Question

I know as a general guideline you should always perform GUI updates on the main thread. But sometimes it’s not that obvious. E.g where should I perform the calls:

setBadgeValue: of a UITabBarItem?

Or

[[UIApplication sharedApplication] setNetworkActivityIndicatorVisible:YES]

In the class reference it’s not specified whether these methods can be run on a non-main thread.

Answer 1

The general rule is that you should keep UIKit updates to the main thread, unless UIKit calls through your implementation from another thread.

Reference: Important: The UIKit classes are generally not thread safe. All drawing-related operations should be performed on your application’s main thread.

From iOS 4 release notes:

• Drawing to a graphics context in UIKit is now thread-safe. Specifically:
  The routines used to access and manipulate the graphics context can now correctly handle contexts residing on different threads.
• String and image drawing is now thread-safe.
• Using color and font objects in multiple threads is now safe to do.

With that information, and unless I or somebody else digs up more docs (which I scanned release notes and saw no more):

setBadgeValue: of a UITabBarItem?

Assume it is safe to call only from the main thread.

[[UIApplication sharedApplication] setNetworkActivityIndicatorVisible:YES]

Assume it is safe to call only from the main thread.