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Understanding SwiftUI

My understanding of SwiftUI

  • The main advantage of SwiftUI over UIKit is to write both your initial UI rendering as well as any updates to your UI in one go.
  • On a very abstract level, this is similar to TEA, but practically, it's quite different: updates in SwiftUI are incremental, and for many things there's no diffing necessary.

In SwiftUI, you define your views as structs that conform to View. The View protocol has a single requirement: you need to implement body, a computed property that returns another type conforming to View.

Just like normal structs in Swift, you can use properties anywhere inside your View. However, unlike normal structs, some properties are special. Specifically, properties that are defined using the property wrappers EnvironmentObject, ObjectBinding, State (and some others). When SwiftUI renders the body, it will subscribe to these properties. When these properties change, they send a notification, and SwiftUI will re-render the necessary parts. I call these special properties "sources of truth".

There's another type called Binding. A Binding<Foo> contains two things: a way to get a value of type Foo, and a way to set a value of type Foo. A binding isn't a source of truth: there's no way for SwiftUI to subscribe to a binding, or to know (directly) that its value changed. Instead, a binding is typically derived from a source of truth. When SwiftUI sets a value through a binding, the source of truth is changed, and this causes the re-render.

  • TODO: difference between List(someCollection) and List { ForEach(someCollection) }.

  • TODO: source of truth / bindings & UIViewRepresentable

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