HTTP 编程（下载文件）

思路：

（1）、刚开始时隐藏进度条

（2）、按下下载按钮后，首先取得URL，用来创建文件，并且构造请求

（3）、发送请求，连接信号与槽（槽是准备读数据、更新进度条、相应结束的槽）

（4）、读取相应信息并且写入文件

（5）、重新隐藏进度条，关闭文件，删除应答等

#ifndef MAINWINDOW_H
#define MAINWINDOW_H

#include <QMainWindow>
#include <QNetworkAccessManager>
#include <QNetworkRequest>
#include <QNetworkReply>
#include <QFile>

namespace Ui {
class MainWindow;
}

class MainWindow : public QMainWindow
{
    Q_OBJECT
    
public:
    explicit MainWindow(QWidget *parent = 0);
    ~MainWindow();
    
private:
    Ui::MainWindow *ui;
    QNetworkAccessManager *manager;
    QFile* file;
    QNetworkReply * reply;//注意要定义成指针的形式

private slots:
    void onBtnClicked();
    void downloadProgress(qint64 bytesReceived, qint64 bytesTotal);
    void readyRead();
    void finished();
};

#endif // MAINWINDOW_H

#include "MainWindow.h"
#include "ui_MainWindow.h"
#include <QUrl>
#include <QString>
#include <QFileInfo>
#include <QFile>
#include <QDebug>

MainWindow::MainWindow(QWidget *parent) :
    QMainWindow(parent),
    ui(new Ui::MainWindow)
{
    ui->setupUi(this);
    manager = new QNetworkAccessManager(this);
    ui->progressBar->hide();
    connect(ui->pushButton, SIGNAL(clicked()), this, SLOT(onBtnClicked()));
}

void MainWindow::onBtnClicked()
{
    QString text = ui->lineEdit->text();
    QUrl url(text);

    QFileInfo info(url.path());
    QString fileName(info.fileName());
    file = new QFile(fileName);

    if( ! file->open(QIODevice::WriteOnly))
    {
        qDebug() << "error";
        delete file;
        file = 0;
        return;
    }

    QNetworkRequest request;//1.构建请求
    request.setUrl(url);

    reply = manager->get(request);//2.发送请求
    connect(reply, SIGNAL(readyRead()), this, SLOT(readyRead()));
    connect(reply, SIGNAL(downloadProgress(qint64,qint64)), this, SLOT(downloadProgress(qint64,qint64)));
    connect(reply, SIGNAL(finished()), this, SLOT(finished()));

}

void MainWindow::downloadProgress(qint64 bytesReceived, qint64 bytesTotal)
{
    ui->progressBar->setMaximum(bytesTotal);
    ui->progressBar->setValue(bytesReceived);
}
void MainWindow::readyRead()
{
    if(file)
    {
        file->write(reply->readAll());//3.处理应答
        ui->progressBar->show();
    }
}
void MainWindow::finished()
{
    ui->progressBar->hide();
    file->flush();
    file->close();
    reply->deleteLater();
}

MainWindow::~MainWindow()
{
    delete ui;
}

#include <QtGui/QApplication>
#include "MainWindow.h"

int main(int argc, char *argv[])
{
    QApplication a(argc, argv);
    MainWindow w;
    w.show();
    
    return a.exec();
}