I haven't posted in a while but here's a quick Swift snippet just so this site doesn't become completely stale.

I spend a lot of my time developing iOS apps that work with our web applications. When I run the app in the simulator I want it to talk to the copy of that web application that's running on my development machine and when I run it on a device (or issue a beta build) it should talk to the staging server. In Objective C this was easy. I'd just use TARGET_IPHONE_SIMULATOR to switch between hosts. Unfortunately, This doesn't work as well in Swift.

If you Google it you'll find this thread on Stack Overflow and this code in particular:

#if (arch(i386) || arch(x86_64)) && os(iOS)
    ...
#endif

From here I ended up with this code:

#if (arch(i386) || arch(x86_64)) && os(iOS)
    let DEVICE_IS_SIMULATOR = true
#else
    let DEVICE_IS_SIMULATOR = false
#endif

This would build and run fine from Xcode but the first time I tried to archive the target for release it would complain that DEVICE_IS_SIMULATOR was undefined.

After some experimentation I came up with the following code that seems to work well:

struct Platform {
    static let isSimulator: Bool = {
        var isSim = false
        #if arch(i386) || arch(x86_64)
            isSim = true
        #endif
        return isSim
    }()
}

// Elsewhere...

if Platform.isSimulator {
    // Do one thing
}
else {
    // Do the other
}

I hope this helps someone. Let me know if you have a better way. I'm off to go grill some burgers.