Correctly account for zero-width spacers in implied distribution (#127)

Currently the implied distribution does not behave as documented in cases where the fixed spacers sum to zero (either by using zero-size spacers or by combining positive- and negative-sized spacers). This updates the logic to correctly identify the use of _any_ fixed spacer.

Resolves #126

Paralayout/ViewDistributionItem.swift

@@ -66,6 +66,7 @@ public enum ViewDistributionItem: ViewDistributionSpecifying, Sendable {
  var distributionItems = [ViewDistributionItem]()
  var totalViewSize: CGFloat = 0
  var totalFixedSpace: CGFloat = 0
+ var hasFixedSpacers: Bool = false
  var totalFlexibleSpace: CGFloat = 0
 
  var subviewsToDistribute = Set<UIView>()
@@ -92,6 +93,7 @@ public enum ViewDistributionItem: ViewDistributionSpecifying, Sendable {
 
  case .fixed:
  totalFixedSpace += layoutSize
+ hasFixedSpacers = true
 
  case .flexible:
  totalFlexibleSpace += layoutSize
@@ -107,7 +109,7 @@ public enum ViewDistributionItem: ViewDistributionSpecifying, Sendable {
 
  // Insert flexible space if necessary.
  if totalFlexibleSpace == 0 {
- if totalFixedSpace == 0 {
+ if !hasFixedSpacers {
  // No spacers at all: insert `1.flexible` between all items.
  for i in 0 ..< (distributionItems.count + 1) {
  distributionItems.insert(1.flexible, at: i * 2)

ParalayoutTests/DistributionTests.swift

@@ -214,4 +214,26 @@ final class DistributionTests: XCTestCase {
  }
  }
 
+ // MARK: - Tests - Spacers
+
+ @MainActor
+ func testZeroSizeFixedSpacer() {
+ let container = UIView(frame: .init(x: 0, y: 0, width: 100, height: 400))
+ container.semanticContentAttribute = .forceRightToLeft
+
+ let firstSubview = UIView(frame: .init(x: 0, y: 0, width: 100, height: 100))
+ container.addSubview(firstSubview)
+ let secondSubview = UIView(frame: .init(x: 0, y: 0, width: 100, height: 100))
+ container.addSubview(secondSubview)
+
+ container.applyVerticalSubviewDistribution {
+ firstSubview
+ 0.fixed
+ secondSubview
+ }
+
+ XCTAssertEqual(firstSubview.frame, .init(x: 0, y: 100, width: 100, height: 100))
+ XCTAssertEqual(secondSubview.frame, .init(x: 0, y: 200, width: 100, height: 100))
+ }
+
 }