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Flexbox & CSS-style Layout in Swift.

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README.md

// repository documentation

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# Core Layout :triangular_ruler: :straight_ruler:

> Flexbox & CSS-style Layout in Swift.

**Core Layout** is a simple system to compute **layout** trees. It wraps the wonderful work done in [facebook/yoga](https://github.com/facebook/yoga) in a **richly typed**, **Swift framework**.

While no assumptions are made about _how_ the **computed layouts** will be used, **Core Layout** does provide a way to apply them to a tree of `UIView`s, since that's likely the most common use case.

## TLDR

Create and configure `Layout`s.

Create and configure other `Layout`s. Add them as children. Repeat.

`compute` the tree of `Layout`s inside a given container `CGSize`.

Use the `CGRect`s from the resulting tree of `ComputedLayout`s for something interesting.

## How it Works

The main type in **Core Layout** is `Layout`. We'll create and nest these types to make a tree of **layouts** that can be computed.

Here's a simple example:

``` swift
let layout = Layout(
  container: ContainerBehavior(primaryAxis: .vertical),
  padding: Edges(all: 8)
)
```

Now, we can `compute` this **layout**, contained within a **size**:

``` swift
let computed = layout.compute(with: CGSize(width: 160, height: 284))

// computed is:

// ComputedLayout(
//   frame: CGRect(x: 0, y: 0, width: 160, height: 284),
// )
```

Nice. Nothing too fancy yet, let's add a second **layout** to the mix:

``` swift
let layout = Layout(
  container: ContainerBehavior(primaryAxis: .vertical),
  padding: Edges(all: 8),
  children: [
    Layout(size: SizeBehavior(minHeight: 44))
  ]
)
```

We've added one child **layout**, with a minimum height of **44** points. Now, lets `compute` it again:

``` swift
let computed = layout.compute(with: CGSize(width: 160, height: 284))

// computed is now:

//  let computed = ComputedLayout(
//    frame: CGRect(x: 0, y: 0, width: 160, height: 284),
//    children: [
//      ComputedLayout(frame: CGRect(x: 8, y: 8, width: 144, height: 44))
//    ]
//  )
```

Neat! That's the basic idea.

## Basic Concepts

Most of this is just plain [Flexbox](https://www.w3.org/TR/css-flexbox-1/) (at least conceptually).

## Containers

Any `Layout` can become a container for _other_ `Layout`s by setting its `container` property to a new `ContainerBehavior` value.

Then, when children are added to the `Layout`, they'll be arranged according to the settings inside the `container` property.

By default, `Layout`s have a `nil` `container` property.

Let's take a look at the many properties of a `CotnainerBehavior`, and how each affects the arrangement of a `Layout`'s `children`.

### Primary Axis

### `.primaryAxis`:

Defines the **primary axis**. This defines which axis any children will be placed along.

We can choose `.vertical`: (the default value)

``` swift
Layout(
  container: ContainerBehavior(primaryAxis: .vertical),
  padding: Edges(all: 8),
  children: [
    Layout(margin: Edges(bottom: 8), size: .absolute(width: 44, height: 44)),
    Layout(margin: Edges(bottom: 8), size: .absolute(width: 44, height: 44)),
    Layout(margin: Edges(bottom: 8), size: .absolute(width: 44, height: 44))
  ]
)
```

Or `.horizontal`:

``` swift
Layout(
  container: ContainerBehavior(primaryAxis: .horizontal),
  padding: Edges(all: 8),
  children: [
    Layout(margin: Edges(bottom: 8), size: .absolute(width: 44, height: 44)),
    Layout(margin: Edges(bottom: 8), size: .absolute(width: 44, height: 44)),
    Layout(margin: Edges(bottom: 8), size: .absolute(width: 44, height: 44))
  ]
)
```

### Primary Axis Distribution

### `.primaryAxisDistribution`:

The `primaryAxisDistribution` property describes how children should be distributed along the **primary axis**.

