ImageCache class

Class for caching images.

Implements a least-recently-used cache of up to 1000 images, and up to 100 MB. The maximum size can be adjusted using maximumSize and maximumSizeBytes.

The cache also holds a list of 'live' references. An image is considered live if its ImageStreamCompleter's listener count has never dropped to zero after adding at least one listener. The cache uses ImageStreamCompleter.addOnLastListenerRemovedCallback to determine when this has happened.

The putIfAbsent method is the main entry-point to the cache API. It returns the previously cached ImageStreamCompleter for the given key, if available; if not, it calls the given callback to obtain it first. In either case, the key is moved to the 'most recently used' position.

A caller can determine whether an image is already in the cache by using containsKey, which will return true if the image is tracked by the cache in a pending or completed state. More fine grained information is available by using the statusForKey method.

Generally this class is not used directly. The ImageProvider class and its subclasses automatically handle the caching of images.

A shared instance of this cache is retained by PaintingBinding and can be obtained via the imageCache top-level property in the painting library.

/// This is the custom implementation of [ImageCache] where we can override
/// the logic.
class MyImageCache extends ImageCache {
  @override
  void clear() {
    print('Clearing cache!');
    super.clear();
  }
}

class MyWidgetsBinding extends WidgetsFlutterBinding {
  @override
  ImageCache createImageCache() => MyImageCache();
}

void main() {
  // The constructor sets global variables.
  MyWidgetsBinding();
  runApp(const MyApp());
}

class MyApp extends StatelessWidget {
  const MyApp({super.key});

  @override
  Widget build(BuildContext context) {
    return Container();
  }
}