2 posts tagged with "learning"

Reference: Demystify SwiftUI WWDC21

When SwiftUI looks at your code, it will look for three things:

• Identity
• Lifetime
• Dependencies

Identity

• Explicit: In this case, .dogTagID represents the ID of the rescueDog data type which conforms to the Identifiable protocol.

  • With a stable ID, SwiftUI can optimize performance.

• Implicit or Structural Identity: If a statement or switch statement in a SwiftUI view's body (@ViewBuilder) creates an implicit ID based on structural differences (i.e., the position of the view relative to its hierarchy).

• Avoid using AnyView if possible.

View Lifetime

• State Lifetime == View Lifetime, meaning when the view's identity changes, the state is destroyed along with that identity.

With this implementation, when date < .now, a different view modifier identity is created, which is not stable.

A better version would be to tie the condition in the value of opacity, so there is just a single View Modifier, which means better performance, more correct code, and SwiftUI is optimized for this kind of code.

#learning Data Essential in SwiftUI - 2020

• @State create a new source of truth
• @Binding create a reference to the source of truth allow write access to that state

Performance consideration: avoid Repeated Heap allocation problem

• @ObservableObject allow multiple SwiftUI view to subscribe to its changes (specifically before the property will change)
• ObservableObject protocol is for class (reference type only), which mean it will be kept alive in the memory if there is reference to it
• Class follow this protocol will usually need to have @Published property that will trigger event that ties to SwiftUI view updates
• ObservableObject can be used to provide a single source of truth through view / app hierarchy
• @Published can be used to mark property can be observed or bound to (Binding)

• ObservableObject can be used to provide a single source of truth through view / app hierarchy, either as a single Instance (one object) or multiple instances

There is 3 way that View can subscribe to changes of @ObservableObjects

• @ObservedObject, the life cycle of the observed object is not managed by view life cycle, but somewhere else
• @StateObject , the object is only initiated just before body of the view is created, and destroyed when the view is no longer rendered. In short, life-cycle of the object is managed by the view
• @EnvironmentObject is usually used when we want to create a globally shared Source of truth

Data Life-cycle

• Tied to the way we defined it

• Scene is new screen or new window instance, think Split view in ipad, windows in macos