@State property wrapper in iOS Swift - Cheat Sheet & Quick Revision

Recall & Review

beginner

What is the purpose of the @State property wrapper in SwiftUI?

The @State property wrapper is used to declare a source of truth for data that can change over time within a SwiftUI view. It allows the view to update its UI automatically when the data changes.

beginner

How does @State help with UI updates in SwiftUI?

When a @State variable changes, SwiftUI automatically re-renders the parts of the view that depend on that variable, keeping the UI in sync with the data.

intermediate

Can you explain why @State variables must be private in SwiftUI views?

Because @State variables are the source of truth owned by the view, they should be private to prevent external modification. This keeps data flow predictable and avoids bugs.

beginner

What happens if you try to modify a regular variable in a SwiftUI view instead of a @State variable?

Modifying a regular variable does not trigger a UI update. The view will not refresh to reflect the change, so the UI and data can get out of sync.

beginner

Show a simple example of declaring a @State variable in a SwiftUI view.

Example:<br>

struct ContentView: View {
  @State private var count = 0

  var body: some View {
    Button("Tap count: \(count)") {
      count += 1
    }
  }
}

What does the @State property wrapper do in SwiftUI?

A Manages network requests automatically

B Defines a constant value that never changes

C Stores data that triggers UI updates when changed

D Creates a global variable accessible anywhere

@State stores data that when changed, causes the SwiftUI view to update its UI automatically.

Why should @State variables usually be private?

A To prevent external code from modifying the view's source of truth

B Because SwiftUI requires all variables to be private

C To make the variable accessible globally

D To improve app performance

Keeping @State variables private ensures the view controls its own data, avoiding unexpected changes from outside.

What happens if you change a regular variable inside a SwiftUI view without @State?

A The UI updates automatically

B The UI will not update to reflect the change

C The app crashes

D The variable becomes a constant

Only changes to @State variables trigger UI updates. Regular variables do not cause the view to refresh.

Which of these is a correct way to declare a @State variable?

A @State private var isOn = false

B var isOn = @State false

C private var @State isOn = false

D let @State isOn = false

The correct syntax is @State private var variableName = value.

When a @State variable changes, what does SwiftUI do?

A Saves the variable to disk

B Deletes the variable

C Does nothing automatically

D Re-renders the view parts that depend on that variable

SwiftUI automatically updates the UI to reflect changes in @State variables.

Explain in your own words what the @State property wrapper does in SwiftUI and why it is important.

Think about how changing data updates what the user sees.

Describe a simple example where you would use a @State variable in a SwiftUI app.

Imagine a button that changes a number on screen when tapped.