qtconcurrentthreadengine.h 8.4 KB
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/****************************************************************************
**
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** Copyright (C) 2016 The Qt Company Ltd.
** Contact: https://www.qt.io/licensing/
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**
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** This file is part of the QtConcurrent module of the Qt Toolkit.
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**
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** $QT_BEGIN_LICENSE:LGPL$
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** Commercial License Usage
** Licensees holding valid commercial Qt licenses may use this file in
** accordance with the commercial license agreement provided with the
** Software or, alternatively, in accordance with the terms contained in
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** a written agreement between you and The Qt Company. For licensing terms
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** and conditions see https://www.qt.io/terms-conditions. For further
** information use the contact form at https://www.qt.io/contact-us.
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**
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** GNU Lesser General Public License Usage
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** Alternatively, this file may be used under the terms of the GNU Lesser
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** General Public License version 3 as published by the Free Software
** Foundation and appearing in the file LICENSE.LGPL3 included in the
** packaging of this file. Please review the following information to
** ensure the GNU Lesser General Public License version 3 requirements
** will be met: https://www.gnu.org/licenses/lgpl-3.0.html.
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**
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** GNU General Public License Usage
** Alternatively, this file may be used under the terms of the GNU
** General Public License version 2.0 or (at your option) the GNU General
** Public license version 3 or any later version approved by the KDE Free
** Qt Foundation. The licenses are as published by the Free Software
** Foundation and appearing in the file LICENSE.GPL2 and LICENSE.GPL3
** included in the packaging of this file. Please review the following
** information to ensure the GNU General Public License requirements will
** be met: https://www.gnu.org/licenses/gpl-2.0.html and
** https://www.gnu.org/licenses/gpl-3.0.html.
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**
** $QT_END_LICENSE$
**
****************************************************************************/

#ifndef QTCONCURRENT_THREADENGINE_H
#define QTCONCURRENT_THREADENGINE_H

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#include <QtConcurrent/qtconcurrent_global.h>
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#if !defined(QT_NO_CONCURRENT) ||defined(Q_CLANG_QDOC)
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#include <QtCore/qthreadpool.h>
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#include <QtCore/qfuture.h>
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#include <QtCore/qdebug.h>
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#include <QtCore/qexception.h>
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#include <QtCore/qwaitcondition.h>
#include <QtCore/qatomic.h>
#include <QtCore/qsemaphore.h>

QT_BEGIN_NAMESPACE


namespace QtConcurrent {

// The ThreadEngineBarrier counts worker threads, and allows one
// thread to wait for all others to finish. Tested for its use in
// QtConcurrent, requires more testing for use as a general class.
class ThreadEngineBarrier
{
private:
    // The thread count is maintained as an integer in the count atomic
    // variable. The count can be either positive or negative - a negative
    // count signals that a thread is waiting on the barrier.

    QAtomicInt count;
    QSemaphore semaphore;
public:
    ThreadEngineBarrier();
    void acquire();
    int release();
    void wait();
    int currentCount();
    bool releaseUnlessLast();
};

enum ThreadFunctionResult { ThrottleThread, ThreadFinished };

// The ThreadEngine controls the threads used in the computation.
// Can be run in three modes: single threaded, multi-threaded blocking
// and multi-threaded asynchronous.
// The code for the single threaded mode is
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class Q_CONCURRENT_EXPORT ThreadEngineBase: public QRunnable
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{
public:
    // Public API:
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    ThreadEngineBase(QThreadPool *pool);
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    virtual ~ThreadEngineBase();
    void startSingleThreaded();
    void startBlocking();
    void startThread();
    bool isCanceled();
    void waitForResume();
    bool isProgressReportingEnabled();
    void setProgressValue(int progress);
    void setProgressRange(int minimum, int maximum);
    void acquireBarrierSemaphore();
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    void reportIfSuspensionDone() const;
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protected: // The user overrides these:
    virtual void start() {}
    virtual void finish() {}
    virtual ThreadFunctionResult threadFunction() { return ThreadFinished; }
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    virtual bool shouldStartThread() { return !shouldThrottleThread(); }
    virtual bool shouldThrottleThread()
    {
        return futureInterface ? (futureInterface->isSuspending() || futureInterface->isSuspended())
                               : false;
    }

