EasyPeasy: a Swift framework that lets you create Auto Layout constraints programmatically

by iOS Example

EasyPeasy is a Swift framework that lets you create Auto Layout constraints programmatically without headaches and never ending boilerplate code. Besides the basics, EasyPeasy resolves most of the constraint conflicts for you and also can attach to a constraint conditional closures that are evaluated before applying a constraint, this way you can install an Auto Layout constraint depending on platform, size classes, orientation… or the state of your controller, easy peasy!

In this quick tour through EasyPeasy we assume that you already know the advantages and disadvantages of the different Auto Layout APIs and therefore you won’t see here a comparison of the code side by side, just read and decide whether EasyPeasy is for you or not.

A touch of EasyPeasy

The example below is quite simple but shows how effortless its implementation result using EasyPeasy.


Features

  • Compatible with iOS, tvOS and OS X.
  • Lightweight and easy to use domain specific language.
  • Resolution of Auto Layout conflicts.
  • Fast and hassle-free update of constraints.
  • Conditional application of constraints.
  • UILayoutGuide and NSLayoutGuide support.

Guides

Table of contents

Installation

Swift compatibility

  • To work with Swift 2.2 use EasyPeasy v.1.2.1 or earlier versions of the library.
  • To work with Swift 2.3 use EasyPeasy v.1.3.1.
  • To work with Swift 3 use EasyPeasy v.1.4.2.
  • To work with Swift 4 use EasyPeasy v.1.8.0.
  • To work with Swift 5 use EasyPeasy v.1.9.0 and above. (thanks Bas van Kuijck).

Cocoapods

EasyPeasy is available through CocoaPods. To install it, simply add the following line to your Podfile:

pod "EasyPeasy"

Carthage

EasyPeasy is Carthage compatible. To add EasyPeasy as a dependency to your project, just add the following line to your Cartfile:

github "nakiostudio/EasyPeasy"

And run carthage update as usual.

Compatibility

EasyPeasy is compatible with iOS (8 and above), tvOS (9 and above) and OS X (10.10 and above). The framework has been tested with Xcode 7 and Swift 2.0, however don’t hesitate to report any issues you may find with different versions.

Usage

EasyPeasy is a set of position and dimension attributes that you can apply to your views. You can manage these from the easy property available within all the UI classes that work with Auto Layout (view subclasses, layout guides, etc).

For instance, to set a width of 200px to a view you would create an attribute of class Width with a constant value of 200, then the attribute is applied to the view by using the easy.layout(_:) method.

myView.easy.layout(Width(200))

Because our view without height is nothing we can apply multiple attributes at once as follows:

myView.easy.layout(
  Width(200),
  Height(120)
)

In the previous example, two attributes have been applied and therefore two constraints created and added: a width constraint with constant = 200 and a height constraint with constant = 120.

Constants

Without really knowing it, we have just created an EasyPeasy Constant struct containing the constant, multipler and the relation of a NSLayoutConstraint.

Relations

EasyPeasy provides an easy way of creating constants with different NSLayoutRelations:

  • .Equal: it is created like in our previous example Width(200).
  • .GreaterThanOrEqual: it is created as easy as this Width(>=200) and it means that our view has a width greater than or equal to 200px.
  • .LessThanOrEqual: it is created as follows Width(<=200).

Multipliers

There is a custom operator that eases the creation of a NSLayoutConstraint multiplier. You can use it like this Width(*2) and means that the width of our view is two times something, we will mention later how to establish the relationship with that something.

In addition, you can combine multipliers with Equal, .GreaterThanOrEqual and LessThanOrEqual relations. i.e. Width(>=10.0*0.5) creates a NSLayoutConstraint with value = 10.0, relation = .GreaterThanOrEqual and multiplier = 0.5, whereas Width(==10.0*0.5) creates a NSLayoutConstraint with value = 10.0, relation = .Equal and multiplier = 0.5.

