summaryrefslogtreecommitdiff
path: root/dom/media/MediaEventSource.h
blob: e184ff33cb69b27d572ad0f1fcbd53537c55db00 (plain)
1
2
3
4
5
6
7
8
9
10
11
12
13
14
15
16
17
18
19
20
21
22
23
24
25
26
27
28
29
30
31
32
33
34
35
36
37
38
39
40
41
42
43
44
45
46
47
48
49
50
51
52
53
54
55
56
57
58
59
60
61
62
63
64
65
66
67
68
69
70
71
72
73
74
75
76
77
78
79
80
81
82
83
84
85
86
87
88
89
90
91
92
93
94
95
96
97
98
99
100
101
102
103
104
105
106
107
108
109
110
111
112
113
114
115
116
117
118
119
120
121
122
123
124
125
126
127
128
129
130
131
132
133
134
135
136
137
138
139
140
141
142
143
144
145
146
147
148
149
150
151
152
153
154
155
156
157
158
159
160
161
162
163
164
165
166
167
168
169
170
171
172
173
174
175
176
177
178
179
180
181
182
183
184
185
186
187
188
189
190
191
192
193
194
195
196
197
198
199
200
201
202
203
204
205
206
207
208
209
210
211
212
213
214
215
216
217
218
219
220
221
222
223
224
225
226
227
228
229
230
231
232
233
234
235
236
237
238
239
240
241
242
243
244
245
246
247
248
249
250
251
252
253
254
255
256
257
258
259
260
261
262
263
264
265
266
267
268
269
270
271
272
273
274
275
276
277
278
279
280
281
282
283
284
285
286
287
288
289
290
291
292
293
294
295
296
297
298
299
300
301
302
303
304
305
306
307
308
309
310
311
312
313
314
315
316
317
318
319
320
321
322
323
324
325
326
327
328
329
330
331
332
333
334
335
336
337
338
339
340
341
342
343
344
345
346
347
348
349
350
351
352
353
354
355
356
357
358
359
360
361
362
363
364
365
366
367
368
369
370
371
372
373
374
375
376
377
378
379
380
381
382
383
384
385
386
387
388
389
390
391
392
393
394
395
396
397
398
399
400
401
402
403
404
405
406
407
408
409
410
411
412
413
414
415
416
417
418
419
420
421
422
423
424
425
426
427
428
429
430
431
432
433
434
435
436
437
438
439
440
441
442
443
444
445
446
447
448
449
450
451
452
453
454
455
456
457
458
459
460
461
462
463
464
465
466
467
468
469
470
471
472
473
474
475
476
477
478
479
480
481
482
483
484
485
486
487
488
489
490
491
492
493
494
495
496
497
498
499
500
501
502
503
504
505
506
507
508
509
510
511
512
513
514
515
516
517
518
519
520
521
522
523
524
525
526
527
528
529
530
531
532
533
534
535
536
537
538
539
540
541
542
543
544
545
546
547
548
549
550
551
552
553
554
555
556
557
558
559
560
561
562
563
564
565
566
567
568
569
570
571
572
573
574
575
576
577
578
579
580
581
582
583
584
585
586
587
588
589
590
591
592
593
594
595
596
597
598
599
600
601
602
603
604
605
606
607
608
609
610
611
612
613
614
615
616
617
/* -*- Mode: C++; tab-width: 2; indent-tabs-mode: nil; c-basic-offset: 2 -*- */
/* 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/IndexSequence.h"
#include "mozilla/Mutex.h"
#include "mozilla/Tuple.h"
#include "mozilla/TypeTraits.h"
#include "mozilla/UniquePtr.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() : mRevoked(false) {}

  void Revoke() {
    mRevoked = true;
  }

  bool IsRevoked() const {
    return mRevoked;
  }

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

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
};

enum class DispatchPolicy : int8_t {
  Sync, // Events are passed synchronously to the listeners.
  Async // Events are passed asynchronously to the listeners.
};

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 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 TakeArgs : public TakeArgsHelper<T>::Type {};

template <DispatchPolicy Dp, typename T> struct EventTarget;