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private:
    bool startThreadInternal();
    void startThreads();
    void threadExit();
    bool threadThrottleExit();
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    void run() override;
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    virtual void asynchronousFinish() = 0;
#ifndef QT_NO_EXCEPTIONS
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    void handleException(const QException &exception);
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#endif
protected:
    QFutureInterfaceBase *futureInterface;
    QThreadPool *threadPool;
    ThreadEngineBarrier barrier;
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    QtPrivate::ExceptionStore exceptionStore;
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    QBasicMutex mutex;
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};


template <typename T>
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class ThreadEngine : public ThreadEngineBase
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{
public:
    typedef T ResultType;

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    ThreadEngine(QThreadPool *pool) : ThreadEngineBase(pool) {}

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    virtual T *result() { return nullptr; }
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    QFutureInterface<T> *futureInterfaceTyped()
    {
        return static_cast<QFutureInterface<T> *>(futureInterface);
    }

    // Runs the user algorithm using a single thread.
    T *startSingleThreaded()
    {
        ThreadEngineBase::startSingleThreaded();
        return result();
    }

    // Runs the user algorithm using multiple threads.
    // This function blocks until the algorithm is finished,
    // and then returns the result.
    T *startBlocking()
    {
        ThreadEngineBase::startBlocking();
        return result();
    }

    // Runs the user algorithm using multiple threads.
    // Does not block, returns a future.
    QFuture<T> startAsynchronously()
    {
        futureInterface = new QFutureInterface<T>();

        // reportStart() must be called before starting threads, otherwise the
        // user algorithm might finish while reportStart() is running, which
        // is very bad.
        futureInterface->reportStarted();
        QFuture<T> future = QFuture<T>(futureInterfaceTyped());
        start();

        acquireBarrierSemaphore();
        threadPool->start(this);
        return future;
    }

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    void asynchronousFinish() override
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    {
        finish();
        futureInterfaceTyped()->reportFinished(result());
        delete futureInterfaceTyped();
        delete this;
    }


    void reportResult(const T *_result, int index = -1)
    {
        if (futureInterface)
            futureInterfaceTyped()->reportResult(_result, index);
    }

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    void reportResults(const QList<T> &_result, int index = -1, int count = -1)
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    {
        if (futureInterface)
            futureInterfaceTyped()->reportResults(_result, index, count);
    }
};

// The ThreadEngineStarter class ecapsulates the return type
// from the thread engine.
// Depending on how the it is used, it will run
// the engine in either blocking mode or asynchronous mode.
template <typename T>
class ThreadEngineStarterBase
{
public:
    ThreadEngineStarterBase(ThreadEngine<T> *_threadEngine)
    : threadEngine(_threadEngine) { }

    inline ThreadEngineStarterBase(const ThreadEngineStarterBase &other)
    : threadEngine(other.threadEngine) { }

    QFuture<T> startAsynchronously()
    {
        return threadEngine->startAsynchronously();
    }

    operator QFuture<T>()
    {
        return startAsynchronously();
    }

protected:
    ThreadEngine<T> *threadEngine;
};


// We need to factor out the code that dereferences the T pointer,
// with a specialization where T is void. (code that dereferences a void *
// won't compile)
template <typename T>
class ThreadEngineStarter : public ThreadEngineStarterBase<T>
{
    typedef ThreadEngineStarterBase<T> Base;
    typedef ThreadEngine<T> TypedThreadEngine;
public:
    ThreadEngineStarter(TypedThreadEngine *eng)
        : Base(eng) { }

    T startBlocking()
    {
        T t = *this->threadEngine->startBlocking();
        delete this->threadEngine;
        return t;
    }
};

// Full template specialization where T is void.
template <>
class ThreadEngineStarter<void> : public ThreadEngineStarterBase<void>
{
public:
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    ThreadEngineStarter(ThreadEngine<void> *_threadEngine)
        : ThreadEngineStarterBase<void>(_threadEngine) {}
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    void startBlocking()
    {
        this->threadEngine->startBlocking();
        delete this->threadEngine;
    }
};

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//! [qtconcurrentthreadengine-1]
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template <typename ThreadEngine>
inline ThreadEngineStarter<typename ThreadEngine::ResultType> startThreadEngine(ThreadEngine *threadEngine)
{
    return ThreadEngineStarter<typename ThreadEngine::ResultType>(threadEngine);
}

} // namespace QtConcurrent


QT_END_NAMESPACE

#endif // QT_NO_CONCURRENT

#endif