Can a single .swift file in Xcode contain entire source code of an iOS app?

Question

In other words, can Swift code in Xcode behave like it does in a Playground file?

Answer 1

It certainly can.

This special file is called main.swift and it behaves just like a Playground file.

From Files and Initialization, a post on Apple Swift Blog:

… The “main.swift” file can contain top-level code, and the order-dependent rules apply as well. In effect, the first line of code to run in “main.swift” is implicitly defined as the main entrypoint for the program. This allows the minimal Swift program to be a single line — as long as that line is in “main.swift”.

However, on iOS, @UIApplicationMain is the application entry point. It does the whole startup work for you automatically. But in main.swift of an iOS app, you have to initialize it manually.

In Swift 3.1, your main.swift file should begin like this:

// main.swift

import Foundation
import UIKit

class AppDelegate: UIResponder, UIApplicationDelegate {
    var window: UIWindow?
}

UIApplicationMain(
    CommandLine.argc,
    UnsafeMutableRawPointer(CommandLine.unsafeArgv)
        .bindMemory(
            to: UnsafeMutablePointer<Int8>.self,
            capacity: Int(CommandLine.argc)),
    nil,
    NSStringFromClass(AppDelegate.self)
)

// Your code begins here

UIApplicationMain initialization provided by Matt Neuburg.

Answer 2

Yes it does, except the AppDelegate though. It is not recommended to do that, but in one file you can have multiple classes.

Answer 3

Yes, you could do this just as you could write all your code on one line of code if you really wanted to with semicolons to end your lines. Although, this is definitely not recommended as it can cause lots of confusion and you have to bother messing with the original organized files.

Answer 4

yes, because all files can see on another as top level code. but for the same reason its pointless to clutter your workspace.