<aside> 💡 The most important difference in SwiftUI is that you do NOT change the UI directly. Instead, views (user interfaces) always reflects the current state, and you update the states to update UI.
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If you are new to Swift (the programming language) but have prior programming experience, you should check out A Swift Tour in The Swift Programming Language book for a quick introduction to the most important concepts and features of the language.
A Swift Tour - The Swift Programming Language (Swift 5.6)
With some basic understanding of the Swift Programming Language, you can start to learn about SwiftUI by watching SwiftUI related WWDC videos. We recommend starting with:
Introduction to SwiftUI - WWDC20 - Videos - Apple Developer
then some more discussion about SwiftUI
SwiftUI Essentials - WWDC19 - Videos - Apple Developer
Data Essentials in SwiftUI - WWDC20 - Videos - Apple Developer
Optionally, you can practice using SwiftUI by following these tutorials:
Other good SwiftUI resources include courses like
CS193p - Developing Apps for iOS
and sample code from Apple, documentations, and blog posts
Hacking with Swift - learn to code iPhone and iPad apps with free Swift 5.4 tutorials