Using alert() and sheet() with optionals

Paul Hudson    @twostraws    March 17th 2024

SwiftUI has two ways of creating alerts and sheets, and so far we’ve mostly only used one: a binding to a Boolean that shows the alert or sheet when the Boolean becomes true.

The second option allows us to bind an optional to the alert or sheet. The key is to use an optional Identifiable object as the condition for showing the sheet, and the closure hands you the non-optional value that was used for the condition, so you can use it safely.

To demonstrate this, we could create a trivial User struct that conforms to the Identifiable protocol:

struct User: Identifiable {
    var id = "Taylor Swift"
}

We could then create a property inside ContentView that tracks which user is selected, set to nil by default:

@State private var selectedUser: User? = nil

Now we can change the body of ContentView so that it sets selectedUser to a value when a button is tapped, then displays a sheet when selectedUser is given a value:

Button("Tap Me") {
    selectedUser = User()
}
.sheet(item: $selectedUser) { user in
    Text(user.id)
}

With that simple code, whenever you tap “Tap Me” a sheet saying “Taylor Swift” appears. As soon as the sheet is dismissed, SwiftUI sets selectedUser back to nil.

Alerts have similar functionality, although you need to pass both the Boolean and optional Identifiable value at the same time. This allows you to show the alert when needed, but also benefit from the same optional unwrapping behavior we have with sheets.

First, add a Boolean property to watch:

@State private var isShowingUser = false

Then toggle that Boolean in your button's action:

isShowingUser = true

And finally change your sheet() modifier for this:

.alert("Welcome", isPresented: $isShowingUser, presenting: selectedUser) { user in
    Button(user.id) { }
}

With that covered, you now know practically all there is to know about sheets and alerts, but there is one last thing I want to sneak in to round out your knowledge.

When we're presenting a sheet, we can add presentation detents to make the sheet take up less than the full screen space. This is done using the presentationDetents() modifier, which accepts a set of sizes to use for the sheet.

For example, we could specify medium and large sizes like this:

Text(selectedUser.id)
    .presentationDetents([.medium, .large])

Because we're specifying two sizes here, the initial size will be used when the sheet is first shown, but iOS will show a little grab handle that lets users pull the sheet up to full size.

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