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

template <>
struct EventTarget<DispatchPolicy::Async, AbstractThread> {
  static void
  Dispatch(AbstractThread* aTarget, already_AddRefed<nsIRunnable> aTask) {
    aTarget->Dispatch(Move(aTask), AbstractThread::DontAssertDispatchSuccess);
  }
};

template <>
struct EventTarget<DispatchPolicy::Sync, nsIEventTarget> {
  static bool IsOnCurrentThread(nsIEventTarget* aTarget) {
    bool current = false;
    auto rv = aTarget->IsOnCurrentThread(&current);
    return NS_SUCCEEDED(rv) && current;
  }
  static void
  Dispatch(nsIEventTarget* aTarget, already_AddRefed<nsIRunnable> aTask) {
    MOZ_ASSERT(IsOnCurrentThread(aTarget));
    nsCOMPtr<nsIRunnable> r = aTask;
    r->Run();
  }
};

template <>
struct EventTarget<DispatchPolicy::Sync, AbstractThread> {
  static void
  Dispatch(AbstractThread* aTarget, already_AddRefed<nsIRunnable> aTask) {
    MOZ_ASSERT(aTarget->IsCurrentThreadIn());
    nsCOMPtr<nsIRunnable> r = aTask;
    r->Run();
  }
};

/**
 * 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;
};

/**
 * A helper class to pass event data to the listeners. Optimized to save
 * copy when Move is possible or |Function| takes no arguments.
 */
template<DispatchPolicy Dp, typename Target, typename Function>
class ListenerHelper {
  // 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.
  template <typename... Ts>
  class R : public Runnable {
  public:
    template <typename... Us>
    R(RevocableToken* aToken, const Function& aFunction, Us&&... aEvents)
      : mToken(aToken)
      , mFunction(aFunction)
      , mEvents(Forward<Us>(aEvents)...) {}

    template <typename... Vs, size_t... Is>
    void Invoke(Tuple<Vs...>& aEvents, IndexSequence<Is...>) {
      // Enable move whenever possible since mEvent won't be used anymore.
      mFunction(Move(Get<Is>(aEvents))...);
    }

    NS_IMETHOD Run() override {
      // Don't call the listener if it is disconnected.
      if (!mToken->IsRevoked()) {
        Invoke(mEvents, typename IndexSequenceFor<Ts...>::Type());
      }
      return NS_OK;
    }

  private:
    RefPtr<RevocableToken> mToken;
    Function mFunction;

    template <typename T>
    using ArgType = typename RemoveCV<typename RemoveReference<T>::Type>::Type;
    Tuple<ArgType<Ts>...> mEvents;
  };

public:
  ListenerHelper(RevocableToken* aToken, Target* aTarget, const Function& aFunc)
    : mToken(aToken), mTarget(aTarget), mFunction(aFunc) {}

  // |F| takes one or more arguments.
  template <typename F, typename... Ts>
  typename EnableIf<TakeArgs<F>::value, void>::Type
  DispatchHelper(const F& aFunc, Ts&&... aEvents) {
    nsCOMPtr<nsIRunnable> r =
      new R<Ts...>(mToken, aFunc, Forward<Ts>(aEvents)...);
    EventTarget<Dp, Target>::Dispatch(mTarget.get(), r.forget());
  }

  // |F| takes no arguments. Don't bother passing aEvent.
  template <typename F, typename... Ts>
  typename EnableIf<!TakeArgs<F>::value, void>::Type
  DispatchHelper(const F& aFunc, Ts&&...) {
    const RefPtr<RevocableToken>& token = mToken;
    nsCOMPtr<nsIRunnable> r = NS_NewRunnableFunction([=] () {
      // Don't call the listener if it is disconnected.
      if (!token->IsRevoked()) {
        aFunc();
      }
    });
    EventTarget<Dp, Target>::Dispatch(mTarget.get(), r.forget());
  }

  template <typename... Ts>
  void Dispatch(Ts&&... aEvents) {
    DispatchHelper(mFunction, Forward<Ts>(aEvents)...);
  }

private:
  RefPtr<RevocableToken> mToken;
  const RefPtr<Target> mTarget;
  Function mFunction;
};