Attributes

EasyPeasy provides as many Attribute classes as attributes NSLayoutConstraint have, plus something that we have called CompoundAttributes (we will explain these attributes later).

DimensionAttributes

There are just two dimension attributes Width and Height. You can create an Auto Layout relationship between your view DimensionAttribute and another view by using the method func like(view: UIView) -> Self. Example:

contentLabel.easy.layout(Width().like(headerView))

That line of code will create a constraint that sets a width for contentLabel equal to the headerView width.

PositionAttributes

The table below shows the different position attributes available. Because they behave like the NSLayoutConstraint attributes, you can find a complete description of them in the Apple docs.

Attribute Attribute Attribute Attribute
Left Right Top Bottom
Leading Trailing CenterX CenterY
LeftMargin RightMargin TopMargin BottomMargin
LeadingMargin TrailingMargin CenterXWithinMargins CenterYWithinMargins
FirstBaseline LastBaseline — —

As well as the DimensionAttributes have the like: method to establish Auto Layout relationships, you can use a similar method to do the same with PositionAttributes. This method is:

Self
“>

func to(view: UIView, _ attribute: ReferenceAttribute? = nil) -> Self

The example below positions contentLabel 10px under headerView with the same left margin as headerView.

contentLabel.easy.layout(
  Top(10).to(headerView),
  Left().to(headerView, .Left)
)

CompoundAttributes

These attributes are the ones that create multiple DimensionAttributes or PositionAttributes under the hood. For example, the Size attribute will create a Width and a Height attributes with their width and height NSLayoutConstraints respectively.

These are the CompoundAttributes available:

  • Size: As mentioned before this attribute will apply a Width and a Height attribute to the view. It can be initialized in many ways and depending on that the result may change. These are some examples:

// Apply width = 0 and height = 0 constraints
view.easy.layout(Size())
// Apply width = referenceView.width and height = referenceView.height constraints
view.easy.layout(Size().like(referenceView))
// Apply width = 100 and height = 100 constraints
view.easy.layout(Size(100))
// Apply width = 200 and height = 100 constraints
view.easy.layout(Size(CGSize(width: 200, height: 100)))
  • Edges: This attribute creates Left, Right, Top and Bottom attributes at once. Examples:

// Apply left = 0, right = 0, top = 0 and bottom = 0 constraints to its superview
view.easy.layout(Edges())
// Apply left = 10, right = 10, top = 10 and bottom = 10 constraints to its superview
view.easy.layout(Edges(10))
// Apply left = 10, right = 10, top = 5 and bottom = 5 constraints to its superview
view.easy.layout(Edges(UIEdgeInsets(top: 5, left: 10, bottom: 5, right: 10)))
  • Center: It creates CenterX and CenterY attributes. Examples:

// Apply centerX = 0 and centerY = 0 constraints to its superview
view.easy.layout(Center())
// Apply centerX = 10 and centerY = 10 constraints to its superview
view.easy.layout(Center(10))
// Apply centerX = 0 and centerY = 50 constraints to its superview
view.easy.layout(Center(CGPoint(x: 0, y: 50)))
  • Margins: This attribute creates LeftMargin, RightMargin, TopMargin and BottomMargin attributes at once. Examples:

// Apply leftMargin = 0, rightMargin = 0, topMargin = 0 and bottomMargin = 0 constraints to its superview
view.easy.layout(Margins())
// Apply leftMargin = 10, rightMargin = 10, topMargin = 10 and bottomMargin = 10 constraints to its superview
view.easy.layout(Margins(10))
// Apply leftMargin = 10, rightMargin = 10, topMargin = 5 and bottomMargin = 5 constraints to its superview
view.easy.layout(Margins(UIEdgeInsets(top: 5, left: 10, bottom: 5, right: 10)))
  • CenterWithinMargins: It creates CenterXWithinMargins and CenterYWithinMargins attributes. Examples:

// Apply centerXWithinMargins = 0 and centerYWithinMargins = 0 constraints to its superview
view.easy.layout(CenterWithinMargins())
// Apply centerXWithinMargins = 10 and centerYWithinMargins = 10 constraints to its superview
view.easy.layout(CenterWithinMargins(10))
// Apply centerXWithinMargins = 0 and centerYWithinMargins = 50 constraints to its superview
view.easy.layout(CenterWithinMargins(CGPoint(x: 0, y: 50)))

Priorities

The Priority enum does the same function as UILayoutPriority and it’s shaped by five cases:

  • low: it creates an Auto Layout priority with Float value 1.

