SnapKit/docs.md

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Requirements

• iOS 8.0+ / Mac OS X 10.11+ / tvOS 9.0+
• Xcode 9.0+
• Swift 4.0+

Communication

• If you need help, use Stack Overflow. (Tag 'snapkit')
• If you'd like to ask a general question, use Stack Overflow.
• If you found a bug, open an issue.
• If you have a feature request, open an issue.
• If you want to contribute, submit a pull request.

Installation

CocoaPods

CocoaPods is a dependency manager for Cocoa projects. You can install it with the following command:

$ gem install cocoapods

CocoaPods 1.1.0+ is required to build SnapKit 4.0.0+.

To integrate SnapKit into your Xcode project using CocoaPods, specify it in your Podfile:

source 'https://github.com/CocoaPods/Specs.git'
platform :ios, '10.0'
use_frameworks!

target '<Your Target Name>' do
    pod 'SnapKit', '~> 4.0'
end

Then, run the following command:

$ pod install

Carthage

Carthage is a decentralized dependency manager that builds your dependencies and provides you with binary frameworks.

You can install Carthage with Homebrew using the following command:

$ brew update
$ brew install carthage

To integrate SnapKit into your Xcode project using Carthage, specify it in your Cartfile:

github "SnapKit/SnapKit" ~> 4.0

Run carthage update to build the framework and drag the built SnapKit.framework into your Xcode project.

Manually

If you prefer not to use either of the aforementioned dependency managers, you can integrate SnapKit into your project manually.

---

Usage

SnapKit is designed to be extremely easy to use. Let's say we want to layout a box that is constrained to it's superview's edges with 20pts of padding.

let box = UIView()
superview.addSubview(box)

box.snp.makeConstraints { (make) -> Void in
    make.top.equalTo(superview).offset(20)
    make.left.equalTo(superview).offset(20)
    make.bottom.equalTo(superview).offset(-20)
    make.right.equalTo(superview).offset(-20)
}

Or even shorter:

let box = UIView()
superview.addSubview(box)

box.snp.makeConstraints { (make) -> Void in
    make.edges.equalTo(superview).inset(UIEdgeInsetsMake(20, 20, 20, 20))
}

Not only does this greatly shorten and increase the readability of constraints SnapKit is also taking care of a few crucial steps in the process:

• Determining the best common superview to install the constraints on.
• Keeping track of the constrainted installed so they can easily be removed later.
• Ensuring setTranslatesAutoresizingMaskIntoConstraints(false) is called on all appropriate views.

Not all things are created equal

.equalTo equivalent to NSLayoutRelation.Equal

.lessThanOrEqualTo equivalent to NSLayoutRelation.LessThanOrEqual

.greaterThanOrEqualTo equivalent to NSLayoutRelation.GreaterThanOrEqual

These three equality constraints accept one argument which can be any of the following:

1. ViewAttribute

make.centerX.lessThanOrEqualTo(view2.snp.left)

ViewAttribute	NSLayoutAttribute
view.snp.left	NSLayoutAttribute.left
view.snp.right	NSLayoutAttribute.right
view.snp.top	NSLayoutAttribute.top
view.snp.bottom	NSLayoutAttribute.bottom
view.snp.leading	NSLayoutAttribute.leading
view.snp.trailing	NSLayoutAttribute.trailing
view.snp.width	NSLayoutAttribute.width
view.snp.height	NSLayoutAttribute.height
view.snp.centerX	NSLayoutAttribute.centerX
view.snp.centerY	NSLayoutAttribute.centerY
view.snp.lastBaseline	NSLayoutAttribute.lastBaseline

2. UIView/NSView

if you want view.left to be greater than or equal to label.left:

// these two constraints are exactly the same
make.left.greaterThanOrEqualTo(label)
make.left.greaterThanOrEqualTo(label.snp.left)

3. Strict Checks

Auto Layout allows width and height to be set to constant values. if you want to set view to have a minimum and maximum width you could pass a primitive to the equality blocks:

// width >= 200 && width <= 400
make.width.greaterThanOrEqualTo(200)
make.width.lessThanOrEqualTo(400)

However Auto Layout does not allow alignment attributes such as left, right, centerY etc to be set to constant values. So if you pass a primitive for these attributes SnapKit will turn these into constraints relative to the view's superview ie:

// creates view.left <= view.superview.left + 10
make.left.lessThanOrEqualTo(10)

You can also use other primitives and structs to build your constraints, like so:

make.top.equalTo(42)
make.height.equalTo(20)
make.size.equalTo(CGSize(width: 50, height: 100))
make.edges.equalTo(UIEdgeInsets(top: 10, left: 0, bottom: 10, right: 0))
make.left.equalTo(view).offset(UIEdgeInsets(top: 10, left: 0, bottom: 10, right: 0))