First let's initialize an array of child `Layout`s (so we don't have to repeat them in each example):

``` swift
let children = [
  Layout(margin: Edges(trailing: 8), size: .absolute(width: 44, height: 44)),
  Layout(margin: Edges(trailing: 8), size: .absolute(width: 44, height: 44)),
  Layout(size: .absolute(width: 44, height: 44)),
]
```

Great. Now we can try out each of the `primaryAxisDistribution` options:

`.leading`: (the default value)

``` swift
Layout(
  container: ContainerBehavior(primaryAxis: .horizontal, primaryAxisDistribution: .leading),
  padding: Edges(all: 8),
  children: children
)
```

`.center`:

``` swift
Layout(
  container: ContainerBehavior(primaryAxis: .horizontal, primaryAxisDistribution: .center),
  padding: Edges(all: 8),
  children: children
)
```

`.trailing`:

``` swift
Layout(
  container: ContainerBehavior(primaryAxis: .horizontal, primaryAxisDistribution: .trailing),
  padding: Edges(all: 8),
  children: children
)
```

`.spaceAround`:

``` swift
Layout(
  container: ContainerBehavior(primaryAxis: .horizontal, primaryAxisDistribution: .spaceAround),
  padding: Edges(all: 8),
  children: children
)
```

`.spaceBetween`:

``` swift
Layout(
  container: ContainerBehavior(primaryAxis: .horizontal, primaryAxisDistribution: .spaceBetween),
  padding: Edges(all: 8),
  children: children
)
```

### Secondary Axis Distribution

### `.secondaryAxisDistribution`:

The `secondaryAxisDistribution` property describes how children should be placed along the **seconday axis**. (i.e. the opposite of the **primary axis**).

If `primaryAxis` is set to `.horizontal`, then the **secondary axis** is **vertical**.

Again, we'll set up some children up front, then try out each option:

``` swift
let size = SizeBehavior(minWidth: 44, minHeight: 44)

let children = [
  Layout(margin: Edges(trailing: 8), size: minSize),
  Layout(margin: Edges(trailing: 8), size: minSize),
  Layout(size: minSize),
]
```

`.leading`:

``` swift
Layout(
  container: ContainerBehavior(primaryAxis: .horizontal, secondaryAxisDistribution: .leading),
  padding: Edges(all: 8),
  children: children
)
```

`.center`:

``` swift
Layout(
  container: ContainerBehavior(primaryAxis: .horizontal, secondaryAxisDistribution: .center),
  padding: Edges(all: 8),
  children: children
)
```

`.trailing`:

``` swift
Layout(
  container: ContainerBehavior(primaryAxis: .horizontal, secondaryAxisDistribution: .trailing),
  padding: Edges(all: 8),
  children: children
)
```

`.stretch`: (the default value)

``` swift
Layout(
  container: ContainerBehavior(primaryAxis: .horizontal, secondaryAxisDistribution: .stretch),
  padding: Edges(all: 8),
  children: children
)
```

### Secondary Axis Distribution (When Wrapping)

### `.secondaryAxisDistributionWhenWrapping`:

This property is only needed when using `shouldWrap: true`, and there are enough children to wrap onto multiple "lines".

Setting `secondaryAxisDistributionWhenWrapping` does nothing if there's only a single "line" of children.

Again, we'll set up some children up front, then try out each option:

``` swift
let size = .absolute(width: 40, height: 40)
let margin = Edges(bottom: 8, trailing: 8)

let children = [
  Layout(margin: margin, size: size),
  Layout(margin: margin, size: size),
  Layout(margin: margin, size: size),
  Layout(margin: margin, size: size),
  Layout(size: size)
]
```

`.leading`:

``` swift
Layout(
  container: ContainerBehavior(
    primaryAxis: .horizontal,
    secondaryAxisDistributionWhenWrapping: .leading,
    shouldWrap: true
  ),
  padding: Edges(top: 8, leading: 8),
  children: children
)
```

`.center`:

``` swift
Layout(
  container: ContainerBehavior(
    primaryAxis: .horizontal,
    secondaryAxisDistributionWhenWrapping: .center,
    shouldWrap: true
  ),
  padding: Edges(top: 8, leading: 8),
  children: children
)
```

`.trailing`:

``` swift
Layout(
  container: ContainerBehavior(
    primaryAxis: .horizontal,
    secondaryAxisDistributionWhenWrapping: .trailing,
    shouldWrap: true
  ),
  padding: Edges(top: 8, leading: 8),
  children: children
)
```

`.stretch`:

`.spaceAround`:

`.spaceBetween`:

### Wrapping

### `.shouldWrap`:

When the `shouldWrap` property is `false` (the default value), children won't wrap to a new row or column. Instead, they'll try to fit themselves into a single "line".

