What's wrong with my #if TARGET_OS_SIMULATOR code for Realm path definition?

As of Xcode 9.3+ Swift now supports #if targetEnvironment(simulator) to check if you're building for Simulator.

Please stop using architecture as a shortcut for simulator. Both macOS and the Simulator are x86_64 which might not be what you want.

// ObjC/C:
#if TARGET_OS_SIMULATOR
    // for sim only
#else
    // for device
#endif


// Swift:
#if targetEnvironment(simulator)
    // for sim only
#else
    // for device
#endif

TARGET_IPHONE_SIMULATOR macro doesn't work in Swift. What you want to do is like the following, right?

#if arch(i386) || arch(x86_64)
let device = false
let RealmDB = try! Realm(path: "/Users/Admin/Desktop/realm/Realm.realm")
#else
let device = true
let RealmDB = try! Realm()
#endif

Please see this post. This is the correct way to do it and it's well explained

https://samsymons.com/blog/detecting-simulator-builds-in-swift/

Basically define a variable named as you like (maybe 'SIMULATOR') to be set during run in simulator. Set it in Build Settings of Target, under Active Compilation Conditions- > Debug then (+) then choose Any iOS Simulator SDK in the dropdown list, then add the variable.

Then in your code

var isSimulated = false
#if SIMULATOR
  isSimulated = true // or your code
#endif