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Thread: QFileIconProvider custom icon slow

  1. #1
    Join Date
    Apr 2021
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    Qt products
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    Platforms
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    Default QFileIconProvider custom icon slow

    I'm using a QFileIconProvider class to show image thumbnails in a QFileDialog by overriding the 'icon' method to load from a file to a QIcon.

    After implementing this, the file dialog is very slow in navigating through sub-folders and hangs for a few seconds before changing directory.

    C++ class is below.

    Why is the rendering being slow and how can I speed it up?

    Qt Code:
    1. QIcon CFileIconProvider::icon(const QFileInfo &info) const
    2. {
    3. const QString &path = info.absoluteFilePath();
    4.  
    5. // check if 'info' is a drive type
    6. if (path.isEmpty())
    7. return QFileIconProvider::icon(QFileIconProvider::Drive);
    8.  
    9. if (info.isFile()) {
    10. if (QFile(info.absoluteFilePath()).exists()) {
    11. // show file as icon
    12. QImage originalImage(info.absoluteFilePath());
    13.  
    14. if (!originalImage.isNull())
    15. return QIcon(info.absoluteFilePath());
    16. }
    17. }
    18.  
    19. // check if 'info' is a directory type
    20. if (info.isDir())
    21. return QFileIconProvider::icon(QFileIconProvider::Folder);
    22.  
    23. return QIcon();
    24. }
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    Qt Code:
    1. #ifndef FILEICON_PROVIDER_H
    2. #define FILEICON_PROVIDER_H
    3.  
    4. #include <QFileIconProvider>
    5. #include <QObject>
    6. #include <QIcon>
    7.  
    8. class CFileIconProvider : public QFileIconProvider
    9. {
    10.  
    11. public:
    12. CFileIconProvider();
    13. QIcon icon(const QFileInfo &info) const override;
    14.  
    15. };
    16.  
    17. #endif // FILEICON_PROVIDER_H
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  2. #2
    Join Date
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    Qt products
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    Default Re: QFileIconProvider custom icon slow

    Hi,
    I think in this part
    Qt Code:
    1. if (info.isFile()) {
    2. if (QFile(info.absoluteFilePath()).exists()) {
    3. // show file as icon
    4. QImage originalImage(info.absoluteFilePath());
    5.  
    6. if (!originalImage.isNull())
    7. return QIcon(info.absoluteFilePath());
    8. }
    9. }
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    you read the file two times, once to create the QImage, and then again to create the QIcon for the return.
    It would also probably help to cache the created icons so that you only have to create them once.

    Ginsengelf

  3. #3
    Join Date
    Apr 2021
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    Qt products
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    Platforms
    Unix/X11 Windows Android

    Default Re: QFileIconProvider custom icon slow

    Thanks Ginsengelf,

    You were right the method was rending the file twice in the loop, which also was taking a lot of time to load into a QIcon.

    As per suggestion, I created a cache of icons that were rendered in a seperate thread in order to not be blocking, hash map was defined via:

    Qt Code:
    1. QMap<QString, QIcon>* iconCache;
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