/**
 * Define whether an event data should be copied or moved to the listeners.
 *
 * @Copy Data will always be copied. Each listener gets a copy.
 * @Move Data will always be moved.
 */
enum class EventPassMode : int8_t {
  Copy,
  Move
};

class ListenerBase {
public:
  ListenerBase() : mToken(new RevocableToken()) {}
  ~ListenerBase() {
    MOZ_ASSERT(Token()->IsRevoked(), "Must disconnect the listener.");
  }
  RevocableToken* Token() const {
    return mToken;
  }
private:
  const RefPtr<RevocableToken> mToken;
};

/**
 * Stored by MediaEventSource to send notifications to the listener.
 * Since virtual methods can not be templated, this class is specialized
 * to provide different Dispatch() overloads depending on EventPassMode.
 */
template <EventPassMode Mode, typename... As>
class Listener : public ListenerBase {
public:
  virtual ~Listener() {}
  virtual void Dispatch(const As&... aEvents) = 0;
};

template <typename... As>
class Listener<EventPassMode::Move, As...> : public ListenerBase {
public:
  virtual ~Listener() {}
  virtual void Dispatch(As... aEvents) = 0;
};

/**
 * Store the registered target thread and function so it knows where and to
 * whom to send the event data.
 */
template <DispatchPolicy Dp, typename Target,
          typename Function, EventPassMode, typename... As>
class ListenerImpl : public Listener<EventPassMode::Copy, As...> {
public:
  ListenerImpl(Target* aTarget, const Function& aFunction)
    : mHelper(ListenerBase::Token(), aTarget, aFunction) {}
  void Dispatch(const As&... aEvents) override {
    mHelper.Dispatch(aEvents...);
  }
private:
  ListenerHelper<Dp, Target, Function> mHelper;
};

template <DispatchPolicy Dp, typename Target, typename Function, typename... As>
class ListenerImpl<Dp, Target, Function, EventPassMode::Move, As...>
  : public Listener<EventPassMode::Move, As...> {
public:
  ListenerImpl(Target* aTarget, const Function& aFunction)
    : mHelper(ListenerBase::Token(), aTarget, aFunction) {}
  void Dispatch(As... aEvents) override {
    mHelper.Dispatch(Move(aEvents)...);
  }
private:
  ListenerHelper<Dp, Target, Function> mHelper;
};

/**
 * Select EventPassMode based on ListenerPolicy.
 *
 * @Copy Selected when ListenerPolicy is NonExclusive because each listener
 * must get a copy.
 *
 * @Move Selected when ListenerPolicy is Exclusive. All types passed to
 * MediaEventProducer::Notify() must be movable.
 */
template <ListenerPolicy Lp>
struct PassModePicker {
  static const EventPassMode Value =
    Lp == ListenerPolicy::NonExclusive ?
    EventPassMode::Copy : EventPassMode::Move;
};

/**
 * Return true if any type is a reference type.
 */
template <typename Head, typename... Tails>
struct IsAnyReference {
  static const bool value = IsReference<Head>::value ||
                            IsAnyReference<Tails...>::value;
};

template <typename T>
struct IsAnyReference<T> {
  static const bool value = IsReference<T>::value;
};

} // namespace detail

template <DispatchPolicy, 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 <DispatchPolicy, ListenerPolicy, typename... Ts>
  friend class MediaEventSourceImpl;

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;
  }

  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 <DispatchPolicy Dp, 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;

  static const detail::EventPassMode PassMode =
    detail::PassModePicker<Lp>::Value;

  typedef detail::Listener<PassMode, ArgType<Es>...> Listener;

  template<typename Target, typename Func>
  using ListenerImpl =
    detail::ListenerImpl<Dp, Target, Func, PassMode, ArgType<Es>...>;

  template <typename Method>
  using TakeArgs = detail::TakeArgs<Method>;

  void PruneListeners() {
    int32_t last = static_cast<int32_t>(mListeners.Length()) - 1;
    for (int32_t i = last; i >= 0; --i) {
      if (mListeners[i]->Token()->IsRevoked()) {
        mListeners.RemoveElementAt(i);
      }
    }
  }

