I download a sample of UICatalog and run it.When I compile it ,it shows the failed message:There is no SDK with the name or path \"iphoneos4.0\".I find the \"Base SDK\" of UICat
Many answers here, none seem to be complete enough about this, and it's all a mess. But they are all helpful. Here, I just want to compile all helpful pieces in one answer. Thanks to Larry (who have almost everything compiled into steps already), Steve, Dan Ray, dkk, geowar and lqf.
This picture from apple is a little decieving and outdated (talking about iOS SDK 4.0 and we're currently at 4.1) but it does show where you should change the project settings. Just select iPhone Simulator 4.0 if it was previously on iPhone Device 4.0.
Other than that, they say "Targets can override build settings defined at the Project level". So, yeah, also go ahead, open Targets and set all of their Base SDK back to default by pressing delete on it. If they were bolded, they'll become regular indicating a default value, which should be iPhone Simulator 4.0 if you did the first step right.
Just realize there are many places where Base SDK can be wrong:
And that under Target it should be set to default, which is value from Project.
There's still something about the overview combo sometimes having and sometimes not having more than 1 option such as "Device" and "Simulator" that I just couldn't understand yet, but this is another potential configuration place for Base SDK being wrong.