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Thread: [QT 5.0] Downloading from a URL then saving to client-defined path/filename?

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  1. 22nd January 2013, 08:11 #1
    anda_skoa
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    Default Re: [QT 5.0] Downloading from a URL then saving to client-defined path/filename?

    Two other things:

    - you don't need a new QNetworkManager for every request, just one will do fine (can handle multiple requests in parallel)
    - your code becomes the owner of the returned QNetworkReply so your code has to delete it. The easiest way to do that is to call reply->deleteLater() in the finished slot.

    Cheers,
    _
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  2. 23rd January 2013, 07:17 #2
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    Salads
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    Default Re: [QT 5.0] Downloading from a URL then saving to client-defined path/filename?

    So if I put the following code where QMainWindow function is it won't have to make a new one every time? Would this help with performance?
    EDIT: What if I set it as a public class definition? I have QNetworkAccessManager *manager; but the program crashes when I try to send the request. (When I press the pushButton originally set to send it)
    Qt Code: 
    1. QNetworkAccessManager *manager = new QNetworkAccessManager(this);
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    Here's my complete .cpp file for reference:
    Qt Code: 
    1. #include "minecraftskindownloader.h"
    2. #include "ui_minecraftskindownloader.h"
    3. #include "nouser.h"
    4. #include <QWidget>
    5. #include <QObject>
    6. #include <QtNetwork/QNetworkAccessManager>
    7. #include <QFileDialog>
    8. #include <QtNetwork/QNetworkRequest>
    9. #include <QDebug>
    10. #include <QUrl>
    11. #include <QFile>
    12. #include <QPixmap>
    13. #include <QMessageBox>
    14.  
    15. MinecraftSkinDownloader::MinecraftSkinDownloader(QWidget *parent) :
    16.     QMainWindow(parent),
    17.     ui(new Ui::MinecraftSkinDownloader)
    18. {
    19.     ui->setupUi(this);
    20.     this->setWindowTitle("Minecraft Skin Downloader");
    21.     setFixedSize(400, 200);
    22.     setStyleSheet("QMainWindow { background-image: url(:/bg.png); } ");
    23.  
    24.     connect(ui->save, SIGNAL(clicked()), this, SLOT(onsaveAs_clicked()));
    25. }
    26.  
    27. MinecraftSkinDownloader::~MinecraftSkinDownloader()
    28. {
    29.     delete ui;
    30. }
    31.  
    32. //http://s3.amazonaws.com/MinecraftSkins/USERNAMEHERE.png
    33.  
    34. void MinecraftSkinDownloader::on_pushButton_clicked()
    35. {
    36.     QString prefix = "http://s3.amazonaws.com/MinecraftSkins/";
    37.     QString usrl = prefix += ui->lineEdit->text() += ".png";
    38.  
    39.     QNetworkAccessManager *manager = new QNetworkAccessManager(this);
    40.     connect(manager, SIGNAL(finished(QNetworkReply*)), this, SLOT(slot_netwManagerFinished(QNetworkReply*)));
    41.  
    42.     QUrl url(usrl);
    43.     QNetworkRequest request(url);
    44.  
    45.     manager->get(request);
    46.  
    47. }
    48.  
    49. void MinecraftSkinDownloader::slot_netwManagerFinished(QNetworkReply *reply)
    50. {
    51.  
    52.     if (reply->error() != QNetworkReply::NoError)
    53.     {
    54.         qDebug() << "Error in" << reply->url() << ":" << reply->errorString();
    55.         NoUser mDiag;
    56.         mDiag.setModal(true);
    57.         mDiag.exec();
    58.         return;
    59.     }
    60.  
    61.     MinecraftSkinDownloader::pngData = reply->readAll();
    62.     QPixmap pixmap;
    63.     pixmap.loadFromData(MinecraftSkinDownloader::pngData);
    64.     QPixmap scaled = pixmap.scaled(128,64,Qt::IgnoreAspectRatio, Qt::FastTransformation);
    65.     ui->preview->show();
    66.     ui->preview->setStyleSheet("QLabel { background-color: white; }");
    67.     ui->preview->setPixmap(scaled);
    68.     reply->deleteLater();
    69.  
    70. }
    71.  
    72. void MinecraftSkinDownloader::onsaveAs_clicked()
    73. {
    74.     QString filename = QFileDialog::getSaveFileName(this, tr("Save Skin"), "C:/Users/", ("PNG Image (*.png)"));
    75.  
    76.     QFile file(filename);
    77.     file.open(QIODevice::WriteOnly);
    78.     file.write(MinecraftSkinDownloader::pngData);
    79.     file.close();
    80. }
    81.  
    82. void MinecraftSkinDownloader::on_pushButton_2_clicked()
    83. {
    84.     ui->lineEdit->clear();
    85.     ui->preview->hide();
    86. }
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    Last edited by Salads; 23rd January 2013 at 07:35.
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  3. 23rd January 2013, 13:06 #3
    anda_skoa
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    Default Re: [QT 5.0] Downloading from a URL then saving to client-defined path/filename?