Learn to prioritize

.priority allows you to specify an exact priority

Priorities are can be tacked on to the end of a constraint chain like so:

make.top.equalTo(label.snp.top).priority(600)

Composition, composition, composition

SnapKit also gives you a few convenience methods to create multiple constraints at the same time.

edges

// make top, left, bottom, right equal view2
make.edges.equalTo(view2);

// make top = superview.top + 5, left = superview.left + 10,
//      bottom = superview.bottom - 15, right = superview.right - 20
make.edges.equalTo(superview).inset(UIEdgeInsets(top: 5, left: 10, bottom: 15, right: 20))

size

// make width and height greater than or equal to titleLabel
make.size.greaterThanOrEqualTo(titleLabel)

// make width = superview.width + 100, height = superview.height + 100
make.size.equalTo(superview).offset(100)

center

// make centerX and centerY = button1
make.center.equalTo(button1)

// make centerX = superview.centerX + 5, centerY = superview.centerY + 5
make.center.equalTo(superview).offset(5)

You can chain view attributes for increased readability:

// All edges but the top should equal those of the superview
make.left.right.bottom.equalTo(superview)
make.top.equalTo(otherView)

Hold on for dear life

Sometimes you need modify existing constraints in order to animate or remove/replace constraints. In SnapKit there are a few different approaches to updating constraints.

1. References

You can hold on to a reference of a particular constraint by assigning the result of a constraint make expression to a local variable or a class property. You could also reference multiple constraints by storing them away in an array.

var topConstraint: Constraint? = nil

...

// when making constraints
view1.snp.makeConstraints { (make) -> Void in
  self.topConstraint = make.top.equalTo(superview).offset(padding.top).constraint
  make.left.equalTo(superview).offset(padding.left)
}

...
// then later you can call
self.topConstraint.uninstall()

// or if you want to update the constraint
self.topConstraint.updateOffset(5)

2. snp.updateConstraints

Alternative if you are only updating the constant value of the constraint you can use the method snp.updateConstraints instead of snp.makeConstraints

// this is Apple's recommended place for adding/updating constraints
// this method can get called multiple times in response to setNeedsUpdateConstraints
// which can be called by UIKit internally or in your code if you need to trigger an update to your constraints
override func updateConstraints() {
    self.growingButton.snp.updateConstraints { (make) -> Void in
        make.center.equalTo(self);
        make.width.equalTo(self.buttonSize.width).priority(250)
        make.height.equalTo(self.buttonSize.height).priority(250)
        make.width.lessThanOrEqualTo(self)
        make.height.lessThanOrEqualTo(self)
    }
    
   // according to Apple super should be called at end of method
	 super.updateConstraints()
}

3. snp.remakeConstraints

snp.remakeConstraints is similar to snp.makeConstraints, but will first remove all existing constraints installed by SnapKit.

func changeButtonPosition() {
  self.button.snp.remakeConstraints { (make) -> Void in 
    make.size.equalTo(self.buttonSize)

    if topLeft {
      make.top.left.equalTo(10)
    } else {
      make.bottom.equalTo(self.view).offset(-10)
      make.right.equalTo(self.view).offset(-10)
    }
  }
}

Snap view to topLayoutGuide and bottomLayoutGuide

topLayoutGuide.snp.bottom is similar to topLayoutGuide.bottomAnchor and you can also use bottomLayoutGuide.snp.top to align view on top of UITabBar.

import SnapKit

class MyViewController: UIVewController {
    
    lazy var tableView = UITableView()
    
    override func viewDidLoad() {
        super.viewDidLoad()

        self.view.addSubview(tableView)
        tableView.snp.makeConstraints { (make) -> Void in
           make.top.equalTo(self.topLayoutGuide.snp.bottom)
           make.left.equalTo(view)
           make.right.equalTo(view)
           make.bottom.equalTo(self.bottomLayoutGuide.snp.top)
        }
    }

}

Snap view to safe layout guide

Just like topLayoutGuide and bottomLayoutGuide using iPhone X's new safeAreaLayoutGuide is easy:

import SnapKit

class MyViewController: UIVewController {
    
    lazy var tableView = UITableView()
    
    override func viewDidLoad() {
        super.viewDidLoad()

        self.view.addSubview(tableView)
        tableView.snp.makeConstraints { (make) -> Void in
           make.top.equalTo(self.safeAreaLayoutGuide.snp.top)
        }
    }

}