Which framework do you suggest for building a 2D platform game for the iPhone? I only need to move assets around the screen and detect collisions etc.
I would think OpenGL. But maybe Quartz 2D is enough? Or is there something else I should be aware of? What are advantages and disadvantages of each? I guess there is a longer learning curve for OpenGL, but that pays off in performance, right?
Try cocos2d:
cocos2d is a framework for building 2D games, demos, and other graphical/interactive applications.
Have a look at cocos2d-iphone. I have heard only good things about it.
cocos2d-iphone is pretty good. I have written some extensions to it which you can read about here and get here. Good luck!
I think you might have two different answers depending on if you want to publish your game in the AppStore. If so, you have to use the provided/accepted frameworks or it won't be accepted. If not, then you are (of course) free to use anything.
There are usually two schools of thoughts here when it comes to included iPhone frameworks:
I haven't tried cocos2d, but that sounds like a great alternative to harness Open GL without having to deal with API.
I thought OpenGL was very easy for 2D. I think it's easier to deal with a well-known API than to try to grok someone's layer on top of it.
Unity3D is not so much a framework as an engine, but 2D can be done (well) with it. Zombieville USA is Unity/2D (http://www.zombievilleusa.com/)
I am currently using and recommend Cocos2d. (http://cocos2d-iphone.org)
The documentation for cocos2d-iphone is pretty good and getting better all the time. There are some full working example games you can download and the API itself comes with quite a number of examples you can use.
Here is a good set of install directions you can use to get started:
http://permadi.com/blog/?p=130
Hope this helps!
You may want to check out Sparrow at http://www.sparrow-framework.org. It is lightweight and very similar to the Flash API.