Watched Stanford’s CS193P Spring 2021 Lecture 1: Getting started with SwiftUI.

Looked for View protocol declaration. Interesting notation View/body-swift.property, View/opacity(_:). Have never seen this.

/// A type that represents part of your app's user interface and provides
/// modifiers that you use to configure views.
///
/// You create custom views by declaring types that conform to the `View`
/// protocol. Implement the required ``View/body-swift.property`` computed
/// property to provide the content for your custom view.
///
///     struct MyView: View {
///         var body: some View {
///             Text("Hello, World!")
///         }
///     }
///
/// Assemble the view's body by combining one or more of the built-in views
/// provided by SwiftUI, like the ``Text`` instance in the example above, plus
/// other custom views that you define, into a hierarchy of views. For more
/// information about creating custom views, see <doc:Declaring-a-Custom-View>.
///
/// The `View` protocol provides a set of modifiers — protocol
/// methods with default implementations — that you use to configure
/// views in the layout of your app. Modifiers work by wrapping the
/// view instance on which you call them in another view with the specified
/// characteristics, as described in <doc:Configuring-Views>.
/// For example, adding the ``View/opacity(_:)`` modifier to a
/// text view returns a new view with some amount of transparency:
///
///     Text("Hello, World!")
///         .opacity(0.5) // Display partially transparent text.
///
/// The complete list of default modifiers provides a large set of controls
/// for managing views.
/// For example, you can fine tune <doc:View-Layout>,
/// add <doc:View-Accessibility> information,
/// and respond to <doc:View-Input-and-Events>.
/// You can also collect groups of default modifiers into new,
/// custom view modifiers for easy reuse.
@available(iOS 13.0, macOS 10.15, tvOS 13.0, watchOS 6.0, *)
public protocol View {

    /// The type of view representing the body of this view.
    ///
    /// When you create a custom view, Swift infers this type from your
    /// implementation of the required ``View/body-swift.property`` property.
    associatedtype Body : View

    /// The content and behavior of the view.
    ///
    /// When you implement a custom view, you must implement a computed
    /// `body` property to provide the content for your view. Return a view
    /// that's composed of built-in views that SwiftUI provides, plus other
    /// composite views that you've already defined:
    ///
    ///     struct MyView: View {
    ///         var body: some View {
    ///             Text("Hello, World!")
    ///         }
    ///     }
    ///
    /// For more information about composing views and a view hierarchy,
    /// see <doc:Declaring-a-Custom-View>.
    @ViewBuilder @MainActor var body: Self.Body { get }
}