ContainerValueKey

protocol · iOS 18.0+ · SDK 27.1

protocol ContainerValueKey

A key for accessing container values. You can create custom container values by extending the ``ContainerValues`` structure with new properties. First declare a new container value key type and specify a value for the required ``defaultValue`` property: private struct MyContainerValueKey: ContainerValueKey { 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 container value property: extension ContainerValues { var myCustomValue: String { get { self[MyContainerValueKey.self] } set { self[MyContainerValueKey.self] = newValue } } } Clients of your container value never use the key directly. Instead, they use the key path of your custom container value property. To set the container value for a view, add the ``View/containerValue(_:_:)`` view modifier to that view: MyView() .containerValue(\.myCustomValue, "Another string") As a convenience, you can also define a dedicated view modifier to apply this container value: extension View { func myCustomValue(_ myCustomValue: String) -> some View { containerValue(\.myCustomValue, myCustomValue) } This improves clarity at the call site: MyView() .myCustomValue("Another string") To read the container value, use `Group(subviews:)` on a containing view, and then access the container value on members of that collection. @ContentBuilder var content: some View { Text("A").myCustomValue("Hello") Text("B").myCustomValue("World") } Group(subviews: content) { subviews in ForEach(subviews) { subview in Text(subview.containerValues.myCustomValue) } } In practice, this will mostly be used by views that contain multiple other views to extract information from their subviews. You could turn the example above into such a container view as follows: struct MyContainer<Content: View>: View { var content: Content init(@ContentBuilder content: () -> Content) { self.content = content() } var body: some View { Group(subviews: content) { subviews in ForEach(subviews) { subview in // Display each view side-by-side with its custom value. HStack { subview Text(subview.containerValues.myCustomValue) } } } } }

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