Unfortunately if you want to leverage/play/enjoy the new SwiftUI hot reload capabilities, then you need to update your beloved macOS Mojave to Catalina alongside with the Xcode 11 beta.
Ok, so let’s image you can’t wait to download the macOS Catalina Beta (6GB) on your existing system with macOS Mojave and Xcode 10. Getting some coffe(s) during the installation, and finally you could downloaded the latest Xcode 11 Beta (7.5GB). You start with trembling fingers a new project, and goes to the ContentView.swift. You click on the “Adjust Editor Options” to see the canvas, and after a short building process instead of seeing the nice hot reloading environment you got the following error:
Failed to code sign ContentView.swift
The info box says exactly the same thing, which is not too much. Your last straw is to click on Diagnostics button, and Voila:
invalid active developer path (/Library/Developer/CommandLineTools), missing xcrun at: /Library/Developer/CommandLineTools/usr/bin/xcrun
failedToCodeSign: xcrun: error: invalid active developer path (/Library/Developer/CommandLineTools), missing xcrun at: /Library/Developer/CommandLineTools/usr/bin/xcrun
the codesign_allocate helper tool cannot be found or used
sudo xcode-select -s /Applications/Xcode-beta.app/Contents/Developer
Obviously the /Applications/Xcode-beta.app should be the path and name where you placed the Xcode 11 Beta.