Concurrency.md

Concurrency

As we all know, updates to user interface elements should be made on the main thread for them to take effect in real-time. The conventional GCD syntax is a bit cumbersome in the middle of code that simply might be running on an inconvenient thread.

dispatch_async(dispatch_get_main_queue()) {
    button.enabled = false
    imageView.image = newImage
}

This is especially ugly if all that you want to do is invoke a single method that has no arguments or return type

dispatch_async(dispatch_get_main_queue()) {
    imageView.startAnimating()
}

In order to clean up the syntax and to emphasize the importance of the code that is being executed, the exclamation point (!) is used as a prefix operator to a closure. The provided closure will be executed on the main thread.

!{
    button.enabled = false
    imageView.image = newImage
}

!imageView.startAnimating

Notice that the final example does not use parentheses.