Development/Tutorials/KPixmapCache

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Warning

From the KPixmapCache apidocs:
KPixmapCache is susceptible to various non-trivial locking bugs and inefficiencies, and is supported for backward compatibility only (since it exposes a QDataStream API for subclasses). Users should port to KImageCache for a very close work-alike, or KSharedDataCache if they need more control

Introduction

KPixmapCache provides disk-caching of QPixmaps. If you're using SVGs or generating pixmaps from some data, you might consider using it as it eliminates the need to generate the same pixmaps over and over again.

Using it

Creating the cache object

Its API is similar to that of the QPixmapCache. To use it, you first need to create the cache object, using a unique name:

KPixmapCache* cache = new KPixmapCache("myapp-images");

Inserting and finding pixmaps

Once that is done you can easily retrieve and insert images with insert() and find() methods:

QPixmap pix("file1.png");
cache->insert("file1.png", pix);

QPixmap pix2;
if (cache->find("file2.png", pix2)) {
    // The pixmap was loaded from cache.
} else {
    // Pixmap was not found in the cache.
    // Load and insert it:
    pix2 = QPixmap("file2.png");
    cache->insert("file2.png", pix2);
}

The Easy Way

As loading pixmaps from files with the aid of cache is a very common operation, KPixmapCache provides a convenience method for that:

QString filename("myfile.png");
QPixmap pix = cache->loadFromFile(filename);

This first checks if the given file is in the cache. If it is, it's loaded from the cache, otherwise it's loaded from the file and inserted into cache.

Similar method is provided for loading SVG files:

QString filename("myfile.svg");
QPixmap pix = cache->loadFromSvg(filename);

This renders contents of the SVG file onto pixmap, using SVG's default size by default. If you need a different size, you can specify it as the second argument of that method:

QString filename("myfile.svg");
QSize size(150, 250);
QPixmap pix = cache->loadFromSvg(filename, size);

This renders the SVG onto a pixmap with the size of 150x250 pixels.

Timestamps

Often you need to make sure the cache is up-to-date and you're not using obsolete pixmaps. For this, the cache provides timestamp() method. By default it returns time when the cache was created, but you can set your own timestamp with setTimestamp() method.

// Our data
MyDataObject data;
// Make sure the cache is up to date
if (cache->timestamp() < data.timestamp()) {
    // Data is newer than the cache
    // Thus the cache is obsolete, delete and reinitialize it
    cache->discard();
}
// Here the cache is always recent enough
QPixmap visualizedData;
QString key("data-visualization");
// Try to find the visualization from the cache
if (!cache->find(key, visualizedData)) {
    // Visualization is not in the cache, recreate it...
    visualizedData = createVisualization(data);
    // ...and put the resulting image into cache
    cache->insert(key, visualizedData);
}
// Show the visualizedData pixmap to user

This example creates a visual representation of some data. If the data is more recent than the cache, then contents of the cache are deleted using the discard() method. Next, we check if cache already contains the necessary pixmap. If it does, then there is no need to recreate it from the data (which might be expensive).

API docs

For a list of all KPixmapCache methods, check out its API docs.