1 files changed, 594 insertions, 0 deletions
diff --git a/dom/media/MediaEventSource.h b/dom/media/MediaEventSource.h
new file mode 100644
index 0000000000..cb0a851ffe
--- /dev/null
+++ b/dom/media/MediaEventSource.h
@@ -0,0 +1,594 @@
+/* -*- 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/DataMutex.h"
+#include "mozilla/Mutex.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;
+
+  virtual void Revoke() = 0;
+  virtual bool IsRevoked() const = 0;
+
+ protected:
+  // Virtual destructor is required since we might delete a Listener object
+  // through its base type pointer.
+  virtual ~RevocableToken() = default;
+};
+
+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);
+  }
+  static bool IsOnTargetThread(nsIEventTarget* aTarget) {
+    bool rv;
+    aTarget->IsOnCurrentThread(&rv);
+    return rv;
+  }
+};
+
+template <>
+struct EventTarget<AbstractThread> {
+  static void Dispatch(AbstractThread* aTarget,
+                       already_AddRefed<nsIRunnable> aTask) {
+    aTarget->Dispatch(std::move(aTask));
+  }
+  static bool IsOnTargetThread(AbstractThread* aTarget) {
+    bool rv;
+    aTarget->IsOnCurrentThread(&rv);
+    return rv;
+  }
+};
+
+/**
+ * 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));
+    }
+  }
+
+ 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>;
+  using SelfType = ListenerImpl<Target, Function, As...>;
+
+ public:
+  ListenerImpl(Target* aTarget, Function&& aFunction)
+      : mData(MakeRefPtr<Data>(aTarget, std::forward<Function>(aFunction)),
+              "MediaEvent ListenerImpl::mData") {
+    MOZ_DIAGNOSTIC_ASSERT(aTarget);
+  }
+
+ protected:
+  virtual ~ListenerImpl() {
+    MOZ_ASSERT(IsRevoked(), "Must disconnect the listener.");
+  }
+
+ private:
+  void DispatchTask(already_AddRefed<nsIRunnable> aTask) override {
+    RefPtr<Data> data;
+    {
+      auto d = mData.Lock();
+      data = *d;
+    }
+    if (NS_WARN_IF(!data)) {
+      // already_AddRefed doesn't allow releasing the ref, so transfer it first.
+      RefPtr<nsIRunnable> temp(aTask);
+      return;
+    }
+    EventTarget<Target>::Dispatch(data->mTarget, 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(
+      Target* aTarget, const F& aFunc, As&&... aEvents) {
+    MOZ_DIAGNOSTIC_ASSERT(EventTarget<Target>::IsOnTargetThread(aTarget));
+    aFunc(std::move(aEvents)...);
+  }
+
+  // |F| takes no arguments.
+  template <typename F>
+  std::enable_if_t<!TakeArgs<F>::value, void> ApplyWithArgsImpl(
+      Target* aTarget, 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.
+    RefPtr<Data> data;
+    {
+      auto d = mData.Lock();
+      data = *d;
+    }
+    if (!data) {
+      return;
+    }
+    MOZ_DIAGNOSTIC_ASSERT(EventTarget<Target>::IsOnTargetThread(data->mTarget));
+    ApplyWithArgsImpl(data->mTarget, data->mFunction, std::move(aEvents)...);
+  }
+
+  // |F| takes one or more arguments.
+  template <typename F>
+  std::enable_if_t<TakeArgs<F>::value, void> ApplyWithNoArgsImpl(
+      Target* aTarget, const F& aFunc) {
+    MOZ_CRASH("Call ApplyWithArgs instead.");
+  }
+
+  // |F| takes no arguments.
+  template <typename F>
+  std::enable_if_t<!TakeArgs<F>::value, void> ApplyWithNoArgsImpl(
+      Target* aTarget, const F& aFunc) {
+    MOZ_DIAGNOSTIC_ASSERT(EventTarget<Target>::IsOnTargetThread(aTarget));
+    aFunc();
+  }
+
+  void ApplyWithNoArgs() override {
+    MOZ_RELEASE_ASSERT(!TakeArgs<Function>::value);
+    // Don't call the listener if it is disconnected.
