how to run code in the UI thread calling it from the others ones

Swift 5+ example

DispatchQueue.main.async {
    // UI code
}


DispatchQueue.main.asyncAfter(deadline: .now() + 0.5) {
    // UI code
}

The preferred way nowadays is using GCD, with the code you quoted in your question:

dispatch_async(dispatch_get_main_queue(), ^{
    // Your code to run on the main queue/thread
});

If you prefer using a more Object-Oriented approach than GCD, you may also use an NSOperation (like an NSBlockOperation) and add it to the [NSOperationQueue mainQueue].

[[NSOperationQueue mainQueue] addOperationWithBlock:^{
    // Your code to run on the main queue/thread
 }];

This does quite the same thing as dispatch_async(dispatch_get_main_queue(), …), has the advantage of being more Objective-C/POO oriented that the plain C GCD function, but has the drawback of needing to allocate memory for creating the NSOperation objects, whereas you can avoid it using plain C and GCD.

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I recommend using GCD, but there are other ways, like those two which allow you to call a selector (method) on a given object from the main thread:

• - (void)performSelectorOnMainThread:(SEL)aSelector withObject:(id)arg waitUntilDone:(BOOL)wait (method of NSObject so it can be called on any object)
• Send - (void)performSelector:(SEL)aSelector target:(id)target argument:(id)anArgument order:(NSUInteger)order modes:(NSArray *)modes on the [NSRunLoop mainRunLoop]

But those solutions are not as flexible as the GCD or NSOperation ones, because they only let you call existing methods (so your object has to have a method that already exists and does what you want to perform), whereas GCD or -[NSOperationQueue addOperationWithBlock:] allows you to pass arbitrary code (using the block).