Detecting if SwiftUI app is in Xcode Previews

There might be a time when you want to detect whether an app is running in Xcode Previews or not.

At first, I expected something like a targetEnvironment() condition that allows us to detect between builds on physical devices and simulators.

#if targetEnvironment(simulator)
   // code for the simulator here
#elseif targetEnvironment(preview) ❌
   // code for Xcode Previews here
#else
   // code for real devices here
#endif

Sadly, Swift doesn't provide such a condition for detecting the Xcode Previews environment.

But we still got some luck. Apple includes a process environment value that tells whether an app is running in the Xcode Previews environment.

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Detecting if a SwiftUI app is in Xcode Previews

Xcode injects the XCODE_RUNNING_FOR_PREVIEWS environment value into the running process when running in the Xcode Previews environment.

We can retrieve this value through the ProcessInfo class.

ProcessInfo.processInfo.environment["XCODE_RUNNING_FOR_PREVIEWS"]

The value of XCODE_RUNNING_FOR_PREVIEWS will be a string of one ("1") if we are in Xcode Previews.

We can create a helper function or computed property for ease of use.

Here is a basic helper computed property to detect if an app is in the Xcode Previews mode.

var isPreview: Bool {
    return ProcessInfo.processInfo.environment["XCODE_RUNNING_FOR_PREVIEWS"] == "1"
}

Then, we can use it like this.

struct ContentView: View {
    var body: some View {
        VStack {
            Image(systemName: "globe")
                .imageScale(.large)
                .foregroundColor(.accentColor)
            Text("Hello, world!")
            if isPreview {
                // 1
                Text("In Preview")
            }
        }
        .padding()
    }
}

1 Put Text("In Preview") if in Xcode Previews.

As a result, we will see the text "In Preview" in Xcode Previews, but not in a simulator or physical device.