WWDC Notes

Data Flow Through SwiftUI

Show Apple's description.
SwiftUI was built from the ground up to let you write beautiful and correct user interfaces free of inconsistencies. Learn how to connect your data as dependencies while keeping the UI fully predictable and error free. Familiarize yourself with SwiftUI’s powerful data flow tools and understand what the best tool is for each situation.

Principles of Data Flow

  • Every time we read a piece of data in our view, we're creating a dependency for that view.
  • Every time the data changes, the view has to change (read: re-render) to reflect the new value.
  • Every piece of data that we're reading in the view hierarchy has a source of truth.
  • The source of truth can live in the view hierarchy (if view-related, like a collapsed status), or externally (our models).
  • We should always have a single source of truth (no duplicate values).

@State

  • When a property has an associated property wrapper @State, SwiftUI knows that it needs to persist the storage across multiple updates of the same view.
  • Mark @State property as private, to really enforce the idea that state is owned and managed by that view specifically.
  • @State properties are a special case of state variables, as SwiftUI knows when they change.
  • And because SwiftUI knows that the @State variables change the view body look, SwiftUI knows that the view rendering depend on their state.
  • When a @State property changes, in the runtime SwiftUI will recompute the body of that view and all of its children.
  • All the changes always flow down through your view hierarchy.
  • We're able to do these re-rendering very efficiently because SwiftUI compares the views and renders again only what is changed.
  • Views are a function of state, not of a sequence of events.

@Binding

  • By using the Binding property wrapper, we define an explicit dependency to a source of truth without owning it.
  • We don't need to provide an initial value because the binding can be derived from a state.
  • Primitives views such as Toggle, TextField, and Slider all expect a binding. We can read and write a @binding property (without owning it!)

Publishers (External changes)

  • In case the event comes from an external source (a.k.a. not within the views), we use Combine’s Publishers
  • It's very important for Views to receive events on the main thread.

ObservableObject (External Data)

  • When the data is outside our View, swiftUI needs to know how to react on changes of that data
  • By making our external data conform to ObservableObject protocol, we are required to have a publisher, accessible via the synthesized objectWillChange property, that fires every time the data is about to change.
  • All we need to do later is to associate an @ObservedObject property wrapper in the model and pass the ObservedObject to the view in its initialization.
  • While SwiftUI Views are value types (Structs), any time we're using a reference type, we should be using the @ObservedObject property wrapper: this way SwiftUI will know when that data changes and can keep our view hierarchy up to date.

Environment

  • Thanks to the .environment modifier, we can add our ObservableObjects into the environment.
  • By using this @EnvironemntObject property wrapper, we can create a dependency to these environment objects.

Object Binding vs Environment

  • We can build your whole app with @Binding, but it can get kind of cumbersome to pass around the model from view to view.
  • EnvironmentObject is a convenience for passing data around your hierarchy indirectly (without the need to pass the object to each view first).

@State vs @ObservedObject

  • @State is great for data that's view local, a value type, managed.
  • @State is allocated, created and managed by SwiftUI
  • @State is value type
  • @ObservedObject is for representing external data to SwiftUI, such as a database.
  • @ObservedObject is storage that we manage, which makes it great for the model that we already have.
  • @ObservedObject is managed by us
  • @ObservedObject is reference type

Missing anything? Corrections? Contributions are welcome 😃

Related

Written by

zntfdr

Federico Zanetello

Federico Zanetello is an iOS Engineer with strong passion for Swift, minimalism, and design. When he’s not busy automating things, he can be found writing at fivestars.blog and/or playing with the latest shiny toys.