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Lankinen

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CS193p Notes - Lecture 9: Data Flow

#ios #swift

Property Wrappers

  • Property wrappers are things like @Publish that are put before var
  • A property wrapper actually creates a struct 
@Published var emojiArt: EmojiArt = EmojiArt() struct Published { var wrappedValue: EmojiArt var projectedValue: Publisher<EmojiArt, Never> } var _emojiArt: Published = Published(wrappedValue: EmojiArt()) var emojiArt: EmojiArt { get { _emojiArt.wrappedValue } set { _emojiArt.wrappedValue = newValue } }
  • Publisher<EmojiArt, Never> is a type that property wrapper decides
    • The point of wrappers is that it's possible to do something when the value is get/set
    • For example when Published wrapperValue is changed, it's calling objectWillChange.send()
  • $ is binding
  • Binding is used a lot and it's very important because it's all about having a single source of the truth 
struct MyView: View { @State var myString = "Hello" var body: View { OtherView(sharedText: $myString) } } struct OtherView: View { @Binding var sharedText: String var body: View { Text(sharedText) } }
  • @EnvironmentObject is similar to .fill() because when it's use in viewModel it's actually added to all of the views under neath the main View where it's called.

Publishers (18:17)

  • Publisher<Output, Failure>
    • Output is the type of thing this Publisher publishes
    • Failure is the type of thing it communicates if it fails while trying to publish

Demo 

import Combine ... @Published private var emojiArt: EmojiArt private var autosaveCancellable: AnyCancellable? ... init() { ... autosaveCancellable = $emojiArt.sink { emojiArt in print("\(emojiArt.json?.utf8 ?? "nill")") UserDefaults.standard.set(emojiArt.json, forKey: EmojiArtDocument.untitled) } ...
  • When sink is stored to a variable, it's going to work as long as this ViewModel exists
    • This way it's auto saving all the changes as long as ViewModel is used

@RealLankinen

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