AnchorKit provides a simple, intuitive way to create layouts using anchors.

Quick Start

Here's some example code:

// Multiple constraints on one line
myView.constrain(.leading, .top, .trailing, to: anotherView)

// One-line edge constraints with insets
myView.constrainEdges(to: anotherView).inset(10)
myView.constrainEdges(to: anotherView).insetVertical(20).insetHorizontal(30)

// Set height/width equal to a constant
myView.constrain(.height, .width, toConstant: 200)
myView.constrainWidth(to: 42)

// Set the height/width equal to a CGSize
myView.constrain(to: CGSize(width: 100, height: 200))

// Set offset (constant)
myView.constrain(.leading, to: .trailing, of: anotherView).offset(20)

// Set insets
myView.constrain(.leading, .trailing, to: anotherView).inset(24)

// Set the relation and multiplier
myView.constrain(.height, relation: .lessThanOrEqual, to: anotherView, multiplier: 1.6)

// Set the priority
myView.constrain(.centerY, to: anotherView, priority: .high).offset(-15)

// Easily center items
myView.constrainCenter(to: anotherView)

// Return value for single constraint is NSLayoutConstraint
let bottomConstraint = myView.constrain(.bottom, to: .top, of: anotherView.layoutMarginsGuide)

// Return value for multiple constraints is [NSLayoutConstraint]
let topAndSideConstraints = myView.constrain(.leading, .trailing, .top, to: anotherView)

Features

• Simple, intuitive, Swifty syntax
• No more isActive = true after every line (constraints are returned pre-activated 🎉)
• Automatically takes care of translatesAutoresizingMaskIntoConstraints = false
• Works with both layout guides and views
• Works on all 3 platforms that support AutoLayout constraints (iOS, macOS, tvOS)
• No proprietary classes to deal with; return values are NSLayoutConstraint or [NSLayoutConstraint]
• Use any number types (Int, Double, Float, etc.), no need to cast to CGFloat

Requirements

• iOS 9.0+, macOS 10.12+, tvOS 9.0+
• Swift 3.1+
• Xcode 8+

Installation

CocoaPods:

pod 'AnchorKit'

Carthage:

github "Weebly/AnchorKit"

Swift Package Manager:

package(url: "https://github.com/Weebly/AnchorKit.git", .from("2.4.0"))

Usage

The Anchor Enum

AnchorKit's API is primarily based on an enum representing the different anchor types available on views and layout guides.

public enum Anchor {
    case leading
    case trailing
    case left
    case right
    case top
    case bottom
    case width
    case height
    case centerX
    case centerY

    // These two are only available on views, not layout guides.
    case firstBaseline
    case lastBaseline
}

AutoLayout defines 3 types of anchors, and you can only constrain anchors to other anchors within the same group.

• X-axis anchors: leading, trailing, left, right, centerX
• Y-axis anchors: top, bottom, centerY, firstBaseline, lastBaseline
• Dimension anchors: width, height

Under the hood, each of these map to an actual NSLayoutAnchor on the view or layout guide you are constraining.

Creating Constraints

The Anchorable Protocol

AnchorKit works on both layout guides and views. To consolidate these two, we use a protocol called Anchorable. Views conform to another protocol, ViewAnchorable, which gives them the ability to use baseline anchors. Mentions of "item" in the docs below refer to views and layout guides.

Anchor to Item Constraints

Constrain anchors of one item to the corresponding anchors of another item.

myView.constrain(.leading, .trailing, .top, to: anotherView)

Calling this method with a single anchor will implicitly return NSLayoutConstraint. Otherwise, the return type is [NSLayoutConstraint]. The full signatures of these methods are shown below.

// Single constraint
@discardableResult
public func constrain<AnchorableType: Anchorable>(_ anchor: Anchor, relation: NSLayoutRelation = .equal, to item: AnchorableType, multiplier: CGFloatRepresentable = 1, priority: LayoutPriority = .required) -> NSLayoutConstraint

// Multiple constraints
@discardableResult
public func constrain<AnchorableType: Anchorable>(_ anchors: Anchor..., relation: NSLayoutRelation = .equal, to item: AnchorableType, multiplier: CGFloatRepresentable = 1, priority: LayoutPriority = .required) -> [NSLayoutConstraint]

Anchor to Anchor Constraints

Constrain an anchor of one item to an anchor of another item.

myView.constrain(.top, to: .bottom, of: anotherView.readableContentGuide)

This method only supports the creation of a single constraint.

