatch(Move(aTask), NS_DISPATCH_NORMAL);
  }
};

template <>
struct EventTarget<AbstractThread> {
  static void
  Dispatch(AbstractThread* aTarget, already_AddRefed<nsIRunnable>&& aTask) {
    aTarget->Dispatch(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;
};

} // namespace detail

template <typename T> class MediaEventSource;

/**
 * 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 <typename T>
  friend class MediaEventSource;

public:
  MediaEventListener() {}

  MediaEventListener(MediaEventListener&& aOther)
    : mToken(Move(aOther.mToken)) {}

  MediaEventListener& operator=(MediaEventListener&& aOther) {
    MOZ_ASSERT(!mToken, "Must disconnect the listener.");
    mToken = Move(aOther.mToken);
    return *this;
  }

  ~MediaEventListener() {
    MOZ_ASSERT(!mToken, "Must disconnect the listener.");
  }

  void Disconnect() {
    mToken->Revoke();
    mToken = nullptr;
  }

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) {}
  nsRefPtr<RevocableToken> mToken;
};

/**
 * A generic and thread-safe class to implement the observer pattern.
 */
template <typename EventType>
class MediaEventSource {
  static_assert(!IsReference<EventType>::value, "Ref-type not supported!");
  typedef typename detail::EventTypeTraits<EventType>::ArgType ArgType;

  /**
   * Stored by MediaEventSource to send notifications to the listener.
   */
  class Listener {
  public:
    Listener() : mToken(new RevocableToken()) {}

    virtual ~Listener() {
      MOZ_ASSERT(Token()->IsRevoked(), "Must disconnect the listener.");
    }

    virtual void Dispatch(const ArgType& aEvent) = 0;

    RevocableToken* Token() const {
      return mToken;
    }

  private:
    const nsRefPtr<RevocableToken> mToken;
  };

  /**
   * Store the registered target thread and function so it knows where and to
   * whom to send the event data.
   */
  template<typename Target, typename Function>
  class ListenerImpl : public Listener {
  public:
    explicit ListenerImpl(Target* aTarget, const Function& aFunction)
      : mTarget(aTarget), mFunction(aFunction) {}

    // |Function| takes one argument.
    void Dispatch(const ArgType& aEvent, TrueType) {
      // Define our custom runnable to minimize copy of the event data.
      // NS_NewRunnableFunction will result in 2 copies of the event data.
      // One is captured by the lambda and the other is the copy of the lambda.
      class R : public nsRunnable {
      public:
        R(RevocableToken* aToken,
          const Function& aFunction, const ArgType& aEvent)
          : mToken(aToken), mFunction(aFunction), mEvent(aEvent) {}

        NS_IMETHOD Run() override {
          // Don't call the listener if it is disconnected.
          if (!mToken->IsRevoked()) {
            // Enable move whenever possible since mEvent won't be used anymore.
            mFunction(Move(mEvent));
          }
          return NS_OK;
        }

      private:
        nsRefPtr<RevocableToken> mToken;
        Function mFunction;
        ArgType mEvent;
      };

      nsCOMPtr<nsIRunnable> r = new R(this->Token(), mFunction, aEvent);
      detail::EventTarget<Target>::Dispatch(mTarget.get(), r.forget());
    }

    // |Function| takes no arguments. Don't bother passing aEvent.
    void Dispatch(const ArgType& aEvent, FalseType) {
      nsRefPtr<RevocableToken> token = this->Token();
      const Function& function = mFunction;
      nsCOMPtr<nsIRunnable> r = NS_NewRunnableFunction([=] () {
        // Don't call the listener if it is disconnected.
        if (!token->IsRevoked()) {
          function();
        }
      });
      detail::EventTarget<Target>::Dispatch(mTarget.get(), r.forget());
    }

    void Dispatch(const ArgType& aEvent) override {
      Dispatch(aEvent, typename detail::TakeArgs<Function>::Type());
    }

  private:
    const nsRefPtr<Target> mTarget;
    Function mFunction;
  };

  template<typename Target, typename Function>
  MediaEventListener
  ConnectInternal(Target* aTarget, const Function& aFunction) {
    MutexAutoLock lock(mMutex);
    auto l = mListeners.AppendElement();
    l->reset(new ListenerImpl<Target, Function>(aTarget, aFunction));
    return MediaEventListener((*l)->Token());
  }

  // |Method| takes one argument.
  template <typename Target, typename This, typename Method>
  MediaEventListener
  ConnectInternal(Target* aTarget, This* aThis, Method aMethod, TrueType) {
    detail::RawPtr<This> thiz(aThis);
    auto f = [=] (ArgType&& aEvent) {
      (thiz.get()->*aMethod)(Move(aEvent));
    };
    return ConnectInternal(aTarget, f);
  }

  // |Method| takes no arguments. Don't bother passing the event data.
  template <typename Target, typename This, typename Method>
  MediaEventListener
  ConnectInternal(Target* aTarget, This* aThis, Method aMethod, FalseType) {
    detail::RawPtr<This> thiz(aThis);
    auto f = [=] () {
      (thiz.get()->*aMethod)();
    };
    return ConnectInternal(aTarget, f);
  }

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, const Function& aFunction) {
    return ConnectInternal(aTarget, aFunction);
  }

  template<typename Function>
  MediaEventListener
  Connect(nsIEventTarget* aTarget, const Function& aFunction) {
    return ConnectInternal(aTarget, 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,
      typename detail::TakeArgs<Method>::Type());
  }

  template <typename This, typename Method>
  MediaEventListener
  Connect(nsIEventTarget* aTarget, This* aThis, Method aMethod) {
    return ConnectInternal(aTarget, aThis, aMethod,
      typename detail::TakeArgs<Method>::Type());
  }

protected:
  MediaEventSource() : mMutex("MediaEventSource::mMutex") {}

  void NotifyInternal(const ArgType& aEvent) {
    MutexAutoLock lock(mMutex);
    for (int32_t i = mListeners.Length() - 1; i >= 0; --i) {
      auto&& l = mListeners[i];
      // Remove disconnected listeners.
      // It is not optimal but is simple and works well.
      if (l->Token()->IsRevoked()) {
        mListeners.RemoveElementAt(i);
        continue;
      }
      l->Dispatch(aEvent);
    }
  }

private:
  Mutex mMutex;
  nsTArray<UniquePtr<Listener>> mListeners;
};

/**
 * 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 EventType>
class MediaEventProducer : public MediaEventSource<EventType> {
public:
  void Notify(const EventType& aEvent) {
    this->NotifyInternal(aEvent);
  }
};

/**
 * 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 */);
  }
};

} // namespace mozilla

#endif //MediaEventSource_h_