mirror of
https://github.com/classilla/tenfourfox.git
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395 lines
10 KiB
C++
395 lines
10 KiB
C++
/* -*- Mode: C++; tab-width: 8; indent-tabs-mode: nil; c-basic-offset: 2 -*- */
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/* vim: set sw=2 ts=8 et ft=cpp : */
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/* This Source Code Form is subject to the terms of the Mozilla Public
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* License, v. 2.0. If a copy of the MPL was not distributed with this file,
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* You can obtain one at http://mozilla.org/MPL/2.0/. */
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#ifndef mozilla_MediaUtils_h
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#define mozilla_MediaUtils_h
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#include "nsAutoPtr.h"
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#include "nsThreadUtils.h"
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#include "nsIAsyncShutdown.h"
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#include "base/task.h"
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namespace mozilla {
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namespace media {
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/*
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* media::Pledge - A promise-like pattern for c++ that takes lambda functions.
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*
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* Asynchronous APIs that proxy to another thread or to the chrome process and
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* back may find it useful to return a pledge to callers who then use
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* pledge.Then(func) to specify a lambda function to be invoked with the result
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* later back on this same thread.
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*
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* Callers will enjoy that lambdas allow "capturing" of local variables, much
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* like closures in JavaScript (safely by-copy by default).
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*
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* Callers will also enjoy that they do not need to be thread-safe (their code
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* runs on the same thread after all).
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*
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* Advantageously, pledges are non-threadsafe by design (because locking and
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* event queues are redundant). This means none of the lambdas you pass in,
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* or variables you lambda-capture into them, need be threasafe or support
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* threadsafe refcounting. After all, they'll run later on the same thread.
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*
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* RefPtr<media::Pledge<Foo>> p = GetFooAsynchronously(); // returns a pledge
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* p->Then([](const Foo& foo) {
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* // use foo here (same thread. Need not be thread-safe!)
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* });
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*
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* See media::CoatCheck below for an example of GetFooAsynchronously().
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*/
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class PledgeBase
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{
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public:
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NS_INLINE_DECL_REFCOUNTING(PledgeBase);
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protected:
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virtual ~PledgeBase() {};
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};
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template<typename ValueType, typename ErrorType = nsresult>
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class Pledge : public PledgeBase
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{
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// TODO: Remove workaround once mozilla allows std::function from <functional>
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// wo/std::function support, do template + virtual trick to accept lambdas
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class FunctorsBase
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{
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public:
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FunctorsBase() {}
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virtual void Succeed(ValueType& result) = 0;
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virtual void Fail(ErrorType& error) = 0;
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virtual ~FunctorsBase() {};
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};
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public:
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explicit Pledge() : mDone(false), mRejected(false) {}
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Pledge(const Pledge& aOther) = delete;
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Pledge& operator = (const Pledge&) = delete;
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template<typename OnSuccessType>
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void Then(OnSuccessType&& aOnSuccess)
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{
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Then(Forward<OnSuccessType>(aOnSuccess), [](ErrorType&){});
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}
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template<typename OnSuccessType, typename OnFailureType>
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void Then(OnSuccessType&& aOnSuccess, OnFailureType&& aOnFailure)
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{
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class Functors : public FunctorsBase
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{
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public:
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Functors(OnSuccessType&& aOnSuccess, OnFailureType&& aOnFailure)
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: mOnSuccess(Move(aOnSuccess)), mOnFailure(Move(aOnFailure)) {}
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void Succeed(ValueType& result)
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{
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mOnSuccess(result);
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}
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void Fail(ErrorType& error)
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{
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mOnFailure(error);
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};
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OnSuccessType mOnSuccess;
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OnFailureType mOnFailure;
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};
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mFunctors = new Functors(Forward<OnSuccessType>(aOnSuccess),
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Forward<OnFailureType>(aOnFailure));
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if (mDone) {
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if (!mRejected) {
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mFunctors->Succeed(mValue);
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} else {
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mFunctors->Fail(mError);
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}
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}
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}
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void Resolve(const ValueType& aValue)
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{
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mValue = aValue;
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Resolve();
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}
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void Reject(ErrorType rv)
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{
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if (!mDone) {
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mDone = mRejected = true;
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mError = rv;
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if (mFunctors) {
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mFunctors->Fail(mError);
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}
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}
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}
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protected:
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void Resolve()
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{
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if (!mDone) {
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mDone = true;
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MOZ_ASSERT(!mRejected);
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if (mFunctors) {
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mFunctors->Succeed(mValue);
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}
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}
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}
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ValueType mValue;
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private:
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~Pledge() {};
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bool mDone;
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bool mRejected;
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ErrorType mError;
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ScopedDeletePtr<FunctorsBase> mFunctors;
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};
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/* media::NewRunnableFrom() - Create an nsRunnable from a lambda.