In this next example, because the children have an `AbsoluteSizeBehavior` for `.size`, and they're not allowed to wrap, they end up trailing off the edge:

``` swift
Layout(
  container: ContainerBehavior(primaryAxis: .horizontal, shouldWrap: false),
  padding: Edges(top: 8, leading: 8),
  children: [
    Layout(margin: Edges(bottom: 8), size: .absolute(width: 44, height: 44)),
    Layout(margin: Edges(bottom: 8), size: .absolute(width: 44, height: 44)),
    Layout(margin: Edges(bottom: 8), size: .absolute(width: 44, height: 44)),
    Layout(margin: Edges(bottom: 8), size: .absolute(width: 44, height: 44)),
    Layout(margin: Edges(bottom: 8), size: .absolute(width: 44, height: 44))
  ]
)
```

When the `wrap` property is `true` however, children will be allowed to _naturally flow_ onto the next row or column (depending on the `primaryAxis` of the `Layout`).

``` swift
Layout(
  container: ContainerBehavior(primaryAxis: .horizontal, shouldWrap: true),
  padding: Edges(top: 8, leading: 8),
  children: [
    Layout(margin: Edges(bottom: 8), size: .absolute(width: 44, height: 44)),
    Layout(margin: Edges(bottom: 8), size: .absolute(width: 44, height: 44)),
    Layout(margin: Edges(bottom: 8), size: .absolute(width: 44, height: 44)),
    Layout(margin: Edges(bottom: 8), size: .absolute(width: 44, height: 44)),
    Layout(margin: Edges(bottom: 8), size: .absolute(width: 44, height: 44))
  ]
)
```

### Overriden Secondary Axis Distribution

### `.overriddenSecondaryAxisDistribution`:

The `overriddenSecondaryAxisDistribution` property is `nil` by default. (Meaning no override takes place). When set to a `non-nil` value, this allows a single child to override its parent container's instructions for how to arrange itself along the **secondary axis**.

Put another way: The value of this property on a child `Layout`, will _supercede_ the value its parent `Layout` has set for its `container?.secondaryAxisDistribution` property when computing the tree of `Layout`s.

Let's take a look at what kind of behavior we can achieve with the `overriddenSecondaryAxisDistribution` property:

`nil` (The default value):

``` swift
let size = SizeBehavior(minWidth: 44, minHeight: 44)

Layout(
  container: ContainerBehavior(primaryAxis: .horizontal, secondaryAxisDistribution: .leading),
  padding: Edges(all: 8),
  children: [
    Layout(size: size, margin: Edges(trailing: 8)),
    Layout(size: size, margin: Edges(trailing: 8)),
    Layout(size: size)
  ]
)
```

`.leading`:

``` swift
let size = SizeBehavior(minWidth: 44, minHeight: 44)

`.center`:

``` swift
let size = SizeBehavior(minWidth: 44, minHeight: 44)

`.trailing`:

``` swift
let size = SizeBehavior(minWidth: 44, minHeight: 44)

`.stretch`:

``` swift
let size = SizeBehavior(minWidth: 44, minHeight: 44)

### Overriding Position

### `.overriddenPosition`:

In many cases, we'll let the container define how a child `Layout` is arranged. Sometimes though, we'd like to specify an exact value for the position of a `Layout`. We can do this through the `Layout`'s `overriddenPosition` property.

We can set values relative to the `Layout`'s container:

``` swift
Layout(
  container: ContainerBehavior(primaryAxis: .vertical),
  padding: Edges(leading: 50),
  children: [
    Layout(
      container: ContainerBehavior(primaryAxis: .horizontal),
      size: .absolute(width: 100, height: 60),
      children: [
        Layout(
          overriddenPosition: Edges(top: 8, leading: 8),
          size: .absolute(width: 44, height: 44)
        )
      ]
    )
  ]
)
```

Note how we've included a **container** `Layout` inside another **container** `Layout`, then included a third `Layout` inside that. The third layout (shown here with a number `3` on it), is positioned relative to its parent **container** `Layout`, using the values we've specified in its `overriddenPosition` property.

### Size Behavior

Every `Layout` has a value for its `.size`. These are `SizeBehavior`s and they have a few different modes. The default `mode` is `.flexibleShrinksButDoesntGrow`.

### Modes

Let's take a look at each mode and how it behaves.

### `.flexibleShrinksButDoesntGrow`:

This is the default value. This mode says the `Layout` (only if needed) will shrink down, but will not be stretched up to fill the available space along the **primary axis**. The `Layout` will also respect the value for its `size.minimum` property.

### `.flexible(units: UInt)`:

This mode says the `Layout` can be shrunken down during computation, and will be stretched up to fill the available space along the **primary axis**. The `Layout` will also respect the value for its `size.minimum`, and `size.maximum` properties.