  • medium: it creates an Auto Layout priority with Float value 500.

  • high: it creates an Auto Layout priority with Float value 750.

  • required: it creates an Auto Layout priority with Float value 1000.

  • custom: it specifies the Auto Layout priority defined by the developer in the case associated value value. Example: .custom(value: 650.0).

In order to apply any of these priorities to an Attribute, the method .with(priority: Priority) must be used. The following example gives an UILayoutPriority of 500 to the Top Attribute applied to view:

=50).with(.medium))
“>

view.easy.layout(Top(>=50).with(.medium))

You can also apply a Priority to an array of Attributes (this operation will override the priorities previously applied to an Attribute).

view.easy.layout([
  Width(200),
  Height(200)
].with(.medium))

Conditions

One of the peculiarities of EasyPeasy is the usage of Conditions or closures that evaluate whether a constraint should be applied or not to the view.

The method when(condition: Condition) sets the Condition closure to an Attribute.

There is plenty of use cases, the example below shows how to apply different constraints depending on a custom variable:

var isCenterAligned = true
...
view.easy.layout(
  Top(10),
  Bottom(10),
  Width(250),
  Left(10).when { !isCenterAligned },
  CenterX(0).when { isCenterAligned }
)

Condition re-evaluation

These Condition closures can be re-evaluated during the lifecycle of a view, to do so you just need to call the convenience method easy.reload().

view.easy.reload()

Bare in mind that these Condition closures are stored in properties therefore you need to capture those variables you access within the closure. For example:

descriptionLabel.easy.layout(
  Height(100).when { [weak self] in
    return self?.expandDescriptionLabel ?? false
  }
)

You can also apply a Condition to an array of Attributes (this operation will override the Conditions previously applied to an Attribute).

view.easy.layout([
  Width(200),
  Height(240)
].when { isFirstItem })

view.easy.layout([
  Width(120),
  Height(140)
].when { !isFirstItem })

ContextualConditions

This iOS only feature is a variant of the Condition closures that receive no parameters and return a boolean value. Instead, a Context struct is passed as parameter providing some extra information based on the UITraitCollection of the UIView the Attributes are going to be applied to.

The properties available on this Context struct are:

  • isPad: true if the current device is iPad.
  • isPhone: true if the current device is iPhone.
  • isHorizontalVerticalCompact: true if both horizontal and vertical size classes are .Compact.
  • isHorizontalCompact: true if the horizontal size class is .Compact.
  • isVerticalCompact: true if the vertical size class is .Compact.
  • isHorizontalVerticalRegular: true if both horizontal and vertical size classes are .Regular.
  • isHorizontalRegular: true if the horizontal size class is .Regular.
  • isVerticalRegular: true if the vertical size class is .Regular.

This is an example of ContextualConditions applied to an array of Attributes:

view.easy.layout([
  Size(250),
  Center(0)
].when { $0.isHorizontalRegular })

view.easy.layout([
  Top(0),
  Left(0),
  Right(0),
  Height(250)
].when { $0.isHorizontalCompact })
ContextualCondition re-evaluation

As we have seen before, you can re-evaluate a Condition closure by calling the easy.reload() convenience method. This also applies to ContextualConditions, therefore if you want your constraints to be updated upon a change on your view UITraitCollection then you need to call the easy.reload() method within traitCollectionDidChange(_:).

