Are you adding a new file to the project and adding that code? If so, you have two main functions, giving you a Duplicate Symbol error. A C program can't have two main functions. Remove the main function in the main.c file that was created when you created the project. When you create a command-line tool project, Xcode provides a main.c file. Why are you creating another file instead of using the main function Xcode created for you.

Apple consolidated a bunch of app project templates into one App template. Choose the App template. You can get more information in the following article: swiftdevjournal.com/where-are-the-single-view-and-master-detail-app-project-templates/

You can present a Game Center view controller without a storyboard. Your SpriteKit scene has a root view controller. let sceneViewController = self.view?.window?.rootViewController Create an instance of GKGameCenterViewController. let viewController = GKGameCenterViewController() Configure the Game Center view controller. Present the Game Center view controller from the scene view controller. sceneViewController?.present(viewController, animated: false, completion: nil) Choose Help Developer Documentation in Xcode to read the Game Center documentation, which is in the Graphics and Games section. There's an article on displaying the Game Center Dashboard and a class reference for GKGameCenterViewController. Ray Wenderlich made their 2D game development book freely available on GitHub. The book has two chapters on using Game Center. github.com/raywenderlich/deprecated-books

The file you are trying to open, AMatrix.txt, is not in the current working directory so the file isn't found. There are multiple ways to fix this. First, you can supply the complete path to the file in your code. Second, you can set the working directory for the Xcode scheme and set it to the directory where the file you want to open resides. In the project window toolbar to the right of the Run and Stop buttons is a path control. The left item in the path control is the name of your project. Click the item and choose Edit Scheme to open the scheme editor. Select the Run step from the left side of the scheme editor. Click the Options button at the top of the scheme editor. Select the Use custom working directory checkbox. Click the folder icon to open an Open panel to choose the working directory.

I cannot reproduce your issue. I do not get the error message on Xcode 12.5 on macOS 11.4 when I create a new iOS game project that uses SpriteKit and run the project on an iOS simulator. The project runs and the spinning nodes appear when tapping/clicking in the simulator.

@testable import is for importing Swift modules, such as your app, not individual classes. Replace the class name with the name of your app (or framework) target, and you should be able to access your app's classes in the unit tests.

Looking at the Xcode 12.5.1 release notes, I see no reason why you would need to update your app. If Xcode 12.5 is working for you, keep using it to develop your app.

When you say "I never see the window", do you mean you do not see the Welcome to Xcode window? If this is the case, Xcode has launched. There happen to be no windows open. You can open the Welcome to Xcode window by choosing Window > Welcome to Xcode or pressing Cmd-Shift-1.

You need to use Xcode 13 to run/debug your project on an iPhone running iOS 15. To use Xcode 12 the phone has to be running iOS 14 or earlier.

New Xcode projects default to using SwiftUI with the SwiftUI app life cycle. The SwiftUI app life cycle does not include an app delegate file. There are multiple ways to get an app delegate file. The first way to get an app delegate is to choose Storyboard from the Interface menu when creating the project. Choosing Storyboard tells Xcode to use UIKIt for the project. UIKIt uses the app delegate file. The second way applies only to Xcode 12. For iOS and Mac SwiftUI projects, there is a Life Cycle menu. Choose either UIKit App Delegate or AppKit App Delegate to get an app delegate file. Xcode 13 does not have a Life Cycle menu. New projects use the SwiftUI app life cycle. To use the app delegate life cycle you will have to add a new file to the project for the app delegate and remove the App file that Xcode creates when building the project.

You need a paid developer account to publish your app on the App Store. You need a paid developer account to have other people use your app. With a free developer account you can put an app you develop on your own device. But you have to reinstall the app every 7 days.

No, UIKit is not dead. Apple has the following video from the most recent WWDC about UIKit What's New in UIKit The choice to use UIKit or SwiftUI for a new project depends on the project and you. Use UIKit if you are comfortable with it or if you need to support iOS 13 and earlier versions of iOS. Use SwiftUI if you are interested in learning it or if you want to make an app that runs on iOS and Mac.

Xcode 12.4 is the latest version that runs on Catalina. You can download it at the following URL: https://developer.apple.com/xcode/resources/ The site Xcode Releases (xcodereleases.com) also has direct download links for every Xcode version Apple has ever released.

Is there a reason you cannot use a tutorial that uses Swift? Most of the Objective-C material is going to be 5+ years old. You are going to have to use Swift if you want to use learning material that is up to date. I recommend going through the Hacking with Swift 100 day course that uses UIKit to learn iOS development. After going through the course, recreate the projects in Objective-C to learn how to make iOS apps in Objective-C.

What version of macOS are you running? What part of the message in the alert are you having trouble understanding? Xcode 12.5, the current release version, requires macOS 11 or later to run. If you are unable to update your Mac to macOS 11, go to the Xcode Releases site (xcodereleases.com) to find direct download links for every version of Xcode Apple has ever released along with the minimum version of macOS you need to run that version. Xcode 12.4 is the latest version that runs on macOS 10.15. Xcode 11.3.1 is the latest version that runs on macOS 10.14. Xcode10.1 is the latest version that run on macOS 10.13.