xpcom/threads/EventTargetCapability.h

   1 /* -*- Mode: C++; tab-width: 8; indent-tabs-mode: nil; c-basic-offset: 2 -*- */
   2 /* vim: set ts=8 sts=2 et sw=2 tw=80: */
   3 /* This Source Code Form is subject to the terms of the Mozilla Public
   4  * License, v. 2.0. If a copy of the MPL was not distributed with this
   5  * file, You can obtain one at http://mozilla.org/MPL/2.0/. */
   6
   7 #ifndef XPCOM_THREADS_EVENTTARGETCAPABILITY_H_
   8 #define XPCOM_THREADS_EVENTTARGETCAPABILITY_H_
   9
  10 #include "mozilla/ThreadSafety.h"
  11 #include "nsIEventTarget.h"
  12
  13 namespace mozilla {
  14
  15 // A helper to ensure that data access and function usage only take place on a
  16 // specific nsIEventTarget.
  17 //
  18 // This class works with Clang's thread safety analysis so that static analysis
  19 // can ensure AssertOnCurrentThread is called before using guarded data and
  20 // functions.
  21 //
  22 // This means using the class is similar to calling
  23 // `MOZ_ASSERT(mTarget->IsOnCurrentThread())`
  24 // prior to accessing things we expect to be on `mTarget`. However, using this
  25 // helper has the added benefit that static analysis will warn you if you
  26 // fail to assert prior to usage.
  27 //
  28 // The following is a basic example of a class using this to ensure
  29 // a data member is only accessed on a specific target.
  30 //
  31 // class SomeMediaHandlerThing {
  32 //  public:
  33 //   SomeMediaHandlerThing(nsIEventTarget* aTarget) : mTargetCapability(aTarget)
  34 //   {}
  35 //
  36 //   void UpdateMediaState() {
  37 //     mTargetCapability.Dispatch(
  38 //         NS_NewRunnableFunction("UpdateMediaState", [this] {
  39 //           mTargetCapability.AssertOnCurrentThread();
  40 //           IncreaseMediaCount();
  41 //         }));
  42 //   }
  43 //
  44 //  private:
  45 //   void IncreaseMediaCount() MOZ_REQUIRES(mTargetCapability) { mMediaCount +=
  46 //   1; }
  47 //
  48 //   uint32_t mMediaCount MOZ_GUARDED_BY(mTargetCapability) = 0;
  49 //   EventTargetCapability<nsIEventTarget> mTargetCapability;
  50 // };
  51 //
  52 // NOTE: If you need a thread-safety capability for specifically the main
  53 // thread, the static `mozilla::sMainThreadCapability` capability exists, and
  54 // can be asserted using `AssertIsOnMainThread()`.
  55
  56 template <typename T>
  57 class MOZ_CAPABILITY("event target") EventTargetCapability final {
  58   static_assert(std::is_base_of_v<nsIEventTarget, T>,
  59                 "T must derive from nsIEventTarget");
  60
  61  public:
  62   explicit EventTargetCapability(T* aTarget) : mTarget(aTarget) {
  63     MOZ_ASSERT(mTarget, "mTarget should be non-null");
  64   }
  65   ~EventTargetCapability() = default;
  66
  67   EventTargetCapability(const EventTargetCapability&) = default;
  68   EventTargetCapability(EventTargetCapability&&) = default;
  69   EventTargetCapability& operator=(const EventTargetCapability&) = default;
  70   EventTargetCapability& operator=(EventTargetCapability&&) = default;
  71
  72   void AssertOnCurrentThread() const MOZ_ASSERT_CAPABILITY(this) {
  73     MOZ_ASSERT(IsOnCurrentThread());
  74   }
  75
  76   // Allow users to check if we're on the same thread as the event target.
  77   bool IsOnCurrentThread() const { return mTarget->IsOnCurrentThread(); }
  78
  79   // Allow users to get the event target, so classes don't have to store the
  80   // target as a separate member to use it.
  81   T* GetEventTarget() const { return mTarget; }
  82
  83   // Helper to simplify dispatching to mTarget.
  84   nsresult Dispatch(already_AddRefed<nsIRunnable> aRunnable,
  85                     uint32_t aFlags = NS_DISPATCH_NORMAL) const {
  86     return mTarget->Dispatch(std::move(aRunnable), aFlags);
  87   }
  88
  89  private:
  90   RefPtr<T> mTarget;
  91 };
  92
  93 }  // namespace mozilla
  94
  95 #endif  // XPCOM_THREADS_EVENTTARGETCAPABILITY_H_