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What is the @ScaledMetric property wrapper?

Paul Hudson    @twostraws   

Updated for Xcode 13.3

New in iOS 14

SwiftUI gives us the @ScaledMetric property wrapper to define numbers that should scale automatically according to the user’s Dynamic Type settings.

In its most basic usage, you give your property a default value and @ScaledMetric will do the rest. For example, this will draw the same picture at different sizes depending on the user’s settings:

struct ContentView: View {
    @ScaledMetric var imageSize = 100.0

    var body: some View {
        Image(systemName: "cloud.sun.bolt.fill")
            .frame(width: imageSize, height: imageSize)

If you need your scaling to match up against a specific other piece of text, you can use the relativeTo parameter for your property wrapper, which lets you specify what font size to match. For example, this will scale alongside the large title size:

@ScaledMetric(relativeTo: .largeTitle) var imageSize = 100.0
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