How do you compute backing pixel alignment in SwiftUI's `Layout`?

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Created 2w

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When performing custom layout in AppKit, it's essential that you pixel align frames using methods like backingAlignedRect. The alignment differs depending on the backingScaleFactor of the parent window.

When building custom Layouts in SwiftUI, how should you compute the alignment of a subview.frame in placeSubviews() before calling subview.place(...)?

Surprisingly, I haven't seen any mention of this in the WWDC videos. However, if I create a Rectangle of width 1px and then position it on fractional coordinates, I get a blurry view, as I would expect.

Rounding to whole numbers works, but on Retina screens you should be able to round to 0.5 as well.

func placeSubviews(
    in bounds: CGRect,
    proposal: ProposedViewSize,
    subviews: Subviews,
    cache: inout Void
) {
  // This should be backing aligned based on the parent window's backing scale factor.
  var frame = CGRect(
    x: 10.3,
    y: 10.8,
    width: 300.6,
    height: 300.1
  )

  subview.place(
    at: frame.origin,
    anchor: .topLeading,
    proposal: ProposedViewSize(frame.size)
  )
}

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Answer 1

kennyc OP

2w

Stumbled across:

I suppose I can pass these into the custom Layout object for use during subview placement.

Still curious if there's any way to tell SwiftUI or Layout to always use backing aligned coordinates instead of having to calculate them all by hand (which I've oddly never seen mentioned in any documentation or videos).

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