Qt开发：QtConcurrent介绍和使用

一、QtConcurrent 简介

QtConcurrent 是 Qt 提供的一个高级并发编程模块，属于 QtConcurrent 命名空间，旨在简化多线程任务的执行。它支持并行执行算法（如 map、filter、reduce），还支持异步任务运行和结果管理，不需要显式管理 QThread。

模块介绍
头文件包含：

#include <QtConcurrent>

模块依赖：

QT += concurrent

常用功能：

二、常用功能分类

2.1 异步运行一个函数（无返回值）

QtConcurrent::run([]() {
    qDebug() << "后台线程中执行任务：" << QThread::currentThread();
});

2.2 异步运行一个带参数的函数（有返回值）

int add(int a, int b) {
    return a + b;
}

QFuture<int> future = QtConcurrent::run(add, 3, 5);

// 后续获取结果
int result = future.result();  // 阻塞直到完成
qDebug() << "结果是：" << result;

2.3 绑定类成员函数

class Worker {
public:
    int multiply(int x, int y) {
        return x * y;
    }
};

Worker worker;
QFuture<int> future = QtConcurrent::run(&worker, &Worker::multiply, 4, 6);

2.4 容器并行处理（map）

并发修改容器元素（原地修改）

QList<int> numbers = {1, 2, 3, 4, 5};

auto doubleIt = [](int &n) {
    n *= 2;
};

QtConcurrent::map(numbers, doubleIt);

// 输出: 2 4 6 8 10

并发映射容器到新容器（mapped）

QList<int> numbers = {1, 2, 3};

auto square = [](int n) {
    return n * n;
};

QList<int> squares = QtConcurrent::mapped(numbers, square).results();
// 输出: 1 4 9

并发过滤（filtered）

QStringList names = {"Alice", "Bob", "Eve"};

auto isShort = [](const QString &name) {
    return name.length() <= 3;
};

QStringList shortNames = QtConcurrent::filtered(names, isShort).results();
// 输出: Bob, Eve

2.5 使用 QFutureWatcher 监听结果（推荐与 UI 配合）

QFutureWatcher<QString> *watcher = new QFutureWatcher<QString>(this);

connect(watcher, &QFutureWatcher<QString>::finished, this, [=]() {
    QString result = watcher->result();
    qDebug() << "计算完成，结果为:" << result;
});

QFuture<QString> future = QtConcurrent::run([]() {
    QThread::sleep(2);
    return QString("Hello from thread");
});

watcher->setFuture(future);

三、线程池控制

QtConcurrent 默认使用全局 QThreadPool，可通过如下方式调整：

QThreadPool::globalInstance()->setMaxThreadCount(8);

或者使用局部线程池：

QThreadPool pool;
QtConcurrent::run(&pool, someFunction);

四、取消任务

可通过 QFuture 中的 cancel() 方法取消任务：

QFuture<void> future = QtConcurrent::run([]{
    for (int i = 0; i < 100; ++i) {
        if (QThread::currentThread()->isInterruptionRequested()) {
            return;
        }
        QThread::msleep(100);
    }
});

// 取消任务
future.cancel();

五、典型应用场景

六、完整示例：并发下载图片

效果目标：

核心类：ImageDownloader

// ImageDownloader.h

#ifndef IMAGEDOWNLOADER_H
#define IMAGEDOWNLOADER_H

#include <QObject>
#include <QNetworkAccessManager>
#include <QNetworkReply>
#include <QSemaphore>

class ImageDownloader : public QObject
{
    Q_OBJECT
public:
    explicit ImageDownloader(QObject *parent = nullptr);

    void setMaxConcurrent(int count);  // 设置最大并发数
    void downloadImages(const QStringList &urls, const QString &saveDir);

signals:
    void imageDownloaded(const QString &url, const QString &filePath);
    void downloadFailed(const QString &url, const QString &error);
    void allFinished();

private:
    void downloadOne(const QString &url, const QString &saveDir, int retryCount = 3);

    QSemaphore m_semaphore;   // 控制并发数
    int m_maxConcurrent = 5;
};

#endif // IMAGEDOWNLOADER_H

// ImageDownloader.cpp

#include "ImageDownloader.h"
#include <QDir>
#include <QFile>
#include <QFileInfo>
#include <QNetworkRequest>
#include <QThread>
#include <QDebug>
#include <QtConcurrent>

ImageDownloader::ImageDownloader(QObject *parent)
    : QObject(parent), m_semaphore(m_maxConcurrent)
{}

void ImageDownloader::setMaxConcurrent(int count)
{
    m_maxConcurrent = count;
    m_semaphore = QSemaphore(m_maxConcurrent);
}

void ImageDownloader::downloadImages(const QStringList &urls, const QString &saveDir)
{
    for (const QString &url : urls) {
        QtConcurrent::run([=]() {
            m_semaphore.acquire(); // 限制并发
            downloadOne(url, saveDir);
            m_semaphore.release();
        });
    }

    // 可选：全部下载完再发信号（略）
}

void ImageDownloader::downloadOne(const QString &urlStr, const QString &saveDir, int retryCount)
{
    QNetworkAccessManager manager;
    QNetworkRequest request(QUrl(urlStr));

    QNetworkReply *reply = manager.get(request);
    QEventLoop loop;

    QObject::connect(reply, &QNetworkReply::finished, &loop, &QEventLoop::quit);
    loop.exec();

    if (reply->error() == QNetworkReply::NoError) {
        QByteArray data = reply->readAll();

        QUrl url(urlStr);
        QString fileName = QFileInfo(url.path()).fileName();
        QString fullPath = QDir(saveDir).filePath(fileName);

        QFile file(fullPath);
        if (file.open(QIODevice::WriteOnly)) {
            file.write(data);
            file.close();
            emit imageDownloaded(urlStr, fullPath);
        } else {
            emit downloadFailed(urlStr, "文件保存失败");
        }
    } else {
        if (retryCount > 0) {
            qDebug() << "下载失败，重试:" << urlStr;
            downloadOne(urlStr, saveDir, retryCount - 1);
        } else {
            emit downloadFailed(urlStr, reply->errorString());
        }
    }

    reply->deleteLater();
}

使用示例：

ImageDownloader *downloader = new ImageDownloader(this);
downloader->setMaxConcurrent(5); // 最多5张图同时下载

QStringList urls = {
    "https://example.com/a.jpg",
    "https://example.com/b.jpg",
    "https://example.com/c.jpg"
};

QString savePath = QDir::currentPath() + "/images";

connect(downloader, &ImageDownloader::imageDownloaded, this, [](const QString &url, const QString &path){
    qDebug() << "下载成功:" << url << " -> " << path;
});

connect(downloader, &ImageDownloader::downloadFailed, this, [](const QString &url, const QString &error){
    qWarning() << "下载失败:" << url << error;
});

downloader->downloadImages(urls, savePath);