src_corelib_thread_qfuture.cpp

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// Copyright (C) 2016 The Qt Company Ltd.
// SPDX-License-Identifier: LicenseRef-Qt-Commercial OR BSD-3-Clause
 
#include <QFuture>
#include <QtConcurrent>
#include <QNetworkReply>
#include <QException>
#include <QImage>
#include <iostream>
 
using namespace std;
 
class Error : public QException
{
public:
    Error() = default;
    explicit Error(const QString &message);
};
 
template<typename T>
void handleResult(const QFuture<T> &future);
 
struct Result {};
 
//! [10]
class Object : public QObject
{
    Q_OBJECT
    //...
signals:
    void noArgSignal();
    void singleArgSignal(int value);
    void multipleArgs(int value1, double value2, const QString &value3);
};
//! [10]
 
void examples(QFuture<QString> someQStringFuture,
                QFuture<int> someIntFuture,
                QFuture<void> someVoidFuture,
                QObject *someObject,
                QUrl url)
{
    {
        //! [0]
        QFuture<QString> future = someQStringFuture;
 
        QFuture<QString>::const_iterator i;
        for (i = future.constBegin(); i != future.constEnd(); ++i)
            cout << qPrintable(*i) << endl;
        //! [0]
    }
 
    {
        //! [1]
        QFuture<QString> future;
        //...
        QFutureIterator<QString> i(future);
        while (i.hasNext())
            QString s = i.next();
        //! [1]
    }
 
    {
        QFuture<QString> future;
        //! [2]
        QFutureIterator<QString> i(future);
        i.toBack();
        while (i.hasPrevious())
            QString s = i.previous();
        //! [2]
    }
 
    {
        //! [3]
        using NetworkReply = std::variant<QByteArray, QNetworkReply::NetworkError>;
 
        enum class IOError { FailedToRead, FailedToWrite };
        using IOResult = std::variant<QString, IOError>;
        //! [3]
 
        //! [4]
        QFuture<IOResult> future = QtConcurrent::run([url] {
                //...
                return NetworkReply(QNetworkReply::TimeoutError);
        }).then([](NetworkReply reply) {
            if (auto error = std::get_if<QNetworkReply::NetworkError>(&reply))
                return IOResult(IOError::FailedToRead);
 
            auto data = std::get_if<QByteArray>(&reply);
            // try to write *data and return IOError::FailedToWrite on failure
            //...
        });
 
        auto result = future.result();
        if (auto filePath = std::get_if<QString>(&result)) {
            // do something with *filePath
        }
        else
        {
            // process the error
        }
        //! [4]
    }
 
    {
        //! [5]
        QFuture<int> future = someIntFuture;
            future.then([](QFuture<int> f) {
                try {
                    //...
                    auto result = f.result();
                    //...
                } catch (QException &e) {
                    // handle the exception
                }
            }).then([](){/*...*/});
        //! [5]
    }
 
    {
        //! [6]
        QFuture<int> future = someIntFuture;
        auto continuation = future.then([](int res1){ /*...*/ return res1; }).then([](int res2){ /*...*/ return res2; })/*...*/;
        //...
        // future throws an exception
        try {
            auto result = continuation.result();
        } catch (QException &e) {
            // handle the exception
        }
        //! [6]
    }
 
    {
        //! [7]
        QFuture<int> future = someIntFuture;
        auto resultFuture = future.then([](int res) {
            //...
            throw Error();
            //...
            return res;
        }).onFailed([](const Error &e) {
            // Handle exceptions of type Error
            //...
            return -1;
        }).onFailed([] {
            // Handle all other types of errors
            //...
            return -1;
        });
 
        auto result = resultFuture.result(); // result is -1
        //! [7]
    }
 
    {
        //! [8]
        QFuture<int> future = someIntFuture;
        future.then([](int res) {
            //...
            throw std::runtime_error("message");
            //...
        }).onFailed([](const std::exception &e) {
            // This handler will be invoked
            return -1;
        }).onFailed([](const std::runtime_error &e) {
            // This handler won't be invoked, because of the handler above.
            return -1;
        });
        //! [8]
    }
 
    {
        //! [9]
        QFuture<int> future = someIntFuture;
        auto resultFuture = future.then([](int res) {
            //...
            throw Error("message");
            //...
            return res;
        }).onFailed([](const std::exception &e) {
            // Won't be invoked
            return -1;
        }).onFailed([](const QException &e) {
            // Won't be invoked
            return -1;
        });
 
        try {
            auto result = resultFuture.result();
        } catch(QException &someException) {
            // Handle the exception
        }
        //! [9]
    }
 
    {
        //! [11]
        Object object;
        QFuture<void> voidFuture = QtFuture::connect(&object, &Object::noArgSignal);
        QFuture<int> intFuture = QtFuture::connect(&object, &Object::singleArgSignal);
 
        using Args = std::tuple<int, double, QString>;
        QFuture<Args> tupleFuture = QtFuture::connect(&object, &Object::multipleArgs);
        //! [11]
    }
 
