associated type.swift

import CoreMedia

protocol PixelContainer { }

protocol Source {

    typealias PixelContainerType: PixelContainer

	func frameAtTime(time: CMTime) -> PixelContainerType?
	
}

class CaptureSource: Source {

    typealias PixelContainerType = CMSampleBuffer

	func frameAtTime(time: CMTime) -> PixelContainerType? {
		let sample: CMSampleBuffer? = nil
		return sample
	}
}

class ImageSource: Source {

    typealias PixelContainerType = CGImage

	func frameAtTime(time: CMTime) -> PixelContainerType? {
		let image: CGImage? = nil
		return image
	}
}

extension CMSampleBuffer: PixelContainer { }
extension CGImage: PixelContainer { }