![]() For example, the if/else above gets translated into a _ConditionalContent. An if/else statement becomes a _ConditionalContent.Import SwiftUI struct ContentView : View gets the type Optional. Here’s the entire SwiftUI code for the counter application: When you see unexpected behavior, make sure to doublecheck on a real device. Note: At the time of writing, Xcode’s built-in previews for SwiftUI, Playgrounds, and the simulator don’t always work. For example, when the user taps the counter button, we should increment our state and let SwiftUI rerender the view tree. SwiftUI then updates the screen to reflect these new view values. To change what’s onscreen, you modify state, and a new tree of view values is computed. To construct views in SwiftUI, you create a tree of view values that describe what should be onscreen. And as a matter of fact, this describes our experience with learning SwiftUI. We believe this advice applies not just to programming languages, but also to complicated frameworks such as SwiftUI. The only way to learn a new programming language is by writing programs in it. We strongly recommend following along by running and modifying the code yourself. The label shows either the number of times the counter was tapped, or a placeholder if the button hasn’t been tapped yet: The example app in its launch state… …and after tapping a few times The app has a button to increase the counter, and below the button is a label. We’ll build a simple counter for our sample application. In subsequent chapters, we’ll dive into more detail on each of the aspects described in this chapter. Hopefully this will give you a first look at the new mental model that’s required for working with SwiftUI. We’ll walk through a simple SwiftUI application and explore how views are constructed, laid out, and updated. SwiftUI is a radical conceptual departure from the previous way of developing apps on Apple’s platforms, and it requires you to rethink how to translate an idea you have in mind into working code. In this chapter, we’ll give you an overview of how SwiftUI works and how it works differently from frameworks like UIKit. Chris would like to thank Erni and Martina for providing a good place to write. Thanks to Natalye Childress for copy editing. Thanks to Javier Nigro, Matt Gallagher, and Ole Begemann for your invaluable feedback on our book. Instead, this book focuses on the concepts behind SwiftUI that we believe are essential to understand and which will prepare you for the next decade of SwiftUI development. For example, we won’t discuss how to use a navigation view on iOS, a split view on macOS, or a carousel on watchOS - especially since specific APIs will change and develop over the coming years. Since SwiftUI is a young framework, this book is not a reference of all the (platform-specific) Swift APIs. We use several examples – ranging from implicit animations to custom ones – to show how to work with this new animation system. Like all view updates in SwiftUI, animations are triggered by state changes. We also show advanced techniques for creating complex custom layouts.įinally, this book covers animations. ![]() To help break this down, we explain the layout behavior of elementary views and view containers and how they can be composed. The layout system is simple at its core, but it can appear complicated at first. SwiftUI also comes with its own layout system that fits its declarative nature. The primary goal of this book is to help you develop and hone your intuition of SwiftUI and the new approach it entails. While this eliminates a whole category of bugs (views getting out of sync with the application’s state), it also means you have to think differently about how to translate an idea into working SwiftUI code. Compared to how they’re handled in object-oriented frameworks, view construction and view updates are expressed in an entirely different, declarative way. In SwiftUI, views are values instead of objects. background(Color.SwiftUI is a radical departure from UIKit, AppKit, and other object-oriented UI frameworks. We can use this to our advantage: by adding a padding, then adding a background to our new View, we can create our own additional layers: Image("cat") ![]() This is also why the order in which you add modifiers matters. Consider this: adding a modifier to a view will return a new View instance that wraps the previous instance.
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