A dictionary describing the format for delivery of preview-sized images alongside the main photo.
UIKit for Mac 13.0+Beta
By default, this property is nil, specifying that the photo output should return only the main image requested. To also receive a preview-sized image, set this property to a dictionary describing the format for that image, containing the following keys and values:
Optionally, you can also include the kCVPixelBufferWidthKey and kCVPixelBufferHeightKey keys to specify the size of the preview image. (If you specify either width or height, you must specify both.) If the size you specify does not match the aspect ratio of the primary photo, the photo output provides a preview image whose size matches the longer of the two specified dimensions, preserving the original aspect ratio.
This documentation contains preliminary information about an API or technology in development. This information is subject to change, and software implemented according to this documentation should be tested with final operating system software.