void Download::on_downloadBtn_clicked()
{
    if(file.exists())
    {
        if(isDownload==false)
        {
            ui->downloadBtn->setText("暫停");
            isDownload=true;
            if(isDisconnect)
            {
                startDownload(ui->urlEdit->document()->toPlainText());
            }
        }
        else
        {
            ui->downloadBtn->setText("下載");
            isDownload=false;
            isDisconnect=true;
            disconnect(reply,0,0,0);//斷開與reply相關的所有連線
            reply->abort();
            reply->deleteLater();
            reply=NULL;
        }
    }
    if(file.exists()&&!isDownLoadError)
    {
        return;
    }
    QString url=ui->urlEdit->document()->toPlainText();
    int index=url.lastIndexOf('/');
    QString filename=url.mid(index+1);
    QString filePath=QFileDialog::getExistingDirectory(NULL,"儲存路徑","./");
    file.setFileName(filePath+'/'+filename);
    if(!file.open(QIODevice::ReadWrite|QIODevice::Append))
    {
        receivedBytes=file.size();
        QMessageBox::information(NULL,"info","open file fail");
        return;
    }
    {
        ui->downloadBtn->setText("暫停");
        isDownload=true;
        isDownLoadError=false;
    }
    startDownload(url);
}

void Download::startDownload(QString url)
{
    QNetworkRequest request;
    request.setUrl(QUrl(url));

    //支援斷點續傳
    QString strRange = QString("bytes=%1-").arg(receivedBytes);
    request.setRawHeader("Range", strRange.toUtf8());

    reply=manager->get(request);
    connect(reply,&QNetworkReply::readyRead,this,&Download::dealData);
    connect(reply,&QNetworkReply::finished,[this]()
    {
        reply->deleteLater();
        if(!isDownLoadError)
        {
            ui->processImg->setPixmap(QPixmap(":/resources/progress/finished.png"));
            ui->downloadBtn->setEnabled(false);
            ui->downloadBtn->setText("完成");
        }
        file.close();
    }
    );
    connect(reply,&QNetworkReply::downloadProgress,this,&Download::downloadProcess);
    connect(reply,QOverload<QNetworkReply::NetworkError>::of(&QNetworkReply::error),this,&Download::downloadError);
}

void Download::dealData()
{
    QNetworkReply*reply=dynamic_cast<QNetworkReply*>(sender());
    file.write(reply->readAll());
    receivedBytes=file.size();
}

void Download::downloadProcess(qint64 received, qint64 total)
{
    if(firstDownload)
    {
        /*
         * 把第一次下載時的檔案長度(即整個檔案的大小)儲存起來
         * 因為斷點續傳total會越來越小
        */
        fileSize=QString::number(total);
        firstDownload=false;
    }

    /*
     * 之所以不用函式提供的兩個引數(received、total),
     * 是因為搞了半天那個進度條顯示不正確，故採用此法
     * 如果你有更好的辦法，部落格留言。。。
     */
    ui->progressBar->setValue(receivedBytes);
    ui->progressBar->setMaximum(fileSize.toInt());
}