Demystify SwiftUI

SwiftUI look on these three things

• Identity: how SwiftUI recognizes each elements. which element is same or different across multiple updates of your app
• Lifetime: SwiftUI tracks the existence of views and data over time
• Dependencies: how SwiftUI understand when the view needs to be updated and why

Views that share the same identity /could/ represent different states of the same UI element

View that represent distinct UI elements will always have different identities.

Types of identity:

1. custom or data driven ID
2. structural ID: differentiate views by type and their position on the view hierarchy

Every view has an ID even if it’s not explicit identity.

Avoid AnyView as much as possible because it will hard for SwiftUI to see structure of the code. This is because AnyView is type-erasing wrapper type which hides the type of the view.

To avoid AnyView we can mark the function with @ViewBuilder and return the view type directly instead of wrapping it in AnyView

@ViewBuilder support switch statement

Identity allows us to define stable element for different values over time. Views can have different states throughout their lifetime.

View value != view identity

@State and @StateObject are persistent storage associated with the view’s identity

Whenever the view identity changes, the state is replaced with the initial value. The persistance of your state is tied to the lifetime of your views.

A dependency is just an input to the view so when the dependency changes, the view is required to produce new view body.

When defining identity, we should look not just it’s stability but also its uniqueness

Related WWDC sessions:

• Data essentials in SwiftUI from WWDC 2020