Why is that constantly animating UIActivityIndicator won't block main thread?

Question

We all show activity indicator while some lengthy operation is happening in background. Though the activity indicator shows a constantly rotating wheel it won't burden the main thread, because other UIComponents in the same screen still react to the touches.

What I think I know:

I know all touch events are handled by main thread, and main Queue is being used to queue the events. Considering main queue is Serialized queue and only one task at a time can run at any given point in time, alley touch events should get queued up in main queue, while my main thread is busy in refreshing the screen/calling drawrect of UIActivityIndicator.

Study:

I have looked into the code of third party activity indicators. Most of them use CABasicAnimation and call repeat always on animation. While few work directly use NSTimer to repeatedly call drawrect with a small delay. Their code works because there is a small delay in calling drawrect and the method drawrect in itself is light weight.

None of it won't take the loads off the main thread but rather they carefully place load on main thread enough to keep their animation going yet keeping main thread free to handle touch events

What I want to know:

1 - Is this strategy to implement activity indicator is correct? or statement like this

self.timer =[NSTimer timerWithTimeInterval:0.1 target:self
selector:@selector(setNeedsDisplay) userInfo:nil repeats:YES];
        [[NSRunLoop currentRunLoop] addTimer:self.timer forMode:NSRunLoopCommonModes];

in one of the third party activity indicator that I saw has any special effect?

2 - If I run CABasicAnimation/transaction and repeat the animation forever will it have any special effects on the load of main thread compared to repeatedly calling setNeedsDispaly/drawrect manually?

Answer 1

I'm not sure whether it will help to implement your own activity indicator, but the system one UIActivitiyIndicatorView is just a UIImageView with an array of 12 images that replace each other over time.

Apple made a pretty neat trick by making their spinner discrete. It allowed them to have a simple implementation that doesn't create any computational load on CPU.

UPD

Returning to the things you want to know: 1 - It's not, because implementing manual frame drawing in drawRect is fully done by CPU. And 2 - I can't say for sure, but if one believes what Apple says in documentation and videos about Core Animation it is heavily optimised and runs on Metal or at least OpenGL underneath, so leverages power of GPU.