The `units` parameter is `1` by default. This means if multiple `.flexible` `Layout`s are arranged in a line, they will all stretch to fill the space evenly. We can alter this behavior by supplying `units` value of some other positive integer. Doing so will adjust how the available space in the line is filled by the `Layout`s.

For example, lets try out a few `.flexible` sized `Layout`s, using the default value (`1`) for `units`:

``` swift
Layout(
  container: ContainerBehavior(primaryAxis: .vertical, secondaryAxisDistribution: .stretch),
  size: .flexible(),
  padding: Edges(top: 8, leading: 8, trailing: 8),
  children: [
    Layout(size: .flexible(units: 1), margin: Edges(bottom: 8)),
    Layout(size: .flexible(units: 1), margin: Edges(bottom: 8)),
    Layout(size: .flexible(units: 1), margin: Edges(bottom: 8))
  ]
)
```

Now, let's try the same example, but pass in different values for the flexible units of the child `Layout`s:

``` swift
Layout(
  container: ContainerBehavior(primaryAxis: .vertical, secondaryAxisDistribution: .stretch),
  size: .flexible(),
  padding: Edges(top: 8, leading: 8, trailing: 8),
  children: [
    Layout(size: .flexible(units: 1), margin: Edges(bottom: 8)),
    Layout(size: .flexible(units: 2), margin: Edges(bottom: 8)),
    Layout(size: .flexible(units: 3), margin: Edges(bottom: 8))
  ]
)
```

### `.absolute(width: Float?, height: Float?)`:

Absolute sizing is pretty straightforward. You supply a width and/or height value, and those values are used explicitly for that dimension of the `Layout`:

``` swift
Layout(
  container: ContainerBehavior(primaryAxis: .vertical),
  children: [Layout(size: .absolute(width: 97, height: 58))]
)
```

### `.relative(closure: RelativeSizeClosure)`:

Relative sizing is a bit more involved. Here we're passed in some options we can use to do our own computations based on the available space of our parent/container `Layout`.

Essentially this means, we're handed the size of our parent, (i.e. the size we're going to be constrained to) and we're expected to return back an exact CGSize for our `Layout`.

This is useful for basic relative sizing (i.e. _my parent's width - X_), however it's most useful in the case of arranging `Layout`s containing variable sized content, such as text.

Let's look at an example of creating a `Layout` tree where one of the `Layout`'s content needs to be "measured" before it can be obtained:

``` swift
Layout(
  container: ContainerBehavior(
    primaryAxis: .vertical,
    secondaryAxisDistribution: .leading
  ),
  children: [
    Layout(
      identifier: "Core Layout",
      size: .relative { (options) -> CGSize in
        let string = NSAttributedString(string: "Core Layout", attributes: [
          NSFontAttributeName : ComputedLayout.debugIdentifierFont
        ])

return string.boundingRect(
          with: options.containerSize,
          options: [.usesLineFragmentOrigin, .usesFontLeading],
          context: nil
        ).size
      },

margin: Edges(leading: 8)
    )
  ]
)
```

We use the `NSAttributedString` function `boundRect(with:options:content:)` to calculate the size of a `String` using a given font. We pass in the `containerSize` property to constrain this measurement to the parent's (already computed) size.

### Margins, Padding & Borders

All 3 of these properties work basically how they do in CSS. Let's take a quick look at them:

### `.margins`:

Adds space on the outside of `Layout`. Pushes sibling `Layout`s out of the way.

``` swift
Layout(
  container: ContainerBehavior(primaryAxis: .vertical),
  children: [
    Layout(identifier: "1", size: .absolute(width: 30, height: 30), margin: Edges(bottom: 10)),
    Layout(identifier: "2", size: .absolute(width: 30, height: 30), margin: Edges(bottom: 20)),
    Layout(identifier: "3", size: .absolute(width: 30, height: 30), margin: Edges(leading: 20))
  ]
)
```

### `.padding`:

Adds space on the _inside_ of a `Layout`. Pushes contained `Layout`s inward.

``` swift
Layout(
  container: ContainerBehavior(primaryAxis: .vertical),
  padding: Edges(all: 20),
  children: [
    Layout(identifier: "1", size: .absolute(width: 30, height: 30)),
    Layout(identifier: "2", size: .absolute(width: 30, height: 30)),
    Layout(identifier: "3", size: .absolute(width: 30, height: 30))
  ]
)
```

### `.border`:

Adds space on the _inside_ of a `Layout`, increasing its size.