    Quote Originally Posted by Salads View Post
    [COLOR="#D3D3D3"]
    EDIT: What if I set it as a public class definition? I have QNetworkAccessManager *manager; but the program crashes when I try to send the request. (When I press the pushButton originally set to send it)
    I am not sure what you mean with that, the QNetworkManager creation in the code you posted seems to be unchanged.

    Btw, you'll also have to deleted the network reply in the error handling case

    Cheers,
    _
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  4. The following user says thank you to anda_skoa for this useful post:

    Salads (23rd January 2013)
  5. 23rd January 2013, 20:02 #4
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    Salads
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    Default Re: [QT 5.0] Downloading from a URL then saving to client-defined path/filename?

    Got the error part thanks

    I'll update my code:

    .h File:
    Qt Code: 
    1. #ifndef MINECRAFTSKINDOWNLOADER_H
    2. #define MINECRAFTSKINDOWNLOADER_H
    3.  
    4. #include <QMainWindow>
    5. #include <QtNetwork/QNetworkReply>
    6. #include <QtNetwork/QNetworkAccessManager>
    7.  
    8. namespace Ui {
    9. class MinecraftSkinDownloader;
    10. }
    11.  
    12. class MinecraftSkinDownloader : public QMainWindow
    13. {
    14.     Q_OBJECT
    15.  
    16. public:
    17.     explicit MinecraftSkinDownloader(QWidget *parent = 0);
    18.     ~MinecraftSkinDownloader();
    19.     QByteArray pngData;
    20.     QNetworkAccessManager *manager;
    21.  
    22. private slots:
    23.  
    24.     void slot_netwManagerFinished(QNetworkReply *reply);
    25.  
    26.     void onsaveAs_clicked();
    27.  
    28.     void on_clear_clicked();
    29.  
    30.     void on_pushButton_clicked();
    31.  
    32. private:
    33.     Ui::MinecraftSkinDownloader *ui;
    34.  
    35.  
    36. };
    37.  
    38. #endif // MINECRAFTSKINDOWNLOADER_H
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    .cpp File
    Qt Code: 
    1. #include "minecraftskindownloader.h"
    2. #include "ui_minecraftskindownloader.h"
    3. #include <QWidget>
    4. #include <QObject>
    5. #include <QtNetwork/QNetworkAccessManager>
    6. #include <QFileDialog>
    7. #include <QtNetwork/QNetworkRequest>
    8. #include <QDebug>
    9. #include <QUrl>
    10. #include <QFile>
    11. #include <QPixmap>
    12. #include <QMessageBox>
    13.  
    14. MinecraftSkinDownloader::MinecraftSkinDownloader(QWidget *parent) :
    15.     QMainWindow(parent),
    16.     ui(new Ui::MinecraftSkinDownloader)
    17. {
    18.     ui->setupUi(this);
    19.     this->setWindowTitle("Minecraft Skin Downloader");
    20.     QIcon icon(":/ico");
    21.     setWindowIcon(icon);
    22.     setFixedSize(400, 200);
    23.     setStyleSheet("QMainWindow { background-image: url(:/bg.png); } ");
    24.  
    25.     connect(ui->save, SIGNAL(clicked()), this, SLOT(onsaveAs_clicked()));
    26.  
    27. }
    28.  
    29. MinecraftSkinDownloader::~MinecraftSkinDownloader()
    30. {
    31.     delete ui;
    32. }
    33.  
    34. //http://s3.amazonaws.com/MinecraftSkins/USERNAMEHERE.png
    35.  
    36. void MinecraftSkinDownloader::on_pushButton_clicked()
    37. {
    38.     QString prefix = "http://s3.amazonaws.com/MinecraftSkins/";
    39.     QString usrl = prefix += ui->lineEdit->text() += ".png";
    40.  
    41.     //QNetworkAccessManager *manager = new QNetworkAccessManager(this);