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

  // |Method| takes one or more arguments.
  template <typename Target, typename This, typename Method>
  typename EnableIf<TakeArgs<Method>::value, MediaEventListener>::Type
  ConnectInternal(Target* aTarget, This* aThis, Method aMethod) {
    detail::RawPtr<This> thiz(aThis);
    auto f = [=] (ArgType<Es>&&... aEvents) {
      (thiz.get()->*aMethod)(Move(aEvents)...);
    };
    return ConnectInternal(aTarget, f);
  }

  // |Method| takes no arguments. Don't bother passing the event data.
  template <typename Target, typename This, typename Method>
  typename EnableIf<!TakeArgs<Method>::value, MediaEventListener>::Type
  ConnectInternal(Target* aTarget, This* aThis, Method aMethod) {
    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);
  }

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

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

  template <DispatchPolicy P, typename... Ts>
  typename EnableIf<P == DispatchPolicy::Async, void>::Type
  NotifyInternal(IntegralConstant<DispatchPolicy, P>, 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->Token()->IsRevoked()) {
        mListeners.RemoveElementAt(i);
        continue;
      }
      l->Dispatch(Forward<Ts>(aEvents)...);
    }
  }

  template <DispatchPolicy P, typename... Ts>
  typename EnableIf<P == DispatchPolicy::Sync, void>::Type
  NotifyInternal(IntegralConstant<DispatchPolicy, P>, Ts&&... aEvents) {
    // Move |mListeners| to a new container before iteration to prevent
    // |mListeners| from being disrupted if the listener calls Connect() to
    // modify |mListeners| in the callback function.
    nsTArray<UniquePtr<Listener>> listeners;
    listeners.SwapElements(mListeners);
    for (auto&& l : listeners) {
      l->Dispatch(Forward<Ts>(aEvents)...);
    }
    PruneListeners();
    // Move remaining listeners back to |mListeners|.
    for (auto&& l : listeners) {
      if (!l->Token()->IsRevoked()) {
        mListeners.AppendElement(Move(l));
      }
    }
    // Perform sanity checks.
    MOZ_ASSERT(Lp == ListenerPolicy::NonExclusive || mListeners.Length() <= 1);
  }

  template <typename... Ts>
  void Notify(Ts&&... aEvents) {
    NotifyInternal(IntegralConstant<DispatchPolicy, Dp>(),
                   Forward<Ts>(aEvents)...);
  }

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

template <typename... Es>
using MediaEventSource =
  MediaEventSourceImpl<DispatchPolicy::Async,
                       ListenerPolicy::NonExclusive, Es...>;

template <typename... Es>
using MediaEventSourceExc =
  MediaEventSourceImpl<DispatchPolicy::Async, 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:
  using MediaEventSource<Es...>::Notify;
};

/**
 * 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() {
    MediaEventSource<void>::Notify(false /* dummy */);
  }
};

/**
 * A producer allowing at most one listener.
 */
template <typename... Es>
class MediaEventProducerExc : public MediaEventSourceExc<Es...> {
public:
  using MediaEventSourceExc<Es...>::Notify;
};

/**
 * Events are passed directly to the callback function of the listeners without
 * dispatching. Note this class is not thread-safe. Both Connect() and Notify()
 * must be called on the same thread.
 */
template <typename... Es>
class MediaCallback
  : public MediaEventSourceImpl<DispatchPolicy::Sync,
                                ListenerPolicy::NonExclusive, Es...> {
public:
  using MediaEventSourceImpl<DispatchPolicy::Sync,
                             ListenerPolicy::NonExclusive, Es...>::Notify;
};

/**
 * A special version of MediaCallback which allows at most one listener.
 */
template <typename... Es>
class MediaCallbackExc
  : public MediaEventSourceImpl<DispatchPolicy::Sync,
                                ListenerPolicy::Exclusive, Es...> {
public:
  using MediaEventSourceImpl<DispatchPolicy::Sync,
                             ListenerPolicy::Exclusive, Es...>::Notify;
};

} // namespace mozilla

#endif //MediaEventSource_h_