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What’s the difference between @ObservedObject, @State, and @EnvironmentObject?

Paul Hudson    @twostraws   

Fully updated for Xcode 11.1

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.

SwiftUI gives us several ways of storing state in our application, but they are subtly different and it’s important to understand how they are different in order to use the framework properly.

In all the state examples we’ve so far we’ve use @State to create properties like this:

struct ContentView: View {
    @State private var score = 0
    // more code
}

This creates a property inside a view, but it uses the @State property wrapper to ask SwiftUI to manage the memory. This matters: all our views are structs, which means they can’t be changed, and if we couldn’t add 1 to a score in a game then it isn’t much of a game.

So, when we say @State to make a property, we hand control over it to SwiftUI so that it remains persistent in memory for as long as the view exists. When that state changes, SwiftUI knows to automatically reload the view with the latest changes so it can reflect its new information.

@State is great for simple properties that belong to a specific view and never get used outside that view, so as a result it’s important to mark those properties as being private to re-enforce the idea that such state is specifically designed never to escape its view.

What is @ObservedObject?

For more complex properties – when you have a custom type you want to use that might have multiple properties and methods, or might be shared across multiple views – you should use @ObservedObject instead.

This is very similar to @State except now we’re using an external reference type rather than a simple local property like a string or an integer. You’re still saying that your view depends on data that will change, except now it’s data you’re responsible for managing yourself – you need to create an instance of the class, create its own properties, and so on.

Whatever type you use with @ObservedObject should conform to the ObservableObject protocol. When you create properties on observable objects you get to decide whether changes to each property should force the view to refresh or not. You usually will, but it’s not required.

There are several ways for an observed object to notify views that important data has changed, but the easiest is using the @Published property wrapper. You can also use custom publishers from the Combine framework if you need more control, but realistically this will be very rare. If the observable object happens to have several views using its data, either option will automatically notify them all.

Warning: When you use a custom publisher to announce that your object has changed, this must happen on the main thread.

What is @EnvironmentObject?

You’ve seen how @State declares simple properties for a type that automatically cause a refresh of the view when it changes, and how @ObservedObject declares a property for an external type that may or may not cause a refresh of the view when it changes. Both of these two must be set by your view, but @ObservedObject might be shared with other views.

There’s a third type of property available to use, which is @EnvironmentObject. This is a value that is made available to your views through the application itself – it’s shared data that every view can read if they want to. So, if your app had some important model data that all views needed to read, you could either hand it from view to view to view or just put it into the environment where every view has instant access to it.

Think of @EnvironmentObject as a massive convenience for times when you need to pass lots of data around your app. Because all views point to the same model, if one view changes the model all views immediately update – there’s no risk of getting different parts of your app out of sync.

Summing up the differences

  • Use @State for simple properties that belong to a single view. They should usually be marked private.
  • Use @ObservedObject for complex properties that might belong to several views. Any time you’re using a reference type you should be using @ObservedObject for it.
  • Use @EnvironmentObject for properties that were created elsewhere in the app, such as shared data.

Of the three you will find that @ObservedObject is both the most useful and the most commonly used, so if you’re not sure which to use start there.

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