qt - 如何通过 http 下载和保存图像？答案

【问题标题】：qt - how to download and save image via http?qt - 如何通过 http 下载和保存图像？
【发布时间】：2011-06-08 21:45:53
【问题描述】：

我正在尝试在控制台应用程序中使用 Qt 下载和保存一些图像。这是我目前得到的，（所有代码正在编译，但运行后，似乎没有进入replyFinished()函数......）

void Test::start()
{
    std::cout << "start1";
    QNetworkAccessManager *manager = new QNetworkAccessManager();
    connect(manager, SIGNAL(finished(QNetworkReply*)), this, SLOT(replyFinished(QNetworkReply*)));
    manager->get(QNetworkRequest(QUrl("http://www.exylum.mydevil.net/firefox.jpg")));
}

void Test::replyFinished(QNetworkReply* reply)
{
    std::cout << "st";
    QImage* img2 = new QImage();
    img2->loadFromData(reply->readAll());

    if(img2->isNull())
        std::cout << "oops";

    if(img2->save("omg2.jpg", "JPG"))
        std::cout << "saved";
    else
        std::cout << "dont...";
}

【问题讨论】：

你能展示一下你是如何使用Test类的吗？
就Test *t = new Test(); t->start();

标签： c++ windows qt

【解决方案1】：

使用 QNetworkAccessManager 下载图片

头文件

#ifndef QDOWNLOADER_H
#define QDOWNLOADER_H

#include <QObject>
#include <QNetworkAccessManager>
#include <QNetworkRequest>
#include <QNetworkReply>
#include <QFile>
#include <QStringList>

class QDownloader : public QObject
{
    Q_OBJECT
public:
    explicit QDownloader(QObject *parent = 0);
    virtual ~QDownloader();
    void setFile(QString fileURL);

private:
    QNetworkAccessManager *manager;
    QNetworkReply *reply;
    QFile *file;

private slots:
    void onDownloadProgress(qint64,qint64);
    void onFinished(QNetworkReply*);
    void onReadyRead();
    void onReplyFinished();
};

#endif // QDOWNLOADER_H

源文件

#include "qdownloader.h"

QDownloader::QDownloader(QObject *parent) :
    QObject(parent)
{
    manager = new QNetworkAccessManager;
}

QDownloader::~QDownloader()
{
    manager->deleteLater();
}

void QDownloader::setFile(QString fileURL)
{
    QString filePath = fileURL;
    QString saveFilePath;
    QStringList filePathList = filePath.split('/');
    QString fileName = filePathList.at(filePathList.count() - 1);
    saveFilePath = QString("C:/Images/" + fileName );

    QNetworkRequest request;
    request.setUrl(QUrl(fileURL));
    reply = manager->get(request);

    file = new QFile;
    file->setFileName(saveFilePath);
    file->open(QIODevice::WriteOnly);

    connect(reply,SIGNAL(downloadProgress(qint64,qint64)),this,SLOT(onDownloadProgress(qint64,qint64)));
    connect(manager,SIGNAL(finished(QNetworkReply*)),this,SLOT(onFinished(QNetworkReply*)));
    connect(reply,SIGNAL(readyRead()),this,SLOT(onReadyRead()));
    connect(reply,SIGNAL(finished()),this,SLOT(onReplyFinished()));
}

void QDownloader::onDownloadProgress(qint64 bytesRead,qint64 bytesTotal)
{
    qDebug(QString::number(bytesRead).toLatin1() +" - "+ QString::number(bytesTotal).toLatin1());
}

void QDownloader::onFinished(QNetworkReply * reply)
{
    switch(reply->error())
    {
        case QNetworkReply::NoError:
        {
            qDebug("file is downloaded successfully.");
        }break;
        default:{
            qDebug(reply->errorString().toLatin1());
        };
    }

    if(file->isOpen())
    {
        file->close();
        file->deleteLater();
    }
}

void QDownloader::onReadyRead()
{
    file->write(reply->readAll());
}

void QDownloader::onReplyFinished()
{
    if(file->isOpen())
    {
        file->close();
        file->deleteLater();
    }
}

