LayoutComputer becomes nil during if statement layout, falling back to DefaultEngine

[Dark-Existed]

I implemented a custom layout similar to the original ZStack to help diagnose the issue. It prints the dimensions during sizeThatFits.

struct MyCustomZStack: Layout {
    public var alignment: Alignment

    @inlinable
    public init(alignment: Alignment = .center) {
        self.alignment = alignment
    }
    
    func sizeThatFits(proposal: ProposedViewSize, subviews: Subviews, cache: inout ()) -> CGSize {
        guard !subviews.isEmpty else {
            return CGSize.zero
        }
        let maxPriority =
            subviews.lazy
            .map { $0.priority }
            .max() ?? 0.0
        return subviews.lazy
            .filter { $0.priority == maxPriority }
            .map {
                let dimension = $0.dimensions(in: proposal)
                print(dimension)
                return dimension
            }
            .reduce(CGSize.zero) { result, dimension in
                CGSize(
                    width: max(result.width, dimension.width),
                    height: max(result.height, dimension.height)
                )
            }
    }
    
    func placeSubviews(in bounds: CGRect, proposal: ProposedViewSize, subviews: Subviews, cache: inout ()) {
        let maxPriority =
            subviews.lazy
                .map { $0.priority }
                .max() ?? 0.0
        let alignmentSize = subviews.lazy
            .filter { $0.priority == maxPriority }
            .map { $0.dimensions(in: ProposedViewSize(bounds.size)) }
            .reduce(CGSize(width: -.infinity, height: -.infinity)) { result, dimension in
                CGSize(
                    width: max(result.width, dimension[alignment.horizontal]),
                    height: max(result.height, dimension[alignment.vertical])
                )
            }
        
        for subview in subviews {
            let dimensions = subview.dimensions(in: ProposedViewSize(bounds.size))
            let horizontalAlignmentValue = dimensions[alignment.horizontal]
            let verticalAlignmentValue = dimensions[alignment.vertical]
            let origin = CGPoint(
                x: alignmentSize.width - horizontalAlignmentValue + bounds.origin.x,
                y: alignmentSize.height - verticalAlignmentValue + bounds.origin.y
            )
            subview.place(at: origin, proposal: proposal)
        }
    }
}

Test Case

struct TestExample: View {
    @State private var showed = true

    var body: some View {
       MyCustomZStack {
            if showed {
                Color.red
                    .frame(width: 300, height: 300)
            }
            Color.blue
                .frame(width: 200, height: 200)
                .onAppear {
                    DispatchQueue.main.asyncAfter(deadline: .now() + 1) {
                        first.toggle()
                    }
                }
        }
    }
}

Expected Behavior

SwiftUI Output

ViewDimensions(guideComputer: SwiftUI.LayoutComputer(box: SwiftUI.(unknown context at $1d4dfec58).LayoutEngineBox<SwiftUI.(unknown context at $1d4df00ec).UnaryLayoutEngine<SwiftUI._FrameLayout>>, seed: 0), size: SwiftUI.ViewSize(value: (300.0, 300.0), _proposal: (440.0, 860.0)))
ViewDimensions(guideComputer: SwiftUI.LayoutComputer(box: SwiftUI.(unknown context at $1d4dfec58).LayoutEngineBox<SwiftUI.(unknown context at $1d4df00ec).UnaryLayoutEngine<SwiftUI._FrameLayout>>, seed: 0), size: SwiftUI.ViewSize(value: (200.0, 200.0), _proposal: (440.0, 860.0)))
ViewDimensions(guideComputer: SwiftUI.LayoutComputer(box: SwiftUI.(unknown context at $1d4dfec58).LayoutEngineBox<SwiftUI.(unknown context at $1d4df00ec).UnaryLayoutEngine<SwiftUI._FrameLayout>>, seed: 1), size: SwiftUI.ViewSize(value: (200.0, 200.0), _proposal: (440.0, 860.0)))
ViewDimensions(guideComputer: SwiftUI.LayoutComputer(box: SwiftUI.(unknown context at $1d4dfec58).LayoutEngineBox<SwiftUI.(unknown context at $1d4df00ec).UnaryLayoutEngine<SwiftUI._FrameLayout>>, seed: 1), size: SwiftUI.ViewSize(value: (200.0, 200.0), _proposal: (440.0, 860.0)))

Observed Behavior

OpenSwiftUI Output

ViewDimensions(guideComputer: OpenSwiftUICore.LayoutComputer(box: OpenSwiftUICore.(unknown context at $1054ca2e8).LayoutEngineBox<OpenSwiftUICore.(unknown context at $1054ca7b0).UnaryLayoutEngine<OpenSwiftUICore._FrameLayout>>, seed: 0), size: OpenSwiftUICore.ViewSize(value: (300.0, 300.0), _proposal: (440.0, 821.6666666666667)))
ViewDimensions(guideComputer: OpenSwiftUICore.LayoutComputer(box: OpenSwiftUICore.(unknown context at $1054ca2e8).LayoutEngineBox<OpenSwiftUICore.(unknown context at $1054ca7b0).UnaryLayoutEngine<OpenSwiftUICore._FrameLayout>>, seed: 0), size: OpenSwiftUICore.ViewSize(value: (200.0, 200.0), _proposal: (440.0, 821.6666666666667)))
ViewDimensions(guideComputer: OpenSwiftUICore.LayoutComputer(box: OpenSwiftUICore.(unknown context at $1054ca2e8).LayoutEngineBox<OpenSwiftUICore.(unknown context at $1054ca7b0).UnaryLayoutEngine<OpenSwiftUICore._FrameLayout>>, seed: 1), size: OpenSwiftUICore.ViewSize(value: (200.0, 200.0), _proposal: (440.0, 821.6666666666667)))
ViewDimensions(guideComputer: OpenSwiftUICore.LayoutComputer(box: OpenSwiftUICore.(unknown context at $1054ca2e8).LayoutEngineBox<OpenSwiftUICore.LayoutComputer.DefaultEngine>, seed: 0), size: OpenSwiftUICore.ViewSize(value: (440.0, 821.6666666666667), _proposal: (440.0, 821.6666666666667)))

Related

The layoutComputer will be nil, and use the DefaultEngine.

package struct LayoutProxy: Equatable {

    ...

    package var layoutComputer: LayoutComputer {
        guard let layoutComputer = attributes.$layoutComputer else {
            return .defaultValue
        }
        return context[layoutComputer]
    }

    ...

    package func dimensions(in proposedSize: _ProposedSize) -> ViewDimensions {
        let computer = layoutComputer
        return ViewDimensions(
            guideComputer: computer,
            size: computer.sizeThatFits(proposedSize),
            proposal: _ProposedSize(
                width: proposedSize.width ?? .nan,
                height: proposedSize.height ?? .nan
            )
        )
    }
}

[Kyle-Ye]

Thanks for the report. I'll look at it later or you can help fix it directly if you have time.