A completed SwiftUI app is constructed from multiple components which are assembled in a hierarchical manner. Before embarking on the creation of even the most basic of SwiftUI projects, it is useful to first gain an understanding of how SwiftUI apps are structured. With this goal in mind, this chapter will introduce the key elements of SwiftUI app architecture, with an emphasis on App, Scene and View elements.
SwiftUI App Hierarchy
When considering the structure of a SwiftUI application, it helps to view a typical hierarchy visually. Figure 18-1, for example, illustrates the hierarchy of a simple SwiftUI app:
Before continuing, it is important to distinguish the difference between the term “app” and the “App” element outlined in the above figure. The software applications that we install and run on our mobile devices have come to be referred to as “apps”. In this chapter reference will be made both to these apps and the App element in the above figure. To avoid confusion, we will use the term “application” to refer to the completed, installed and running app, while referring to the App element as “App”. The remainder of the book will revert to using the more common “app” when talking about applications.
The App object is the top-level element within the structure of a SwiftUI application and is responsible for handling the launching and lifecycle of each running instance of the application.
The App element is also responsible for managing the various Scenes that make up the user interface of the application. An application will include only one App instance.
Each SwiftUI application will contain one or more scenes. A scene represents a section or region of the application’s user interface. On iOS and watchOS a scene will typically take the form of a window which takes up the entire device screen. SwiftUI applications running on macOS and iPadOS, on the other hand, will likely be comprised of multiple scenes. Different scenes might, for example, contain context specific layouts to be displayed when tabs are selected by the user within a dialog, or to design applications that consist of multiple windows.
SwiftUI includes some pre-built primitive scene types that can be used when designing applications, the most common of which being WindowGroup and DocumentGroup. It is also possible to group scenes together to create your own custom scenes.
Views are the basic building blocks that make up the visual elements of the user interface such as buttons, labels and text fields. Each scene will contain a hierarchy of the views that make up a section of the application’s user interface. Views can either be individual visual elements such as text views or buttons, or take the form of containers that manage other views. The Vertical Stack view, for example, is designed to display child views in a vertical layout. In addition to the Views provided with SwiftUI, you will also create custom views when developing SwiftUI applications. These custom views will comprise groups of other views together with customizations to the appearance and behavior of those views to meet the requirements of the application’s user interface.
Figure 18-2, for example, illustrates a scene containing a simple view hierarchy consisting of a Vertical Stack containing a Button and TextView combination:
SwiftUI applications are constructed hierarchically. At the top of the hierarchy is the App instance which is responsible for the launching and lifecycle of the application. One or more child Scene instances contain hierarchies of the View instances that make up the user interface of the application. These scenes can either be derived from one of the SwiftUI primitive Scene types such as WindowGroup, or custom built.
On iOS or watchOS, an application will typically contain a single scene which takes the form of a window occupying the entire display. On a macOS or iPadOS system, however, an application may comprise multiple scene instances, often represented by separate windows which can be displayed simultaneously or grouped together in a tabbed interface.