ImageView OnDraw-method when moving from Xamarin.Android to Xamarin.Forms

I have a Xamarin.Anrdoid App which I'm trying to move to Xamarin.Forms in order to get a common codebase for Android and iOS. In Xamarin.Android I'm using a class called ImageView overriding the "Draw"-method in order to draw my own images using different parts of .png-files. What's the easiest way to accomplish this in Xamarin.Forms?