How to set UserDefaults value with Launch Arguments

Learn how to use a launch argument to override UserDefaults value and test your apps.

What is a launch argument

In programming, we can provide different inputs to a method by passing an argument. Launch argument is a way to pass arguments to the iOS application on launch.

Launch Argument has many functions, but I will use a launch argument to set UserDefaults values.

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How to set a launch argument

Before we explore how to set a UserDefaults value, let's learn how to set launch argument in Xcode.

To set launch argument in Xcode:

1. Go to Edit Scheme screen either by select Product Menu > Scheme > Edit Scheme... (⌘ - command + <) or click on scheme name and select Edit Scheme.
   
2. Select Run on the left panel, then select Arguments tab.
   
3. Under Arguments Passed On Launch section, click plus button (+) to add a new argument.
   

How to set UserDefaults key value with launch argument

You don't have to do anything here. UserDefaults contains all arguments parsed from the application's arguments. So, all we need to do is add a launch argument and access it from UserDefaults.

Add a launch argument

You can set the Xcode launch argument with the following syntax.

-argument_name argument_value

In the following example, we add -launch-argument 1. Our argument name is launch-argument, and our value is 1.

Read a launch argument

To read it, you simply reference the passing argument with its name.

If you add a launch argument like this:

-launch-argument 1

You can read it using the name launch-argument (Don't include -).

UserDefaults.standard.string(forKey: "launch-argument")
// Optional("1")

UserDefaults also support basic type conversion like String, bool, Int, and Double.

As an example, I will set five launch arguments with different values and try to access them using provided UserDefaults methods.

The values are 0, 1, true, false, and hello, world.

I access it through four different getter methods.

UserDefaults.standard.string(forKey: "string-true")
    UserDefaults.standard.integer(forKey: "string-true")
    UserDefaults.standard.double(forKey: "string-true")
    UserDefaults.standard.bool(forKey: "string-true")

And here is the result.

| Commands | 1 | 0 | "true" | "false" | "hello, world!" |
| ------------------ | ----------- | ---- | ---------- | ---- |
| .string(forKey:) | 1 | 0 | true | false | hello, world! |
| .integer(forKey:)|1 | 0 | 0 | 0 | 0 |
| .double(forKey:) |1.0 | 0.0 | 0.0 | 0.0 | 0.0 |
| .bool(forKey:) |true | false | true | false | false |

Behaviors

Setting UserDefaults with launch argument doesn't really set the value. It is just a temporary override of the value for that particular running session.

You should know two behaviors when setting UserDefaults value via launch arguments.

Tempolary effect

The key/value that is set from the launch argument doesn't register into a UserDefaults. The value is gone if you remove launch arguments in the subsequence run.

Override value

A key/value set with a launch argument will take precedence over the one set in your app.

So with the launch argument set to -number-one 1, the following code will print out 1.

UserDefaults.standard.set("Hello, world!", forKey: "number-one")
print(UserDefaults.standard.string(forKey: "number-one") ?? "n/a")
// 1, not Hello, world!

The "Hello, world!" text is already set, but the launch argument shadows it. Once you remove -number-one 1, it will print out the actual value.

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Conclusion

The fact that we can temporarily override UserDefaults with launch argument makes it a convenient tool to test out screen or part of an app that relies on a UserDefaults value.

Here is a sample SwiftUI view that uses the skipOnboarding flag to determine which view to show.

struct ContentView: View {

    @AppStorage("skipOnboarding")
    var skipOnboarding = false
    
    var body: some View {
        if skipOnboarding {
            Text("Home")
        } else {
            Text("Onboarding")
            Button("Skip") {
                skipOnboarding = true
            }
        }
    }
}

You can use a launch argument to directly run the app with skipOnboarding set to true or false.

-skipOnboarding false

-skipOnboarding true

Once you finish testing, you can simply remove the launch argument without polluting your current skipOnboarding value.