The error message says you have two main functions in your project. You can have only one. When you create the project, Xcode includes a main.cpp file that has a main function. If you add a second main function, you are going to get an error. Remove one of the two main functions. If you added your own C++ file to the project, delete the main.cpp function Xcode included with the project.

In a followup reply, the OP provided the following error message: duplicate symbol '_main' in:   /Users/lamiaalkhunini/Library/Developer/Xcode/DerivedData/c-awfbaiszjqjdyzfyhvlstvnfmpbe/Build/Intermediates.noindex/c.build/Debug/c.build/Objects-normal/x86_64/l.o   /Users/lamiaalkhunini/Library/Developer/Xcode/DerivedData/c-awfbaiszjqjdyzfyhvlstvnfmpbe/Build/Intermediates.noindex/c.build/Debug/c.build/Objects-normal/x86_64/ll.o ld: 1 duplicate symbol for architecture x86_64 clang: error: linker command failed with exit code 1 (use -v to see invocation) The duplicate symbol _main told me there were two main functions in the project. I didn't guess it from the original question.

Using the Core Text framework is going to be the easiest way to create a PDF with large amounts of text. Core Text has a function to determine how much text can fit on a page, CTFramesetterSuggestFrameSizeWithConstraints. It's been a while since I used Core Text, but the following article shows how to create a PDF with Core Text and Quartz (Core Graphics): meandmark.com/blog/2016/08/creating-pdfs-with-core-text-and-quartz/

You created an action to go from A to B. Create a second action to go from B to A. Create an SKAction sequence with the two actions and run the sequence. let sequence = SKAction.sequence([firstAction, secondAction]) sprite.run(sequence) firstAction and secondAction are the actions you created to move from A to B and B to A. sprite is the sprite node to move with the action sequence.

Hacking with Swift(hackingwithswift.com) has a free 100 day course on SwiftUI.

If you want an answer, you need to show the code that pauses the workout and the timer when using the Foundation timer.

Xcode 10.1 is not officially supported on Catalina and Big Sur. I recommend installing Xcode 12.4 and checking if that fixes things.

If by versioning you mean creating a new version of the Core Data model, create a new version of the data model when you reach the point in development that you want to keep the existing data in your app. In the early stages of making your app it's easier to trash the existing Core Data store (file) and starting with a fresh store. Eventually you will hit a point where starting with a fresh store causes problems, such as when other people start using your app. That's when you need to create a new version of the model and migrate from the old version to the new version.

For an iOS app deleting the app and reinstalling it will trash the existing data. For a Mac app, the Core Data store is in the Application Support folder. For a Core Data app of mine, the file has the extension .storedata. Move the store file to the Trash to delete it.

There is no Xcode beta available for Catalina. Just use Xcode 12.4, which you can download from the App Store or from Apple's developer downloads page. If you can't find Apple's developer downloads page, the site Xcode Releases (xcodereleases.com) has Apple's download links for every Xcode version.

If you are stuck on High Sierra, you cannot learn SwiftUI. Either learn UIKit for iOS development or AppKit (Cocoa) for Mac development. You will not be able to submit iOS apps to the App Store with Xcode 10, but you can use Xcode 10 to learn iOS development. The following articles provide resources for learning iOS and Mac development, most of which do not use SwiftUI: swiftdevjournal.com/learning-ios-development/ swiftdevjournal.com/resources-for-learning-mac-development/ Unforunately these forums don't let us add clickable links so you will have to copy and paste the URLs in your browser.

If you option-click the Add button in the project window toolbar, the Library window will stay open. I don't know if that's what you mean by "pin the library view on the Xcode desktop", but if option-clicking isn't enough for you, you will have to explain what you mean by pinning the library view.

You can set the delegate from the storyboard instead of in the viewDidLoad function. In the storyboard, make a connection from the text field to the view controller and choose the delegate outlet. If that does not work, you should tell us the book you're following. The following article provides a detailed explanation of the error message you are getting: swiftdevjournal.com/crashing-with-swift-optionals/

Create an Objective-C command line tool project. Choose Command Line Tool from the list of Mac app project templates. Click the Next button. Choose Objective-C from the Language menu.

To save when the app exits, go to the AppDelegate.swift file. For an iOS app there will be a function called applicationWillResignActive. In a Mac app, the function will be called applicationWillTerminate. These functions are called when quitting the app. Call the saveContext function in either applicationWillResignActive or applicationWillTerminate.