![]() Both of these two must be set by your view, but might be shared with other views. What is seen how declares simple properties for a type that automatically cause a refresh of the view when it changes, and how declares a property for an external type that may or may not cause a refresh of the view when it changes. All other views must use to tell SwiftUI they want to watch the object for changes but don’t own it directly. ![]() The rule is this: whichever view is the first to create your object must use to tell SwiftUI it is the owner of the data and is responsible for keeping it alive. There is one important difference between and which is ownership – which view created the object, and which view is just watching it. What is between and lies This is a specialized version of and it works in almost exactly the same way: you must conform to the ObservableObject protocol, you can use to mark properties as causing change notifications, and any views that watch an will refresh their body when the object changes. Warning: When you use a custom publisher to announce that your object has changed, this must happen on the main thread. If the observable object happens to have several views using its data, either option will automatically notify them all. You can also use custom publishers from the Combine framework if you need more control, but realistically this will be very rare. There are several ways for an observed object to notify views that important data has changed, but the easiest is using the property wrapper. When you add properties to observable objects you get to decide whether changes to each property should force views that are watching your object to refresh or not. Whatever type you use with should conform to the ObservableObject protocol. 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. This is very similar to except now we’re using an external reference type rather than a simple local property like a string or an integer. What is 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 will often use instead. ![]() When that state changes, SwiftUI knows to automatically reload the view with the latest changes so it can reflect its new 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. So, when we say 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. This matters: all our views are structs, which means they can’t be changed, and if we weren’t even able to modify an integer in our apps then there wouldn’t be much we could do. This creates a property inside a view, but it uses the property wrapper to ask SwiftUI to manage the memory. The simplest way of working with state is the property wrapper, used like this: struct ContentView: View private var tapCount = 0 ![]() 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. 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. What’s the difference between and Hudson September 3rd 2021
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