What's the relationship between UI animation and the main runloop - objective-c

Question

i have this code to wait for a loading task, showing a activityIndicator view
if (isLoading) {
self.tipView = [[BBTipsView alloc] initWithMessage:#"loading..." showLoading:YES parentView:self.view autoClose:NO];
self.tipView.needsMask = YES;
[self.tipView show];
while (isLoading) {
[[NSRunLoop currentRunLoop] runMode:NSDefaultRunLoopMode beforeDate:[NSDate distantFuture]];
}
[self.tipView close];
}
the loading view will animate until the isLoading become false.here is my problem:
running a runloop in main thread will block the main thread until there is a source event comes or timer fire. but why the loading view keep animating while the main runloop didn't return?
-----edit by bupo----
I found that when timer fire the runloop won't return. This will make sense that the animation refresh ui by CADisplayLink timer fire.
Note that from the perspective of NSRunloop, NSTimer objects are not "input"—they are a special type, and one of the things that means is that they do not cause the run loop to return when they fire.

Answer 1

The NSRunLoop method runMode:beforeDate: runs until the given date OR until it finds a single event to deal with - after which the call returns. You're calling it on the main run loop ([NSRunLook currentRunLoop]). Hence, even though you think you're blocking the main run loop, you're not - you're causing events to be serviced. Hence, the animation timer can function, even though you might think you're 'blocking' the main run loop.
To confirm this, comment out the call to runMode:beforeDate: and you should see that the UI freezes until the operation completes.
Edit: see CodaFi's comment on your question. What actually happens if you comment out the call to runMode:beforeDate:, out of interest?
Original answer:
This style of code isn't recommended for starting and stopping UI animations. Don't mess around with run loops unless you have to. And having a tight loop that checks for boolean flag changing from elsewhere is often a code smell that means there's a better way.
Instead, do it asynchronously and without sitting on the main thread:
// on main thread
self.tipView = [[BBTipsView alloc] initWithMessage:#"loading..." showLoading:YES parentView:self.view autoClose:NO];
self.tipView.needsMask = YES;
[self.tipView show];
} // end of the method
- (void)loadingHasFinished {
// assuming this method called on main thread
[self.tipView close];
}
Obviously you'll have to ensure the loadingHasFinished is called as appropriate.
If loadingHasFinished is called on a background thread rather than the main thread, you'll want something like this:
- (void)loadingHasFinished {
dispatch_async(dispatch_get_main_queue(), ^{
[self.tipView close];
});
};