Note: **Core Layout** doesn't handle any kind of rendering, just layout computation.

``` swift
Layout(
  container: ContainerBehavior(primaryAxis: .horizontal),
  children: [
    Layout(identifier: "1", size: .absolute(width: 30, height: 30), border: Edges(all: 20)),
    Layout(identifier: "2", size: .absolute(width: 30, height: 30), border: Edges(all: 20)),
    Layout(identifier: "3", size: .absolute(width: 30, height: 30), border: Edges(all: 20))
  ]
)
```

## More Advanced Example

We can combine multiple `Layout`s, nesting them inside one another, to achieve complex arrangements with ease.

Here's an example representing a row of content in an iOS app:

``` swift
Layout(
  container: ContainerBehavior(primaryAxis: .vertical, secondaryAxisDistribution: .stretch),
  padding: Edges(all: 8),
  children: [
    Layout(
      container: ContainerBehavior(
        primaryAxis: .horizontal,
        primaryAxisDistribution: .spaceBetween,
        secondaryAxisDistribution: .center
      ),
      padding: Edges(all: 8),
      children: [
        Layout(
          container: ContainerBehavior(
            primaryAxis: .horizontal,
            secondaryAxisDistribution: .center
          ),

children: [
            Layout(size: .absolute(width: 30, height: 30)),

Layout(
              size: .relative(closure: { (options) -> CGSize in /* ...hidden for brevity... */ }),

margin: Edges(leading: 8)
            )
          ]
        ),

Layout(size: .absolute(width: 8, height: 8))
      ]
    )
  ]
)

```

## Using `ComputedLayout`s

We've seen how we can turn `Layout`s into `ComputedLayout`s, but... then what?

**Core Layout** intentionally tries not to assume anything about how `ComputedLayout`s will be used, but _does_ provide a very basic way to use `ComputedLayout`s to arrange `UIView`s:

``` swift
let rootView = createViewTree()
let layoutTree = createLayoutTree()

let computedLayout = layoutTree.compute(with: view.bounds.size)

rootView.apply(layout: computedLayout)
```

This will set all the `frame` properties of `rootView` and its `subviews` (and so on) to match those in the `ComputedLayout` tree.

Note: This _will_ **throw** if the two trees ever don't match up.

## Debugging

Care has been taken to include a couple of useful debugging tools. `ComputedLayout` types have been extended to render an initializer of themselves when passed to `print`. This allows us to easily copy/paste **computed layouts** into **tests** (for example).

``` swift
let layout = Layout(
  container: ContainerBehavior(primaryAxis: .vertical),
  children: [
    Layout(
      overriddenPosition: .absolute(Edges(top: 8, leading: 8)),
      size: .absolute(width: 44, height: 44)
    )
  ]
)

let computed = layout.compute(with: CGSize(width: 160, height: 284))

print(computed)

// Prints:
//
//  ComputedLayout(
//    frame: CGRect(x: 0.0, y: 0.0, width: 160.0, height: 284.0),
//    children: [
//      ComputedLayout(frame: CGRect(x: 8.0, y: 8.0, width: 44.0, height: 44.0))
//    ]
//  )
```

Last but not least, `ComputedLayout` implements the `debugQuickLookObject()` **function** and renders a custom `UIImage` representing the **computed layout**.

This is great (for example) when we're stopped at a break point:

Sharp-eyed readers have probably already guessed, but these **quick look images** are where the diagrams in this **README** came from. :sunglasses:

## CostumeKit :tophat:

**Core Layout** works great with [CostumeKit](http://github.com/jakemarsh/costumekit).

For example, we can expose "standard padding" in a `Costume`:

``` swift
open class MyAppCostume : Costume {
  let spacing = CGFloat(8)

// standard padding is 2 spacing units in all directions
  public func wearStandardPadding(_ layout: Layout) {
    layout.padding = Edges(all: spacing * 2)
  }

public var name: String { return "Default" }
  public var description: String { return "The default costume." }

public init() { }
}
```

Cheers.

## Requirements

- iOS 15.0+ / macOS 12.0+ / tvOS 15.0+ / watchOS 8.0+
- Swift 5.9+
- Xcode 15+

## Installation

### Swift Package Manager (Recommended)

Add CoreLayout to your project via Xcode:

1. File → Add Package Dependencies...
2. Enter: `https://github.com/jakemarsh/core-layout`

Or add to your `Package.swift`:

```swift
dependencies: [
    .package(url: "https://github.com/jakemarsh/core-layout", from: "2.0.0")
]
```

### Carthage

Add `github "jakemarsh/core-layout"` to your `Cartfile`

## Building Core Layout

```bash
git clone https://github.com/jakemarsh/core-layout.git
cd core-layout
swift build
swift test
```

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