What is `GeometryReader` and what are its pitfalls?

GeometryReader is a container view that exposes the size and coordinate space of its parent, letting you build size-dependent layouts.
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Answer: GeometryReader is a container view that exposes the size and coordinate space of its parent, letting you build size-dependent layouts. However it comes with notable downsides.

Code Example:

struct ProportionalView: View {
    var body: some View {
        GeometryReader { geometry in
            Rectangle()
                .frame(width: geometry.size.width * 0.5,
                       height: geometry.size.height * 0.3)
        }
    }
}

Pitfalls:

Expands to fill all available space by default — can break layouts
Triggers layout recalculation which can hurt performance
Avoid wrapping simple views in it unnecessarily

Better alternatives in many cases:

// Use .containerRelativeFrame for proportional sizing (iOS 17+)
Rectangle()
    .containerRelativeFrame(.horizontal) { size, _ in size * 0.5 }