Start using systemLayoutSizeFittingSize

This way we can tell the view whether a measurement is
exactly (which it should honor unconditionally), or not
(which it might honor if its constraint is Fill).

Author: squarejesse

redwood-layout-uiview/src/commonMain/kotlin/app/cash/redwood/layout/uiview/UIViewMeasureCallback.kt

@@ -23,6 +23,8 @@ import kotlinx.cinterop.CValue
 import kotlinx.cinterop.useContents
 import platform.CoreGraphics.CGSize
 import platform.CoreGraphics.CGSizeMake
+import platform.UIKit.UILayoutPriorityDefaultLow
+import platform.UIKit.UILayoutPriorityRequired
 import platform.UIKit.UIView
 import platform.UIKit.UIViewNoIntrinsicMetric
 
@@ -44,15 +46,18 @@ internal object UIViewMeasureCallback : MeasureCallback {
       else -> height.toDouble()
     }
 
-    // The default implementation of sizeThatFits: returns the existing size of
-    // the view. That means that if we want to layout an empty UIView, which
-    // already has a frame set, its measured size should be CGSizeZero, but
-    // UIKit returns the existing size. See https://github.com/facebook/yoga/issues/606
-    // for more information.
+    // The default implementation of sizeThatFits: returns the existing size of the view. That means
+    // that if we want to layout an empty UIView, which already has a frame set, its measured size
+    // should be CGSizeZero, but UIKit returns the existing size. For more information, see
+    // https://github.com/facebook/yoga/issues/606
     val sizeThatFits = if (view.isMemberOfClass(UIView.`class`()) && view.typedSubviews.isEmpty()) {
       Size(0f, 0f)
     } else {
-      view.sizeThatFits(CGSizeMake(constrainedWidth, constrainedHeight)).toSize()
+      view.systemLayoutSizeFittingSize(
+        targetSize = CGSizeMake(constrainedWidth, constrainedHeight),
+        withHorizontalFittingPriority = widthMode.toFittingPriority(),
+        verticalFittingPriority = heightMode.toFittingPriority(),
+      ).toSize()
     }
 
     return Size(
@@ -65,3 +70,8 @@ internal object UIViewMeasureCallback : MeasureCallback {
 private fun CValue<CGSize>.toSize() = useContents {
   Size(width.toFloat(), height.toFloat())
 }
+
+private fun MeasureMode.toFittingPriority() = when (this) {
+  MeasureMode.Exactly -> UILayoutPriorityRequired
+  else -> UILayoutPriorityDefaultLow
+}

redwood-layout-uiview/src/commonMain/kotlin/app/cash/redwood/layout/uiview/YogaUIView.kt

@@ -21,6 +21,8 @@ import platform.CoreGraphics.CGRectMake
 import platform.CoreGraphics.CGRectZero
 import platform.CoreGraphics.CGSize
 import platform.CoreGraphics.CGSizeMake
+import platform.UIKit.UILayoutPriority
+import platform.UIKit.UILayoutPriorityRequired
 import platform.UIKit.UIScrollView
 import platform.UIKit.UIScrollViewContentInsetAdjustmentBehavior.UIScrollViewContentInsetAdjustmentNever
 import platform.UIKit.UIScrollViewDelegateProtocol
@@ -107,6 +109,27 @@ internal class YogaUIView : UIScrollView(cValue { CGRectZero }), UIScrollViewDel
     }
   }
 
+  override fun systemLayoutSizeFittingSize(
+    targetSize: CValue<CGSize>,
+    withHorizontalFittingPriority: UILayoutPriority,
+    verticalFittingPriority: UILayoutPriority,
+  ): CValue<CGSize> {
+    return targetSize.useContents<CGSize, CValue<CGSize>> {
+      calculateLayout(
+        width = when {
+          withHorizontalFittingPriority == UILayoutPriorityRequired -> width.toYoga()
+          widthConstraint == Constraint.Fill -> width.toYoga()
+          else -> Size.UNDEFINED
+        },
+        height = when {
+          verticalFittingPriority == UILayoutPriorityRequired -> height.toYoga()
+          heightConstraint == Constraint.Fill -> height.toYoga()
+          else -> Size.UNDEFINED
+        },
+      )
+    }
+  }
+
   override fun layoutSubviews() {
     super.layoutSubviews()