@discardableResult
public func constrain<AnchorableType: Anchorable>(_ anchor: Anchor, relation: NSLayoutRelation = .equal, to otherAnchor: Anchor, of item: AnchorableType, multiplier: CGFloatRepresentable = 1, priority: LayoutPriority = .required) -> NSLayoutConstraint

Anchor to Constant Constraints

Constrain an anchor of an item to a constant. This is is especially useful (and more readable) for the width and height anchors.

myView.constrain(.height, toConstant: 200)
myBoxView.constrain(.width, .height, toConstant: 50) // Creates a box

// Even easier:
myView.constrainHeight(to: 42)
myView.constrainWidth(to: 60)

Calling this method with a single anchor will implicitly return NSLayoutConstraint. Otherwise, the return type is [NSLayoutConstraint]. The full signatures of these methods are shown below.

// Single constraint
@discardableResult
public func constrain(_ anchor: Anchor, relation: NSLayoutRelation = .equal, toConstant constant: CGFloatRepresentable, priority: LayoutPriority = .required) -> NSLayoutConstraint

// Multiple constraints
@discardableResult
public func constrain(_ anchors: Anchor..., relation: NSLayoutRelation = .equal, toConstant constant: CGFloatRepresentable, priority: LayoutPriority = .required) -> [NSLayoutConstraint]

This method can also work on items with anchors other than width and height. The resulting behavior is equivalent to constraining the anchor to the corresponding anchor on the view's superview (or layout guide's owningView). So myView.constrain(.leading, toConstant: 10) translates to myView.constrain(.leading, to: myView.superview!).offset(10).

Constrain to Edges

Constrains the edges of the current item to another item.

myView.constrainEdges(to: anotherView)

This is just a convenience method for constraining the leading, trailing, top, and bottom anchors. For better autocomplete behavior, there is a separate equivalent method if you would like to add a relation other than .equal to the constraints.

@discardableResult
public func constrainEdges<AnchorableType: Anchorable>(to item: AnchorableType, priority: LayoutPriority = .required) -> [NSLayoutConstraint]

// With a relation other than .equal
@discardableResult
public func constrainEdges<AnchorableType: Anchorable>(_ relation: NSLayoutRelation, to item: AnchorableType, priority: LayoutPriority = .required) -> [NSLayoutConstraint]

Constrain to Center

Constraints the center of the current item to another item.

myView.constrainCenter(to: anotherView)

Thiis is just a convenience method for constraining the centerX and centerY anchors. For better autocomplete behavior, there is a separate equivalent method if you would like to add a relation other than .equal to the constraints.

@discardableResult
public func constrainCenter<AnchorableType: Anchorable>(to item: AnchorableType, priority: LayoutPriority = .required) -> [NSLayoutConstraint]

// With a relation other than .equal
@discardableResult
public func constrainCenter<AnchorableType: Anchorable>(_ relation: NSLayoutRelation, to item: AnchorableType, priority: LayoutPriority = .required) -> [NSLayoutConstraint]

Constrain to Size

Constrains the width and height of an item to a specific CGSize.

myView.constrain(to: CGSize(width: 10, height: 20))

This is a convenience method for setting the width and height anchors to the respective width and height of a CGSize. For better autocomplete behavior, there is a separate equivalent method if you would like to add a relation other than .equal to the constraints.

@discardableResult
public func constrain(to size: CGSize, priority: LayoutPriority = .required) -> [NSLayoutConstraint]

@discardableResult
public func constrain(_ relation: NSLayoutRelation, to size: CGSize, priority: LayoutPriority = .required) -> [NSLayoutConstraint]

For views, because width and height constraints belong to the view itself, you can update the those constraints directly from the view:

myView.updateSize(newSize)
myView2.updateWidth(100)
myView3.updateHeight(200)

Constrain to UIViewController topLayoutGuide and bottomLayoutGuide

The topLayoutGuide and bottomLayoutGuide properties on UIViewController have a type of UILayoutSupport, which is a protocol. Constrain to these just as you would to any other item.

Note that the only anchors available on these are height, top, and bottom.

myView.constrain(.top, to: .bottom, of: topLayoutGuide).offset(10)
otherView.constrain(.height, to: bottomLayoutGuide)

The signatures for these methods are identical to the anchor-to-anchor and anchor-to-item methods except that the second item has a type of UILayoutSupport.

