Hello,

First of all I like the idea behind this library! However in practice it doesn't look usable yet:

This is what I get in some toy project, however in a real use case this issue is actually hitting hard with a crash both in debug and release due to this precondition failure:

[error] precondition failure: setting value during update: 2072

As far as I see, all methods exposed by UIViewRepresentable are called during the view update, so no state can be changed there and the current approach seems to hit a dead end.

I'm using Xcode 13.2.1 on macOS 12.1.

What

• Resolves #1

Trying to update ideal size state right after UIView generation caused a run-time warning, since this was effectively modifying a @State during body access.

How

• Don't update ideal size on UIView generation
  • We didn't really need this anyway, as we will always get at least one more update after the initial layout pass.
• Don't mutate UIViewHolder state, instead hold and update the calculated ideal size inside UIViewAdaptor

Review Points

• Does this PR resolve the issue?
• Are there any other problems with this fix?

Something wrong with UIStackView inside UIViewAdaptor, but I can't figure out what exactly. I tried to use different flexibility, idealSizeCalculator but still got this errors in console:

Unable to simultaneously satisfy constraints. Probably at least one of the constraints in the following list is one you don't want. Try this: (1) look at each constraint and try to figure out which you don't expect; (2) find the code that added the unwanted constraint or constraints and fix it. ( "<NSLayoutConstraint:0x28241fed0 UIView:0x13dd10ce0.height == 100 (active)>", "<NSLayoutConstraint:0x28241fb60 'UISV-alignment' UIView:0x13dd0ec50.bottom == UIView:0x13dd10ce0.bottom (active)>", "<NSLayoutConstraint:0x28241fb10 'UISV-alignment' UIView:0x13dd0ec50.top == UIView:0x13dd10ce0.top (active)>", "<NSLayoutConstraint:0x28241fc00 'UISV-canvas-connection' UIStackView:0x13dd10950.top == UIView:0x13dd0ec50.top (active)>", "<NSLayoutConstraint:0x28241fbb0 'UISV-canvas-connection' V:[UIView:0x13dd0ec50]-(0)-| (active, names: '|':UIStackView:0x13dd10950 )>", "<NSLayoutConstraint:0x28241f610 'UIView-Encapsulated-Layout-Height' UIStackView:0x13dd10950.height == 711 (active)>" )

Will attempt to recover by breaking constraint <NSLayoutConstraint:0x28241fed0 UIView:0x13dd10ce0.height == 100 (active)>

Make a symbolic breakpoint at UIViewAlertForUnsatisfiableConstraints to catch this in the debugger. The methods in the UIConstraintBasedLayoutDebugging category on UIView listed in <UIKitCore/UIView.h> may also be helpful.

var body: some View {
    VStack {
        UIViewAdaptor {
            let stack = UIStackView()
            
            let firstView = UIView()
            firstView.backgroundColor = .red
            
            let secondView = UIView()
            secondView.backgroundColor = .blue
            
            
            [firstView, secondView].forEach {
                $0.translatesAutoresizingMaskIntoConstraints = false
            }
        
            NSLayoutConstraint.activate([
                firstView.heightAnchor.constraint(equalToConstant: 100),
                secondView.heightAnchor.constraint(equalToConstant: 100)
            ])
            
            stack.addArrangedSubview(firstView)
            stack.addArrangedSubview(secondView)
            
            return stack
        }
        Spacer()
    }
}