How display progressively an UIImageView

I find a w开发者_高级运维ay to display progressively an UIImageView like that :

I don't necessarily want to animate it, but I can't find a way to display only a part of an UIImageView by only play with its frame or contentMode property.

Any suggestion ?

Put a UIView over the imageview and change the uiview frame accordingly.

Well, I've found my response :

UIImageView in UIView and not to display image parts out of bounds