The CGFloatRepresentable Protocol

This protocol allows you to use any number types for constraint offsets, insets, and multipliers without the need to cast all values to CGFloat.

public protocol CGFloatRepresentable {
    var cgFloatValue: CGFloat { get }
}

Adopted by the following types:

• Int, Int8, Int16, Int32, Int64
• UInt, UInt8, UInt16, UInt32, UInt64
• Double
• Float, Float80
• CGFloat
• NSNumber

Offsets and Insets

To set constants on constraints, AnchorKit uses offsets and insets. An offset has equivalent behavior to constant on NSLayoutConstraint. An inset, however, negates the constant for the trailing, right, bottom, and lastBaseline anchors and behaves normally for all other anchors.

// The top of bottomView will be 20 points below the top of topView
bottomView.constrain(.top, of: topView).offset(20)

// The innerView will be "inside" the outerView, with 10 points of padding on the sides
innerView.constrain(.leading, .trailing, to: outerView).inset(10)

As you may have noticed in the second example, offset(_:) and inset(_:) work on both single constraints and sequences of constraints.

When updating constraints, for the sake of proper Swift naming conventions, AnchorKit also provides the methods updateOffset(_:) and updateInset(_:).

You can also use UIEdgeInsets (or EdgeInsets on macOS) to set the insets. The insets will be applied to the corresponding constraints.

innerView.constrainEdges(to: outerView).inset(UIEdgeInsets(top: 10, left: 12, bottom: 14, right: 16))

For multiple constraints, it's also possible to only set the horizontal and vertical insets, like so:

// The innerView will have an inset of 10 on the top and bottom sides and an inset of 20 on the leading and trailing sides
innerView.constrainEdges(to: outerView).insetVertical(10).insetHorizontal(20)`

When updating horizontal/vertical inset, use updateHorizontalInsets(_:) and updateVerticalInsets(_:).

Layout Priorities

To set priorities on constraints, AnchorKit provides an enum called LayoutPriority.

public enum LayoutPriority: RawRepresentable {
    case low        // Priority: 250
    case medium     // Priority: 500
    case high       // Priority: 750
    case required   // Priority: 1000
    case custom(Float)
}

All of the constraint creation methods have a default priority of required, but you can set your own when needed.

myView.constrain(.centerX, to: anotherView, priority: .medium)

For complex layouts, sometimes you want just a slightly higher priority for one constraint. Good news: you can use addition and subtraction operators with layout priorities.

myView.constrain(.left, to: anotherView, priority: .low + 1) // Priority = 251

You can get/set the layout priority on NSLayoutConstraints with the layoutPriority extension method.

let topConstraint = myView.constrain(.top, to: anotherView, priority: .medium)
topConstraint.layoutPriority = .high

And finally, use layout priorites to set your content compression resistance and content hugging:

myView.hug(with: .low, for: .vertical)
myView.resistCompression(with: .high, for: .horizontal)

Other Goodies

Activation & Deactivation

AnchorKit constraints come preactivated. However, you can also deactivate constraints and capture them on the same line:

let centerYConstraint = myView.constraint(.centerY, to: anotherView).deactivate()
// ...
centerYConstraint.activate()

These methods also work on sequences of constraints.

iOS 11 System Spacing Constraints

In iOS 11 and tvOS 11, UIKit gave us a new way to define constraints using system-defined spacing with methods such as constraintEqualToSystemSpacingAfter(_:multiplier:). These are useful when constraining items to the new safeAreaLayoutGuide found on UIView, which is now the recommended API to use instead of topLayoutGuide and bottomLayoutGuide on UIViewController (those are now deprecated).

AnchorKit provides a higher level method for system spacing constraints:

myView.constrainUsingSystemSpacing(.top, .below, .bottom, of: anotherView.safeAreaLayoutGuide)
myView.constrainUsingSystemSpacing(.leading, .after, .trailing, of: anotherView.safeAreaLayoutGuide)

The full signature for this method is below:

@discardableResult
public func constrainUsingSystemSpacing<AnchorableType: Anchorable>(_ anchor: Anchor, relation: Relation = .equal, _ position: SystemSpacingPosition, _ otherAnchor: Anchor, of item: AnchorableType, multiplier: CGFloatRepresentable = 1, priority: LayoutPriority = .required) -> NSLayoutConstraint

Support

For questions, support, and suggestions, please open up an issue.

License

AnchorKit is available under the MIT license. See the LICENSE file for more info.