Differences between ItemizedOverlay and Overlay class

Can someone tell my when to use Overlay or when to use ItemizedOverlay class!

What are d开发者_如何学Goifferences between this two classes?

Draw methods do the same thing?

Can I use only ItemizedOverlay class in my project or I must use and Overlay as base class!

Thanks

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Overlay is a general overlay. ItemizedOverlay is a subclass that makes it easier to create an overlay that is a discrete series of marked points on the map. So, if you are trying to show a bus route, or shade a region, or something like that, Overlay is the class you want. If you are trying to show a collection of restaurants, or churches, or whatever, ItemizedOverlay works.

Note, though, that ItemizedOverlay is designed for modest numbers of points (e.g., dozens). If you have a large number of points, you may need to create your own Overlay just for performance reasons.