Text mirroring in SwiftUI

(Xcode 13.4.1)

In this project we use SwiftUI properties wrappers to reflect a textfield's value in a separated view.

Properties wrappers

State is inevitable in any modern app, but with SwiftUI it’s important to remember that all of our views are simply functions of their state – we don’t change the views directly, but instead manipulate the state and let that dictate the result. (Paul Husdon, on hackingwithswift)

Properties wrappers are one of the biggest features of SwiftUI. Some of them allows you to manage the state of your application.

• @StateObject private var entry in swiftui_testApp.swift

  This creates and owns the property entry that we will place in the environment later on.

• @Published var value: String in Entry.swift

  The @Published keyword tells to SwiftUI we want to notify every views that observe this property.

• @EnvironmentObject var entry: Entry in Card.swift and Mirror.swift

  An @EnvironmentObject catch a property we previously placed in the environment (the entry in swiftui_testApp.swift). This way we will get notified every time the @Published value is updated. In Card.swift we write the value using text-binding $entry.value and in Mirror.swift we read the value.

Using those 3 wrappers, Mirror.swift gets notified every time the entry is updated by Card.swift so the text is automatically reflected.

Different approach

We could also have used an @ObservedObject property (see a different implementation on swift_ui-observed-object branch).

Here, we use an @ObservedObject instead of using an @AppState object. As we saw earlier, the difference between these two wrapper is ownership. @AppState owns the object and handles its lifetime while @ObservedObject — as its name suggests — observe the object without owning it directly.

Note: While @EnvironmentObject can be catch everywhere in your application (assuming that it has been previously placed in the environment), when we use @ObservedObject, if we want to pass the object from view A to D, it has to be passed through each views.

Few references

• https://www.swiftbysundell.com/articles/property-wrappers-in-swift/
• https://www.hackingwithswift.com/quick-start/swiftui/whats-the-difference-between-observedobject-state-and-environmentobject
• https://www.hackingwithswift.com/quick-start/swiftui/what-is-the-observedobject-property-wrapper
• https://www.hackingwithswift.com/quick-start/swiftui/what-is-the-published-property-wrapper
• https://www.hackingwithswift.com/quick-start/swiftui/what-is-the-state-property-wrapper
• https://www.hackingwithswift.com/quick-start/swiftui/what-is-the-stateobject-property-wrapper
• https://www.hackingwithswift.com/quick-start/swiftui/what-is-the-environmentobject-property-wrapper