    {
        Object object;
        //! [12]
        QtFuture::connect(&object, &Object::singleArgSignal).then([](int value) {
            // do something with the value
        });
        //! [12]
    }
 
    {
        Object object;
        //! [13]
        QtFuture::connect(&object, &Object::singleArgSignal).then(QtFuture::Launch::Async, [](int value) {
            // this will run in a new thread
        });
        //! [13]
    }
 
    {
        Object object;
        //! [14]
        QtFuture::connect(&object, &Object::singleArgSignal).then([](int value) {
            //...
            throw std::exception();
            //...
        }).onFailed([](const std::exception &e) {
            // handle the exception
        }).onFailed([] {
            // handle other exceptions
        });
        //! [14]
    }
 
    {
        //! [15]
        QFuture<int> testFuture = someIntFuture;
        auto resultFuture = testFuture.then([](int res) {
            // Block 1
        }).onCanceled([] {
            // Block 2
        }).onFailed([] {
            // Block 3
        }).then([] {
            // Block 4
        }).onFailed([] {
            // Block 5
        }).onCanceled([] {
            // Block 6
        });
        //! [15]
    }
 
    {
        //! [16]
        QFuture<int> testFuture = someIntFuture;
        auto resultFuture = testFuture.then([](int res) {
            // Block 1
        }).onFailed([] {
            // Block 3
        }).then([] {
            // Block 4
        }).onFailed([] {
            // Block 5
        }).onCanceled([] {
            // Block 6
        });
        //! [16]
    }
 
    {
        Result result;
        bool cachedResultsReady;
 
        //! [20]
        QObject *context /*...*/;
        auto future = cachedResultsReady ? QtFuture::makeReadyVoidFuture()
                                        : QtConcurrent::run([] { /* compute result */});
        auto continuation = future.then(context, [] {
            // Runs in the context's thread
        }).then([] {
            // May or may not run in the context's thread
        });
        //! [20]
    }
 
    {
        //! [21]
        QFuture<int> testFuture /*...*/;
        auto resultFuture = testFuture.then([](int res) {
            // Block 1
            //...
            return 1;
        }).then([](int res) {
            // Block 2
            //...
            return 2;
        }).onCanceled([] {
            // Block 3
            //...
            return -1;
        });
        //! [21]
    }
 
    {
        //! [22]
        QList<QFuture<int>> inputFutures {/*...*/};
 
        // whenAll has type QFuture<QList<QFuture<int>>>
        auto whenAll = QtFuture::whenAll(inputFutures.begin(), inputFutures.end());
 
        // whenAllVector has type QFuture<std::vector<QFuture<int>>>
        auto whenAllVector =
                QtFuture::whenAll<std::vector<QFuture<int>>>(inputFutures.begin(), inputFutures.end());
        //! [22]
    }
 
    {
        //! [23]
        QList<QFuture<int>> inputFutures {/*...*/};
 
        QtFuture::whenAll(inputFutures.begin(), inputFutures.end())
                .then([](const QList<QFuture<int>> &results) {
                    for (auto future : results) {
                        if (future.isCanceled())
                        {
                            // handle the cancellation (possibly due to an exception)
                        }
                        else
                        {
                            // do something with the result
                        }
                    }
                });
        //! [23]
    }
 
    {
        //! [24]
        QFuture<int> intFuture /*...*/;
        QFuture<QString> stringFuture /*...*/;
        QFuture<void> voidFuture /*...*/;
 
        using FuturesVariant = std::variant<QFuture<int>, QFuture<QString>, QFuture<void>>;
 
        // whenAll has type QFuture<QList<FuturesVariant>>
        auto whenAll = QtFuture::whenAll(intFuture, stringFuture, voidFuture);
 
        // whenAllVector has type QFuture<std::vector<FuturesVariant>>
        auto whenAllVector =
                QtFuture::whenAll<std::vector<FuturesVariant>>(intFuture, stringFuture, voidFuture);
        //! [24]
    }
 
    {
        //! [25]
        QFuture<int> intFuture /*...*/;
        QFuture<QString> stringFuture /*...*/;
        QFuture<void> voidFuture /*...*/;
 
        using FuturesVariant = std::variant<QFuture<int>, QFuture<QString>, QFuture<void>>;
 
        QtFuture::whenAll(intFuture, stringFuture, voidFuture)
                .then([](const QList<FuturesVariant> &results) {
                    //...
                    for (auto result : results)
                    {
                        // assuming handleResult() is overloaded based on the QFuture type
                        std::visit([](auto &&future) { handleResult(future); }, result);
                    }
                    //...
                });
        //! [25]
    }
 
    {
        //! [26]
        QList<QFuture<int>> inputFutures /*...*/;
 