    42.     connect(manager, SIGNAL(finished(QNetworkReply*)), this, SLOT(slot_netwManagerFinished(QNetworkReply*)));
    43.  
    44.     QUrl url(usrl);
    45.     QNetworkRequest request(url);
    46.  
    47.     manager->get(request);
    48.  
    49. }
    50.  
    51. void MinecraftSkinDownloader::slot_netwManagerFinished(QNetworkReply *reply)
    52. {
    53.  
    54.     if (reply->error() != QNetworkReply::NoError)
    55.     {
    56.         qDebug() << "Error in" << reply->url() << ":" << reply->errorString();
    57.         QPixmap noUsr(":/voidUser.png"); // Void User path
    58.         ui->preview->show();
    59.         ui->preview->setStyleSheet("background: transparent;");
    60.         ui->preview->setPixmap(noUsr);
    61.         reply->deleteLater();
    62.         return;
    63.     }
    64.  
    65.     pngData = reply->readAll();
    66.     QPixmap pixmap;
    67.     pixmap.loadFromData(pngData);
    68.     QPixmap scaled = pixmap.scaled(128,64,Qt::IgnoreAspectRatio, Qt::FastTransformation);
    69.     ui->preview->show();
    70.     ui->preview->setStyleSheet("QLabel { background-color: white; }");
    71.     ui->preview->setPixmap(scaled);
    72.     reply->deleteLater();
    73.  
    74. }
    75.  
    76. void MinecraftSkinDownloader::onsaveAs_clicked()
    77. {
    78.     if(pngData.isEmpty())
    79.     {
    80.         ui->preview->show();
    81.         QPixmap error(":/noSaveData.png");
    82.         ui->preview->setPixmap(error);
    83.         return;
    84.     }
    85.  
    86.     QString filename = QFileDialog::getSaveFileName(this, tr("Save Skin"), "C:/Users/", ("PNG Image (*.png)"));
    87.  
    88.     QFile file(filename);
    89.     file.open(QIODevice::WriteOnly);
    90.     file.write(pngData);
    91.     file.close();
    92. }
    93.  
    94. void MinecraftSkinDownloader::on_clear_clicked()
    95. {
    96.     pngData.clear();
    97.     ui->preview->setStyleSheet("background: transparent;");
    98.     ui->lineEdit->clear();
    99.     ui->preview->hide();
    100.     return;
    101. }
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    It crashes the moment I press that on_pushButton_clicked(), and doesn't even get to display the error message (voidUser.png) when I leave it blank. Same thing happens when I put a valid username in there too.
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  6. 24th January 2013, 02:52 #5
    ChrisW67
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    Default Re: [QT 5.0] Downloading from a URL then saving to client-defined path/filename?

    If you ran the program in your debugger you would know exactly which line your program was crashing on (probably 42, maybe 47), which is more useful than the "somewhere in on_pushButton_clicked()" level of information you have now.

    You have made a pointer to QNetworkAccessManager member variable but you never initialise that pointer. When you subsequently use it... BOOM! You need to move the equivalent of line 41 (also 42) into the constructor. A good habit to get into is: any time you add a member variable to a class immediately add an initialisation of the variable into the constructor. In the case of pointers that can be setting it to 0 or allocating a new object; either way it forces you to think about where it should be set.
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    Salads (31st January 2013)
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