【讨论】：

这会在我调用 setFile 时创建文件，但它们是空文件。因为在我关闭程序之前我无法删除它们，所以我假设我需要调用其他东西才能完成下载过程？

【解决方案2】：

这是一个老问题，但作为参考，我将发布@lwinhtooko 代码的工作版本：

下载器.h：

#pragma once

#include <QFile>
#include <QNetworkAccessManager>
#include <QNetworkRequest>
#include <QNetworkReply>
#include <QObject>
#include <QUrl>

class Downloader : public QObject {
    Q_OBJECT
    QFile *m_file;
    bool m_isReady = true;

public:
    explicit Downloader(QObject *parent = 0) : QObject(parent) {}
    virtual ~Downloader() { delete m_file; }

    void downloadFileFromURL(const QString &url, const QString &filePath);

private slots:
    void onDownloadFileComplete(QNetworkReply *reply);
};

下载器.cpp：

#include "downloader.h"

void Downloader::downloadFileFromURL(const QString &url, const QString &filePath) {
    if (!m_isReady)
        return;
    m_isReady = false;

    const QString fileName = filePath + url.right(url.size() - url.lastIndexOf("/")); // your filePath should end with a forward slash "/"
    m_file = new QFile();
    m_file->setFileName(fileName);
    m_file->open(QIODevice::WriteOnly);
    if (!m_file->isOpen()) {
        m_isReady = true;
        return; // TODO: permission check?
    }

    QNetworkAccessManager *manager = new QNetworkAccessManager;

    QNetworkRequest request;
    request.setUrl(QUrl(url));

    connect(manager, SIGNAL(finished(QNetworkReply *)), this, SLOT(onDownloadFileComplete(QNetworkReply *)));

    manager->get(request);
}

void Downloader::onDownloadFileComplete(QNetworkReply *reply) {
    if (!m_file->isWritable()) {
        m_isReady = true;
        return; // TODO: error check
    }

    m_file->write(reply->readAll());
    m_file->close(); // TODO: delete the file from the system later on
    m_isReady = true;
}

问题似乎是 QNetworkReply 的使用方式。

【讨论】：

很棒的答案。 ty

【解决方案3】：

以上解决方案都不适用于我。但我发现这个 Qt wiki 页面非常适合所有类型的数据。

Download Data from URL

【讨论】：

【解决方案4】：

头文件

#ifndef IMAGEDOWNLOAD_H
#define IMAGEDOWNLOAD_H

#include <QWidget>
#include <QHttp>
#include <QFile>
#include <QUrl>

class ImageDownload : public QWidget
{
    Q_OBJECT
public:
    explicit ImageDownload(QWidget *parent = 0);

private:
    int httpGetId;
    QHttp http;
    QFile myfile;

private slots:
    void httpRequestFinished(int, bool);
    void progress(int,int);
};

#endif // IMAGEDOWNLOAD_H

源文件

#include "imagedownload.h"

ImageDownload::ImageDownload(QWidget *parent) :
    QWidget(parent)
{
    QUrl url("url of image.");

    myfile.setFileName("C:/Qt/imagefilename");
    myfile.open(QIODevice::WriteOnly);

    connect(&http,SIGNAL(requestFinished(int,bool)),this,SLOT(httpRequestFinished(int,bool)));
    connect(&http,SIGNAL(dataReadProgress(int,int)),this,SLOT(progress(int,int)));
    http.setHost(url.host(),QHttp::ConnectionModeHttp,url.port());
    httpGetId = http.get(url.path(),&myfile);
}

void ImageDownload::httpRequestFinished(int id, bool error)
{
    if(id == httpGetId)
    {
        myfile.close();
    }
    if(error)
    {
        qDebug(http.errorString().toLatin1());
    }
}

void ImageDownload::progress(int a, int b)
{
    qDebug(QString::number(a).toLatin1()+" : "+QString::number(b).toLatin1());
}

