Warn on calls to UIKit from background threads

Question

iOS\'s UIKit is not thread-safe, let us call this fact well known. I know the rule, I\'m careful, but I still get bitten - and every now and then the resulting crash is far

Answer 1

I try not to introduce multi threading unless I have tried a single threaded approach first but it depends on the problem you are trying to solve.

Even when multithreading is the only option I usually avoid long running background operations or operations that that perform several unrelated tasks.

Just my opinion.

Edit

An example of doing work on the main thread whilst displaying a loading spinner:

MBProgressHUD *hud = [MBProgressHUD customProgressHUDInView:view dim:dim];
[hud show:NO];
//Queue it so the ui has time to show the loading screen before the op starts
NSBlockOperation *blockOp = [NSBlockOperation blockOperationWithBlock:block];
NSBlockOperation *finOp = [NSBlockOperation blockOperationWithBlock:^{
    [MBProgressHUD hideAllHUDsForView:view animated:NO];
}];
[finOp addDependency:blockOp];
[[NSOperationQueue mainQueue] addOperations:@[blockOp, finOp] waitUntilFinished:NO];

Answer 2

Personally anytime I open the box to a multithreaded approach I start wrapping ALL interface based calls in performSelectorOnMainThread: so that there is never an issue. If we've already made it to the main there this call shouldn't cause any significant slow down, but if its called from a background thread I can rest easy knowing its safe.

Answer 3

This code (just add to project and compile this file without ARC) causes assertions on UIKit access outside of the main thread: https://gist.github.com/steipete/5664345

I've just used it to pickup numerous UIKit/main thread issues in some code I've just picked up.