EnvironmentKey

Overview 概要

You can create custom environment values by extending the EnvironmentValues structure with new properties. First declare a new environment key type and specify a value for the required defaultValue property:

private struct MyEnvironmentKey: EnvironmentKey {
    static let defaultValue: String = "Default value"
}

The Swift compiler automatically infers the associated Value type as the type you specify for the default value. Then use the key to define a new environment value property:

extension EnvironmentValues {
    var myCustomValue: String {
        get { self[MyEnvironmentKey.self] }
        set { self[MyEnvironmentKey.self] = newValue }
    }
}

Clients of your environment value never use the key directly. Instead, they use the key path of your custom environment value property. To set the environment value for a view and all its subviews, add the environment(_:_:) view modifier to that view:

MyView()
    .environment(\.myCustomValue, "Another string")

As a convenience, you can also define a dedicated view modifier to apply this environment value:

extension View {
    func myCustomValue(_ myCustomValue: String) -> some View {
        environment(\.myCustomValue, myCustomValue)
    }
}

This improves clarity at the call site:

MyView()
    .myCustomValue("Another string")

To read the value from inside MyView or one of its descendants, use the Environment property wrapper:

struct MyView: View {
    @Environment(\.myCustomValue) var customValue: String


    var body: some View {
        Text(customValue) // Displays "Another string".
    }
}