【讨论】：

对不起，这不起作用，（在我修复了一些错误之后）它仍然没有下载任何东西...... :(
exylum.mydevil.net/firefox.jpg 不是有效的网址。你试过其他网址吗？
是的，我正在尝试，现在我还添加了 file.open(QFile::WriteOnly);
您可以将 QWidget 更改为您的类名，即 QObject 的子类。
但我仍然发现它无法下载某些图像文件并显示错误“连接被拒绝（或超时）”

【解决方案5】：

这个问题很老，但我遇到了类似的问题，现在我有了 Qt 5.8 的解决方案。

代码旨在快速运行，网络请求以异步方式完成。因此，您必须为每个调用提供一个 ID，以了解哪个重放已完成。

此外，此代码使用 SSL。如果您不想使用 SSL 加密，请使用 QSslConfiguartion 删除 4 行。

文件下载器.h：

#ifndef FILEDOWNLOADER_H
#define FILEDOWNLOADER_H

#include <QObject>
#include <QStringList>
#include <QFile>
#include <QDir>

#include <QNetworkAccessManager>
#include <QNetworkRequest>
#include <QNetworkReply>

class FileDownloader : public QObject
{
    Q_OBJECT

public:
    explicit FileDownloader(QObject *parent = 0);
    virtual ~FileDownloader();
    void downloadFile(QUrl url, QString id, QString dir_absolute_path);

signals:
    // emits error string
    void error(QString);
    // Emits path to img on disk and id
    void downloaded(QString, QString);

private slots:
    void fileDownloaded();
    void onReadyRead();

private:
    QNetworkAccessManager *webCtrl;
    QMap<QNetworkReply*, QFile*> replytofile;
    QMap<QNetworkReply*, QPair<QString, QString> > replytopathid;

    const QByteArray userAgent = "Mozilla/5.0 (Windows NT 6.1; Win64; x64) AppleWebKit/537.36 (KHTML, like Gecko) Chrome/62.0.3202.94 Safari/537.36";
};

#endif // FILEDOWNLOADER_H

文件下载器.cpp：

#include "filedownloader.h"
#include <QDebug>

FileDownloader::FileDownloader(QObject *parent) :
    QObject(parent),
    webCtrl(new QNetworkAccessManager(this))
{

}

FileDownloader::~FileDownloader() 
{
    delete webCtrl;
}

void FileDownloader::downloadFile(QUrl url, QString id, QString dir_absolute_path)
{
    QString url_string = url.toString();
    QString path = dir_absolute_path + url_string.right(url_string.size() - url_string.lastIndexOf("/"));

    QFile *file = new QFile(path, this);
    if(!file->open(QIODevice::WriteOnly))
    {
        return;
    }

    QNetworkRequest request(url);
    request.setRawHeader("User-Agent", userAgent);

    QSslConfiguration sslConfiguration(QSslConfiguration::defaultConfiguration());
    sslConfiguration.setPeerVerifyMode(QSslSocket::VerifyNone);
    sslConfiguration.setProtocol(QSsl::AnyProtocol);
    request.setSslConfiguration(sslConfiguration);

    QNetworkReply *reply = webCtrl->get(request);
    replytofile.insert(reply, file);
    replytopathid.insert(reply, QPair<QString, QString>(path, id));

    QObject::connect(reply, &QNetworkReply::finished, this, &FileDownloader::fileDownloaded);
    QObject::connect(reply, &QNetworkReply::readyRead, this, &FileDownloader::onReadyRead);
}

void FileDownloader::fileDownloaded()
{
    QNetworkReply* reply = qobject_cast<QNetworkReply*>(sender());

    if (replytofile[reply]->isOpen())
    {
        replytofile[reply]->close();
        replytofile[reply]->deleteLater();
    }

    switch(reply->error())
    {
    case QNetworkReply::NoError:
        break;

    default:
        emit error(reply->errorString().toLatin1());
        break;
    }

    emit downloaded(replytopathid[reply].first, replytopathid[reply].second);

    replytofile.remove(reply);
    replytopathid.remove(reply);
    delete reply;
}

void FileDownloader::onReadyRead()
{
    QNetworkReply* reply = qobject_cast<QNetworkReply*>(sender());

    replytofile[reply]->write(reply->readAll());
}

【讨论】：