CS193p Notes - Lecture 3: Reactive UI + Protocols + Layout

• ViewModel is never calling View but View calls ViewModel
  • One ViewModel can be used by many Views and that is why it's a class
• Holding option and clicking a variable name allows to do all kind of things like rename.
• mutating keyword is used front of func when it's changing self
• in struct EmojiMemoryGameView: View { View is called protocol (constrains and gains). The idea is that we gain something (functions) from using it but it also contrains us to do something like have var body: some View

Reactive

• ObservableObject is constraint and gain that gives a class (ViewModel) ability to follow changes. Every time something changes it updates the UI.
  • objectWillChange.send() is can be added to functions where some object will change.
  • It's more common to add @Published property wrapper front of the variables that will change because that way you don't forget to add objectWillChange.send() to some position.
    • Behind the scenes it checks that every time that variable is changes it calls objectWillChange.send()
• @ObservedObject is added front of ModelView in View class to redraw it every time the ModelView changes.
• protocol is like stuct/class that can contain vars and functions but it doesn't had implementation to them.
  • Classes and structs can use protocol following way class Car: Vehicle It's possible to implement multiple protocols by separating them with commas.
  • Protocols can also inherit other protocols protocol Vehicle: Moveable ...
  • protocol offers constrains and gains
  • The idea in protocols is that multiple structs/classes can use it and then if it's extended all of them get the new feature
• extension is used to add new functionalities to structs, classes, protocols
• Protocols are useful because they say what the types (structs/classes/other protocols) are capable of and also demand certain behavior.

protocol Greatness {
    func isGreaterThan(other: Self) -> Bool
}

extension Array where Element: Greatness {
    var greatest: Element {
        // for-loop through all the elements
        // which (inside this extension) we know each implementes the Greatness protocol
        // and figure out which one is greatest by calling isGreaterThan(other:) on them
        return the greatest by calling isGreaterThan on each Element
    }
}

• Self means that type comes from the thing that is implementing that protocol.

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Layout (49:57)

• HStack and VStack
  • Offers space to its "least flexible" subviews first
    • Image: "inflexible"
    • Text: "slightly more flexible"
    • RoudedRectangle: "flexible"
  • Defaults work a little bit differently in different devices and that is why we should keep them and not put specific numbers because those probably doesn't work as many device
  • Spacer takes the space given and draws nothing
  • Divider Draws line (horizontal or vertical depending the stack)

HStack {
    Text("Important").layoutPriority(100)
    Image(systemName: "arrow.up") // default layout priority is 0
    Text("Unimportant")
}

• Important text takes the space it wants
• Then image takes the space it wants
• Finally Unimportant text takes the remaining space

---

Demo (1:12:04)

struct CardView: View {
  GeometryReader { geometry in 
    ZStack {
      if self.card.isFaceUp {
        RoundedRectangle(cornerRadius: 10.0).fill(Color.white)
        RoundedRectangle(cornerRadius: 10.0).stroke(lineWidth: 3)
      } else {
        RoundedRectangle(cornerRadius: 10.0).fill()
      }
    }
    .font(Font.system(size: min(geometry.size.width, geometry.size.height) * 0.75))
}

It's common to move the numbers to a "control panel". In one place it's easy to modify them and the main code becomes cleaner.

@RealLankinen

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Originally published here