Xcode 8 beta 6: main.swift won't compile

问题:

We have a custom UIApplication object, so our main.swift was

import Foundation
import UIKit

UIApplicationMain(Process.argc, Process.unsafeArgv, NSStringFromClass(MobileUIApplication), NSStringFromClass(AppDelegate))

and that didn't work in Xcode 8 beta 5 so we used this

//TODO Swift 3 workaround? https://forums.developer.apple.com/thread/46405
UIApplicationMain( Process.argc, UnsafeMutablePointer<UnsafeMutablePointer<CChar>>(Process.unsafeArgv), nil, NSStringFromClass(AppDelegate.self))

On Xcode 8 beta 6 we get Use of unresolved identifier 'Process'

What do we need to do in Xcode 8 beta 6/Swift 3 to define the UIApplicationMain?

回答1:

I write it this way:

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

To change the UIApplication class, substitute NSStringFromClass(MobileUIApplication.self) for nil in that formulation.

However, if your only purpose here is to substitute a UIApplication subclass as the shared application instance, there's an easier way: in the Info.plist, add the "Principal class" key and set its value to the string name of your UIApplication subclass, and mark your declaration of that subclass with an @objc(...) attribute giving it the same Objective-C name.

EDIT This problem is now solved in iOS 12 / Xcode 10. CommandLine.unsafeArgv now has the correct signature, and one can call UIApplicationMain easily:

UIApplicationMain(
    CommandLine.argc, CommandLine.unsafeArgv, 
    nil, NSStringFromClass(AppDelegate.self)
)

回答2:

It seems Process has been renamed to CommandLine in beta 6.

CommandLine

But the type of CommandLine.unsafeArgv is mismatching the second argument of UIApplication, so you may need to write something like this:

CommandLine.unsafeArgv.withMemoryRebound(to: UnsafeMutablePointer<Int8>.self, capacity: Int(CommandLine.argc)) {argv in
    _ = UIApplicationMain(CommandLine.argc, argv, NSStringFromClass(MobileUIApplication.self), NSStringFromClass(AppDelegate.self))
}

(UPDATE)This mismatching should be considered as a bug. Generally, you'd better send a bug report when you find "this-should-not-be" things, like the third parameter in beta 5. I hope this "bug" will be fixed soon.

---

If you just want to designate your custom UIApplication class, why don't you use Info.plist?

NSPrincipalClass | String | $(PRODUCT_MODULE_NAME).MobileUIApplication

(Shown as "Principal class" in non-Raw Keys/Values view.)

With this in your Info.plist, you can use your MobileUIApplication with normal way using @UIApplicationMain.

(ADDITION) Header doc of UIApplicationMain:

// If nil is specified for principalClassName, the value for NSPrincipalClass from the Info.plist is used. If there is no
// NSPrincipalClass key specified, the UIApplication class is used. The delegate class will be instantiated using init.