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The easiest way to understand the compound dimension types in the Layout API is to see all the units in relation to one another.

Values (CGFloat, ASDimension)

ASDimension is essentially a normal CGFloat with support for representing either a point value, a relative percentage value, or an auto value.

This unit allows the same API to take in both fixed values, as well as relative ones.

Swift Objective-C

// dimension returned is relative (%)
ASDimensionMake(@"50%");  
ASDimensionMakeWithFraction(0.5);
// dimension returned in points
ASDimensionMake(@"70pt");
ASDimensionMake(70);

ASDimensionMakeWithPoints(70);

// dimension returned is relative (%)
ASDimensionMake("50%")
ASDimensionMakeWithFraction(0.5)

// dimension returned in points
ASDimensionMake("70pt")
ASDimensionMake(70)
ASDimensionMakeWithPoints(70)

Example using ASDimension

ASDimension is used to set the flexBasis property on a child of an ASStackLayoutSpec. The flexBasis property specifies an object's initial size in the stack dimension, where the stack dimension is whether it is a horizontal or vertical stack.

In the following view, we want the left stack to occupy 40% of the horizontal width and the right stack to occupy 60% of the width.

We do this by setting the .flexBasis property on the two childen of the horizontal stack:

Swift Objective-C

self.leftStack.style.flexBasis = ASDimensionMake(@"40%");
self.rightStack.style.flexBasis = ASDimensionMake(@"60%");
[horizontalStack setChildren:@[self.leftStack, self.rightStack]];

self.leftStack.style.flexBasis = ASDimensionMake("40%")
self.rightStack.style.flexBasis = ASDimensionMake("60%")

horizontalStack.children = [self.leftStack, self.rightStack]

Sizes (CGSize, ASLayoutSize)

ASLayoutSize is similar to a CGSize, but its width and height values may represent either a point or percent value. The type of the width and height are independent; either one may be a point or percent value.

Swift Objective-C

ASLayoutSizeMake(ASDimension width, ASDimension height);

ASLayoutSizeMake(_ width: ASDimension, _ height: ASDimension)

`ASLayoutSize` is used for setting a layout element's `.preferredLayoutSize`, `.minLayoutSize` and `.maxLayoutSize` properties. It allows the same API to take in both fixed sizes, as well as relative ones.

Swift Objective-C

// Dimension type "Auto" indicates that the layout element may 
// be resolved in whatever way makes most sense given the circumstances
ASDimension width = ASDimensionMake(ASDimensionUnitAuto, 0);  
ASDimension height = ASDimensionMake(@"50%");
layoutElement.style.preferredLayoutSize = ASLayoutSizeMake(width, height);

// Dimension type "Auto" indicates that the layout element may 
// be resolved in whatever way makes most sense given the circumstances
let width = ASDimensionMake(.auto, 0)
let height = ASDimensionMake("50%")
        
layoutElement.style.preferredLayoutSize = ASLayoutSizeMake(width, height)

If you do not need relative values, you can set the layout element's `.preferredSize`, `.minSize` and `.maxSize` properties. The properties take regular `CGSize` values.

Swift Objective-C

layoutElement.style.preferredSize = CGSizeMake(30, 160);

layoutElement.style.preferredSize = CGSize(width: 30, height: 60)

Most of the time, you won't want to constrain both width and height. In these cases, you can individually set a layout element's size properties using `ASDimension` values.

Swift Objective-C

layoutElement.style.width     = ASDimensionMake(@"50%");
layoutElement.style.minWidth  = ASDimensionMake(@"50%");
layoutElement.style.maxWidth  = ASDimensionMake(@"50%");
layoutElement.style.height    = ASDimensionMake(@"50%");
layoutElement.style.minHeight = ASDimensionMake(@"50%");
layoutElement.style.maxHeight = ASDimensionMake(@"50%");

layoutElement.style.width     = ASDimensionMake("50%")
layoutElement.style.minWidth  = ASDimensionMake("50%")
layoutElement.style.maxWidth  = ASDimensionMake("50%")

layoutElement.style.height    = ASDimensionMake("50%")
layoutElement.style.minHeight = ASDimensionMake("50%")
layoutElement.style.maxHeight = ASDimensionMake("50%")

Size Range (ASSizeRange)

UIKit doesn't provide a structure to bundle a minimum and maximum CGSize. So, ASSizeRange was created to support a minimum and maximum CGSize pair.

ASSizeRange is used mostly in the internals of the layout API. However, the constrainedSize value passed as an input to layoutSpecThatFits: is an ASSizeRange.

Swift Objective-C

- (ASLayoutSpec *)layoutSpecThatFits:(ASSizeRange)constrainedSize;

func layoutSpecThatFits(_ constrainedSize: ASSizeRange) -> ASLayoutSpec

The constrainedSize passed to an ASDisplayNode subclass' layoutSpecThatFits: method is the minimum and maximum sizes that the node should fit in. The minimum and maximum CGSizes contained in constrainedSize can be used to size the node's layout elements.