我的 Swift 应用程序完成处理程序似乎不起作用

Question

I'm using a method found here to convert a street address to coordinate points. My completion handler, when successful, should add the coordinate points to a list that I've initialized, but when I go into debug mode, the completion handler is ignored completely causing an Index Out Of Range Error. I think I'm doing something wrong with my completion handler which can be found below:

func setUp() {
    for a in addresses {
        print(a)
        getCoordinate(address: a, completionHandler: {coord , error in
            
            if error == nil {
            items.append(Location(coordinate: CLLocationCoordinate2D(latitude: coord.latitude, longitude: coord.longitude), address: a))
                print("resulting coordinate = (\(coord.latitude),\(coord.longitude))")
            
            } else {
                print(error as Any)
            }
            
        })
        
    }
        
}

So to summarize, when I go to debug this problem, the debugger gets to line 4 in the snippet and then skips over the rest of the function. Thanks for any help!

Answer 1

That is how async code works. You make the call, and pass it a completion handler. The function call returns immediately. At some future date, the aysnc function finishes its work, and then it calls your completion handler.

Think of it like asking your kid to run to the store to buy some ingredients you need to finish making dinner. You give your kid the list send them on their way, and then go back to cooking the rest of dinner. When they come back, after an unpredictable delay, they tell you they have the stuff (call your completion handler) and at that point you take the items they gave you (Your completion handler can take parameters from the async function.)

If you set a breakpoint inside the completion handler, on the function call, and after the function call, you'll see the debugger first hit the function call, then the after-function-call breakpoint, and then at some future time you'll see it hit the completion handler code's breakpoint.