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* media::NewTaskFrom() - Create a Task from a lambda.
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*
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* Passing variables (closures) to an async function is clunky with nsRunnable:
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*
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* void Foo()
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* {
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* class FooRunnable : public nsRunnable
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* {
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* public:
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* FooRunnable(const Bar &aBar) : mBar(aBar) {}
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* NS_IMETHOD Run()
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* {
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* // Use mBar
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* }
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* private:
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* RefPtr<Bar> mBar;
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* };
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*
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* RefPtr<Bar> bar = new Bar();
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* NS_DispatchToMainThread(new FooRunnable(bar);
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* }
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*
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* It's worse with more variables. Lambdas have a leg up with variable capture:
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*
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* void Foo()
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* {
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* RefPtr<Bar> bar = new Bar();
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* NS_DispatchToMainThread(media::NewRunnableFrom([bar]() mutable {
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* // use bar
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* });
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* }
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*
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* Capture is by-copy by default, so the nsRefPtr 'bar' is safely copied for
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* access on the other thread (threadsafe refcounting in bar is assumed).
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*
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* The 'mutable' keyword is only needed for non-const access to bar.
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*/
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template<typename OnRunType>
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class LambdaRunnable : public nsRunnable
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{
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public:
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explicit LambdaRunnable(OnRunType&& aOnRun) : mOnRun(Move(aOnRun)) {}
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private:
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NS_IMETHODIMP
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Run()
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{
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return mOnRun();
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}
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OnRunType mOnRun;
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};
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template<typename OnRunType>
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LambdaRunnable<OnRunType>*
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NewRunnableFrom(OnRunType&& aOnRun)
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{
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return new LambdaRunnable<OnRunType>(Forward<OnRunType>(aOnRun));
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}
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template<typename OnRunType>
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class LambdaTask : public Task
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{
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public:
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explicit LambdaTask(OnRunType&& aOnRun) : mOnRun(Move(aOnRun)) {}
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private:
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void
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Run()
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{
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return mOnRun();
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}
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OnRunType mOnRun;
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};
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template<typename OnRunType>
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LambdaTask<OnRunType>*
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NewTaskFrom(OnRunType&& aOnRun)
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{
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return new LambdaTask<OnRunType>(Forward<OnRunType>(aOnRun));
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}
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/* media::CoatCheck - There and back again. Park an object in exchange for an id.
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*
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* A common problem with calling asynchronous functions that do work on other
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* threads or processes is how to pass in a heap object for use once the
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* function completes, without requiring that object to have threadsafe
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* refcounting, contain mutexes, be marshaled, or leak if things fail
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* (or worse, intermittent use-after-free because of lifetime issues).
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*
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* One solution is to set up a coat-check on the caller side, park your object
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* in exchange for an id, and send the id. Common in IPC, but equally useful
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* for same-process thread-hops, because by never leaving the thread there's
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* no need for objects to be threadsafe or use threadsafe refcounting. E.g.
