Swift compiler directives examples from source: https://needone.app/compiler-directives-in-swift/

compiler_directives_in_swift.swift

/*
For explanation please go to the post: https://needone.app/compiler-directives-in-swift/
*/

// Environment Checking
var url = ""
#if DEBUG
// use localhost while under debug mode
url = "https://localhost"
#else
// use production url while under release
url = "production url"
#endif

// You can also negate the flag
#if !RELEASE
// use localhost while under debug mode
url = "https://localhost"
#else
// use production url while under release
url = "production url"
#endif


// Platform Detection
#if os(OSX)
// compiles for OS X
#elseif os(iOS)
// compiles for iOS
#elseif os(tvOS)
// compiles for TV OS
#elseif os(watchOS)
// compiles for Apple watch
#endif


// Warnings and Errors
#warning("Needs improve on the performance")
#error("Needs remove the duplicated code")
import Foundation

class MyClass {
    // Some code here
    #warning("This is a warning message")
    var myVariable: Int = 0
    // Some more code here
}


// Language version Check
// Check the Swift version
#if swift(<5)
#endif


// Check environments like Simulator or Catalyst
#if targetEnvironment(simulator)
#endif


// Module check
// Check if a module presents
#if canImport(UIKit)
#endif


// iOS Version Check
if #available(iOS 10.0, *) {
}

@available(iOS 10, macOS 10.15, *)
func newMethod() {
	//  this method will be only available when iOS > 10, macOS > 10.15
}


// #function, #line and #file
import UIKit

class ViewController: UIViewController {

  override func viewDidLoad() {
    super.viewDidLoad()
    print(#file, #line, #function)
  }
}