+    RefPtr<Data> data;
+    {
+      auto d = mData.Lock();
+      data = *d;
+    }
+    if (!data) {
+      return;
+    }
+    MOZ_DIAGNOSTIC_ASSERT(EventTarget<Target>::IsOnTargetThread(data->mTarget));
+    ApplyWithNoArgsImpl(data->mTarget, data->mFunction);
+  }
+
+  void Revoke() override {
+    {
+      auto data = mData.Lock();
+      *data = nullptr;
+    }
+  }
+
+  bool IsRevoked() const override {
+    auto data = mData.Lock();
+    return !*data;
+  }
+
+  struct RefCountedMediaEventListenerData {
+    // Keep ref-counting here since Data holds a template member, leading to
+    // instances of varying size, which the memory leak logging system dislikes.
+    NS_INLINE_DECL_THREADSAFE_REFCOUNTING(RefCountedMediaEventListenerData)
+   protected:
+    virtual ~RefCountedMediaEventListenerData() = default;
+  };
+  struct Data : public RefCountedMediaEventListenerData {
+    Data(RefPtr<Target> aTarget, Function&& aFunction)
+        : mTarget(std::move(aTarget)),
+          mFunction(std::forward<Function>(aFunction)) {}
+    const RefPtr<Target> mTarget;
+    FunctionStorage mFunction;
+  };
+
+  // Storage for target and function. Also used to track revocation.
+  mutable DataMutex<RefPtr<Data>> mData;
+};
+
+/**
+ * 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 MOZ_UNANNOTATED;
+  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)...);
+  }
+};
+
+/**
+ * A class that facilitates forwarding MediaEvents from multiple sources of the
+ * same type into a single source.
+ *
+ * Lifetimes are convenient. A forwarded source is disconnected either by
+ * the source itself going away, or the forwarder being destroyed.
+ *
+ * Not threadsafe. The caller is responsible for calling Forward() in a
+ * threadsafe manner.
+ */
+template <typename... Es>
+class MediaEventForwarder : public MediaEventSource<Es...> {
+ public:
+  template <typename T>
+  using ArgType = typename detail::EventTypeTraits<T>::ArgType;
+
+  explicit MediaEventForwarder(nsCOMPtr<nsISerialEventTarget> aEventTarget)
+      : mEventTarget(std::move(aEventTarget)) {}
+
+  MediaEventForwarder(MediaEventForwarder&& aOther)
+      : mEventTarget(aOther.mEventTarget),
+        mListeners(std::move(aOther.mListeners)) {}
+
+  ~MediaEventForwarder() { MOZ_ASSERT(mListeners.IsEmpty()); }
+
+  MediaEventForwarder& operator=(MediaEventForwarder&& aOther) {
+    MOZ_RELEASE_ASSERT(mEventTarget == aOther.mEventTarget);
+    MOZ_ASSERT(mListeners.IsEmpty());
+    mListeners = std::move(aOther.mListeners);
+  }
+
+  void Forward(MediaEventSource<Es...>& aSource) {
+    // Forwarding a rawptr `this` here is fine, since DisconnectAll disconnect
+    // all mListeners synchronously and prevents this handler from running.
+    mListeners.AppendElement(
+        aSource.Connect(mEventTarget, [this](ArgType<Es>&&... aEvents) {
+          this->NotifyInternal(std::forward<ArgType<Es>...>(aEvents)...);
+        }));
+  }
+
+  template <typename Function>
+  void ForwardIf(MediaEventSource<Es...>& aSource, Function&& aFunction) {
+    // Forwarding a rawptr `this` here is fine, since DisconnectAll disconnect
+    // all mListeners synchronously and prevents this handler from running.
+    mListeners.AppendElement(aSource.Connect(
+        mEventTarget, [this, func = aFunction](ArgType<Es>&&... aEvents) {
+          if (!func()) {
+            return;
+          }
+          this->NotifyInternal(std::forward<ArgType<Es>...>(aEvents)...);
+        }));
+  }
+
+  void DisconnectAll() {
+    for (auto& l : mListeners) {
+      l.Disconnect();
+    }
+    mListeners.Clear();
+  }
+
+ private:
+  const nsCOMPtr<nsISerialEventTarget> mEventTarget;
+  nsTArray<MediaEventListener> mListeners;
+};
+
+}  // namespace mozilla
+
+#endif  // MediaEventSource_h_