demo-image-viewer

It's a demonstration only image viewer made with Qt6. The binaries are available.

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Features

• Opens an image in a time independent of the count of images in a directory where it is located.
• Handles the images of the directory in a separated thread with QtConcurrent::run.
• The directory parsing is fast. It does not use QFileInfo in std::sort, but a custom struct.
• Handles the directory only once, until it is changed. (For example, drag'n'dropping a file from the same directory does not trigger the directory parsing, since it's not needed to do, just find the file by name in the list structure.)
• Decodes 8.3 file names to long file names with Win API (GetLongPathNameW). An 8.3 can be faced on a user input of a file with long path (260+ chars) from disc C from Windows Explorer: on Drag'n'Drop and when a user opens an image with a double click (the image path is passed as a command line argument to the program.)
• Opens images with a long path starts with \\?\ (\\?\UNC\).
• Preloades the adjacent images in a separate thread.
• Sorts by mtime, btime, size.
• Updates the image position (in the title) on the sorting change.
• Lists hidden files (QDir::Hidden).
• All long time taking operations log the execution time in the console with qDebug().

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Here is the example of the console log, after the program is started (~70 ms):

The speed results for 350 KB image with a directory with 30000+ files (Run it with Qt Creator to look at the qDebug() logs):

• [timer][displayImage]: 84 ms — time to display the image.

At this moment the image is visible.

That is performed in a background thread:

• [timer][entryInfoList]: 143 ms — time to get the list with QFileInfo of all files (31973) in the directory
• [timer][filterBySupportedExts]: 6 ms — time filter that list to exclude the unsupported files
• [filterBySupportedExts] fileInfoList.size: 31973
• [filterBySupportedExts] fileInfoListFiltered.size: 30006
• [timer][initFileEntryList]: 200 ms — time to create a list of data structure with all necessary information for all filtered files (30006)
• [timer][initFileList]: 352 ms — the total time of the work with directory files ~(143 + 6 + 200)
• [timer][sortByMtime]: 6 ms — to sort all files by mtime

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Note: Do NOT use code that calls methods of QFileInfo in std::sort. It's incredibly slow.

That is up to 600 times slower

        std::sort(fileEntryList.begin(), fileEntryList.end(),
                  [](const FileEntry& a, const FileEntry& b) {
                      return a.fileInfo.lastModified() < b.fileInfo.lastModified();
                  }
        );

than that

        std::sort(fileEntryList.begin(), fileEntryList.end(),
                  [](const FileEntry& a, const FileEntry& b) {
                      return a.mtime < b.mtime;
                  }
        );

3200-3600 ms vs 6 ms.

So, store all required fields of QFileInfo in your own data struct.

Try by yourself (uncomment the line 307 and comment lines 298-304): https://github.com/AlttiRi/demo-image-viewer/blob/ad9ac9b1c9d78244462064983e676542700e96d9/core.h#L296-L311