        QtFuture::whenAny(inputFutures.begin(), inputFutures.end())
                .then([](const QtFuture::WhenAnyResult<int> &result) {
                    qsizetype index = result.index;
                    QFuture<int> future = result.future;
                    //...
                });
        //! [26]
    }
 
    {
        //! [27]
        QFuture<int> intFuture /*...*/;
        QFuture<QString> stringFuture /*...*/;
        QFuture<void> voidFuture /*...*/;
 
        using FuturesVariant = std::variant<QFuture<int>, QFuture<QString>, QFuture<void>>;
 
        QtFuture::whenAny(intFuture, stringFuture, voidFuture).then([](const FuturesVariant &result) {
            //...
            // assuming handleResult() is overloaded based on the QFuture type
            std::visit([](auto &&future) { handleResult(future); }, result);
            //...
        });
        //! [27]
    }
 
    {
        //! [28]
        QFuture<QFuture<int>> outerFuture /*...*/;
        QFuture<int> unwrappedFuture = outerFuture.unwrap();
        //! [28]
    }
 
    {
        auto scale = [](const QImage &img) { return img.scaled(100, 100); };
        auto reduceImages = [](QImage &result, const QImage &img) { return result; };
 
        //! [29]
        auto downloadImages = [] (const QUrl &url) {
            QList<QImage> images;
            //...
            return images;
        };
 
        auto processImages = [scale, reduceImages](const QList<QImage> &images) {
            return QtConcurrent::mappedReduced(images, scale, reduceImages);
        };
 
        auto show = [](const QImage &image) { /*...*/ };
 
        auto future = QtConcurrent::run(downloadImages, url)
                    .then(processImages)
                    .unwrap()
                    .then(show);
        //! [29]
    }
 
    {
        //! [30]
        QFuture<QFuture<QFuture<int>>> outerFuture;
        QFuture<int> unwrappedFuture = outerFuture.unwrap();
        //! [30]
    }
 
    {
        //! [31]
        QPromise<int> p;
 
        QFuture<int> f1 = p.future();
        f1.then([](int) { qDebug("first"); });
 
        QFuture<int> f2 = p.future();
        f2.then([](int) { qDebug("second"); });
 
        p.start();
        p.addResult(42);
        p.finish();
        //! [31]
    }
 
    {
        //! [32]
        const std::vector<int> values{1, 2, 3};
        auto f = QtFuture::makeReadyRangeFuture(values);
        //! [32]
    }
 
    {
        //! [33]
        auto f = QtFuture::makeReadyRangeFuture({1, 2, 3});
        //! [33]
    }
 
    {
        auto f = QtFuture::makeReadyRangeFuture({1, 2, 3});
        //! [34]
        const int count = f.resultCount(); // count == 3
        const auto results = f.results(); // results == { 1, 2, 3 }
        //! [34]
    }
 
    {
        //! [35]
        auto f = QtFuture::makeReadyValueFuture(std::make_unique<int>(42));
        //...
        const int result = *f.takeResult(); // result == 42
        //! [35]
    }
 
    {
        //! [36]
        auto f = QtFuture::makeReadyVoidFuture();
        //...
        const bool started = f.isStarted(); // started == true
        const bool running = f.isRunning(); // running == false
        const bool finished = f.isFinished(); // finished == true
        //! [36]
    }
 
    {
        //! [37]
        QObject *context /*...*/;
        QFuture<Result> future /*...*/;
        auto continuation = future.then(context, [context](Result result) {
                                    // ...
                                }).onCanceled([context = QPointer(context)] {
                                    if (!context)
                                        return;  // context was destroyed already
                                    // handle cancellation
                                });
 
        //! [37]
    }
 
    {
        //! [38]
        auto f = QtConcurrent::run([] {/*...*/})
                        .then([]{
                            // Then 1
                        })
                        .then([]{
                            // Then 2
                        })
                        .onCanceled([]{
                            // OnCanceled 1
                        })
                        .then([]{
                            // Then 3
                        })
                        .then([]{
                            // Then 4
                        })
                        .onCanceled([]{
                            // OnCanceled 2
                        });
        //...
        f.cancelChain();
        //! [38]
    }
}
 
class SomeClass : public QObject
{
    Q_OBJECT
 
public:
    void someMethod()
    {
        {
            //! [17]
            // somewhere in the main thread
            auto future = QtConcurrent::run([] {
                // This will run in a separate thread
                //...
            }).then(this, [] {
                // Update UI elements
            });
            //! [17]
        }
 
        {
            //! [18]
            auto future = QtConcurrent::run([] {
                //...
            }).then(this, [] {
                // Update UI elements
            }).then([] {
                // This will also run in the main thread
            });
            //! [18]
        }
 
        {
            //! [19]
            // somewhere in the main thread
            auto future = QtConcurrent::run([] {
                // This will run in a separate thread
                //...
                throw std::exception();
            }).onFailed(this, [] {
                // Update UI elements
            });
            //! [19]
        }
 
    }
};