Alternatively, EasyPeasy can do this step for you automatically. This is disabled by default as it requires method swizzling; to enable it simply compile the framework adding the compiler flags -D EASY_RELOAD.

UILayoutGuides

Since the version v.0.2.3 (and for iOS 9 projects and above) EasyPeasy integrates UILayoutGuides support.

Applying constraints

Applying a constraint to an UILayoutGuide is as easy as we have discussed in the previous sections, just apply the EasyPeasy attributes you want using the easy.layout(_:) method.

func viewDidLoad() {
  super.viewDidLoad()

  let layoutGuide = UILayoutGuide()
  self.view.addLayoutGuide(layoutGuide)

  layoutGuide.easy.layout(
    Top(10),
    Left(10),
    Right(10),
    Height(100).when { Device() == .iPad },
    Height(60).when { Device() == .iPhone }
  )
}

As you can see, all the different attributes and goodies EasyPeasy provides for UIViews are also applicable to UILayoutGuides.

Connecting UILayoutGuides and UIViews

As mentioned in the Attributes section you can create constraint relationships between an UIView attribute and other UIViews attributes using the methods to(_:_) and like(_:_). Now you can take advantage of those methods to create a relationship between your UIView attributes and an UILayoutGuide.

let layoutGuide = UILayoutGuide()
let separatorView: UIView
let label: UILabel

func setupLabel() {
  self.label.easy.layout(
    Top(10).to(self.layoutGuide),
    CenterX(0),
    Size(60)
  )

  self.separatorView.easy.layout(
    Width(0).like(self.layoutGuide),
    Height(2),
    Top(10).to(self.label),
    CenterX(0).to(self.label)
  )
}

Lastly

Finally but not less important in this section we will explain how to interact with Attributes once they have been applied to an UIView using the easy.layout(_:) method.

Updating constraints

We briefly mentioned in the introductory section that EasyPeasy solves most of the constraint conflicts and it’s true. Usually, in order to update a constraint or the constant of a constraint you have to keep a reference to your NSLayoutConstraint and update the constant when needed. With EasyPeasy you just need to apply another Attribute to your UIView of the same or different type. In the example below we have two methods, the one in which we setup our constraints viewDidLoad() and a method in which we want to update the Top attribute of our headerView.

func viewDidLoad() {
  super.viewDidLoad()

  headerView.easy.layout(
    Top(0),
    Left(0),
    Right(0),
    Height(60)
  )
}

func didTapButton(sender: UIButton?) {
  headerView.easy.layout(Top(100))
}

That’s it! we have updated our Top constraint without caring about keeping references or installing/uninstalling new constraints.

However, there is some cases in which EasyPeasy cannot prevent a conflict (at least for now). This is when multiple constraints cannot be satisfied, i.e. existing a Left and Right constraints it’s also applied a Width constraint (all of them with the same priority). But EasyPeasy is smart enough to prevent conflicts, i.e. when replacing a Left and Right attributes with a CenterX attribute.

Clearing constraints

EasyPeasy provides a method extending UIView that clears all the constraints installed in an UIView by the framework. This method is func easy.clear().

view.easy.clear()

Animating constraints

Animating constraints with EasyPeasy is very straightforward, just apply one or more Attributes to your view within an animation block and you are ready to go, without worrying about constraint conflicts. Example:

UIView.animateWithDuration(0.3) {
  view.easy.layout(Top(10))
  view.layoutIfNeeded()
}

Example projects

Don’t forget to clone the repository and run the iOS and OS X example projects to see EasyPeasy in action.

Note: the messages in the demo app aren’t real and the appearance of those Twitter accounts no more than a tribute to some kickass developers ?

EasyPeasy playground

Alternatively, you can play with EasyPeasy cloning the Playground project available here.

Autogenerated documentation

EasyPeasy is a well documented framework and therefore all the documented classes and methods are available in Cocoadocs.

Author

Carlos Vidal – @nakiostudio

License

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

GitHub

https://github.com/nakiostudio/EasyPeasy