CGImageSource

CGImageSource is a part of the Image I/O framework in Apple's Core Graphics that provides an abstract, read-only interface for image data. It can represent a single image or a sequence of images contained in a source such as a file, a data blob, or a data provider. The structure allows clients to discover metadata, access image properties, and create bitmap representations on demand without fully decoding the source upfront. This makes it suitable for loading images from disk, network, or other data streams in a memory-efficient way, including animated and multi-frame formats.

A CGImageSource is created from various data sources, including CFData, CFURL, or a data provider, using functions such as CGImageSourceCreateWithData, CGImageSourceCreateWithURL, or CGImageSourceCreateWithDataProvider. An optional type hint can be supplied to help identify the image type when the data does not clearly indicate it. The source can also be built incrementally with CGImageSourceCreateIncremental for progressive data loading.

Key capabilities include querying the number of images in the source with CGImageSourceGetCount, and retrieving the overall type with CGImageSourceGetType. Image properties can be read per image via CGImageSourceCopyPropertiesAtIndex, and actual image data can be produced with CGImageSourceCreateImageAtIndex. These APIs support both single-frame images and multi-frame formats such as GIF, PNG, and TIFF, enabling frame-by-frame decoding or extraction of metadata without decoding the entire dataset.

Incremental loading and on-demand decoding enable efficient handling of large or remote image data, while standard option dictionaries allow customization of caching and decoding behavior. CGImageSource thus serves as a flexible, low-level entry point for image I/O operations.