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*
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* class FooDoer
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* {
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* CoatCheck<Foo> mOutstandingFoos;
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*
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* public:
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* void DoFoo()
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* {
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* RefPtr<Foo> foo = new Foo();
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* uint32_t requestId = mOutstandingFoos.Append(*foo);
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* sChild->SendFoo(requestId);
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* }
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*
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* void RecvFooResponse(uint32_t requestId)
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* {
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* RefPtr<Foo> foo = mOutstandingFoos.Remove(requestId);
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* if (foo) {
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* // use foo
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* }
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* }
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* };
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*
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* If you read media::Pledge earlier, here's how this is useful for pledges:
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*
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* class FooGetter
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* {
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* CoatCheck<Pledge<Foo>> mOutstandingPledges;
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*
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* public:
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* already_addRefed<Pledge<Foo>> GetFooAsynchronously()
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* {
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* RefPtr<Pledge<Foo>> p = new Pledge<Foo>();
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* uint32_t requestId = mOutstandingPledges.Append(*p);
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* sChild->SendFoo(requestId);
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* return p.forget();
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* }
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*
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* void RecvFooResponse(uint32_t requestId, const Foo& fooResult)
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* {
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* RefPtr<Foo> p = mOutstandingPledges.Remove(requestId);
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* if (p) {
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* p->Resolve(fooResult);
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* }
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* }
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* };
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*
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* This helper is currently optimized for very small sets (i.e. not optimized).
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* It is also not thread-safe as the whole point is to stay on the same thread.
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*/
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template<class T>
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class CoatCheck
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{
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public:
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typedef std::pair<uint32_t, RefPtr<T>> Element;
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uint32_t Append(T& t)
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{
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uint32_t id = GetNextId();
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mElements.AppendElement(Element(id, RefPtr<T>(&t)));
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return id;
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}
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already_AddRefed<T> Remove(uint32_t aId)
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{
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for (auto& element : mElements) {
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if (element.first == aId) {
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RefPtr<T> ref;
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ref.swap(element.second);
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mElements.RemoveElement(element);
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return ref.forget();
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}
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}
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MOZ_ASSERT_UNREACHABLE("Received id with no matching parked object!");
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return nullptr;
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}
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private:
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static uint32_t GetNextId()
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{
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static uint32_t counter = 0;
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return ++counter;
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};
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nsAutoTArray<Element, 3> mElements;
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};
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/* media::Refcountable - Add threadsafe ref-counting to something that isn't.
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*
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* Often, reference counting is the most practical way to share an object with
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* another thread without imposing lifetime restrictions, even if there's
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* otherwise no concurrent access happening on the object. For instance, an
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* algorithm on another thread may find it more expedient to modify a passed-in
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* object, rather than pass expensive copies back and forth.
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*
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* Lists in particular often aren't ref-countable, yet are expensive to copy,
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* e.g. nsTArray<RefPtr<Foo>>. Refcountable can be used to make such objects
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* (or owning smart-pointers to such objects) refcountable.
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*
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* Technical limitation: A template specialization is needed for types that take
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* a constructor. Please add below (ScopedDeletePtr covers a lot of ground though).
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*/
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template<typename T>
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class Refcountable : public T
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{
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public:
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NS_INLINE_DECL_THREADSAFE_REFCOUNTING(Refcountable<T>)
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private:
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~Refcountable<T>() {}
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};
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template<typename T>
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class Refcountable<ScopedDeletePtr<T>> : public ScopedDeletePtr<T>
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{
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public:
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explicit Refcountable<ScopedDeletePtr<T>>(T* aPtr) : ScopedDeletePtr<T>(aPtr) {}
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NS_INLINE_DECL_THREADSAFE_REFCOUNTING(Refcountable<T>)
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private:
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~Refcountable<ScopedDeletePtr<T>>() {}
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};
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/* media::ShutdownBlocker - Async shutdown helper.
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*/
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class ShutdownBlocker : public nsIAsyncShutdownBlocker
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{
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public:
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ShutdownBlocker(const nsString& aName) : mName(aName) {}
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NS_IMETHOD
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BlockShutdown(nsIAsyncShutdownClient* aProfileBeforeChange) override = 0;
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NS_IMETHOD GetName(nsAString& aName) override
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{
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aName = mName;
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return NS_OK;
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}
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NS_IMETHOD GetState(nsIPropertyBag**) override
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{
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return NS_OK;
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}
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NS_DECL_ISUPPORTS
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protected:
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virtual ~ShutdownBlocker() {}
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private:
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const nsString mName;
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};
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} // namespace media
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} // namespace mozilla
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#endif // mozilla_MediaUtils_h
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