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Understanding property wrappers in Swift and SwiftUI

Paul Hudson    @twostraws   

Fully updated for Xcode 11.4

SwiftUI relies heavily on property wrappers to make our code easier to read, write, and maintain, but if you’ve never used them before you’d be forgiven for wondering where all the @ and $ signs have come from – they can seem alien at first.

Although property wrappers are a general Swift feature since they were introduced in Swift 5.1, they are particularly common in SwiftUI – you’ll see @Published, @ObservedObject, @EnvironmentObject and many more, all with the purpose of helping reduce the amount of boilerplate in our code.

We’ll be going into detail on each one of SwiftUI’s property wrappers in the next few chapters, but just briefly it’s worth summarizing the basics:

  • Some property wrappers allow us to achieve effects that would otherwise not be possible, such as the way @State lets us modify properties inside a struct.
  • Some property wrappers specifically require you to have done extra work elsewhere, and may crash your app if that work isn’t done. For example, @FetchRequest expects you to have placed a Core Data managed object context into the environment ahead of time.
  • You may apply only one property wrapper at a time, which means @ObservedObject @Binding var value = SomeClass() is not allowed.
  • Even though some property wrappers look similar (looking at you, @Environment and @EnvironmentObject!) they are all different and it’s important you use them appropriately.
  • You can create your own property wrappers if you want, but this isn’t required to use SwiftUI.

That covers the basics, but to really understand how these property wrappers work it’s worth investigating them individually.

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