How to create a piano using iOS 9 Auto Layout

Issue #22

In the beginning, people use frame and Autoresizing Mask, then they use Auto Layout, then iOS 9 encourages them to use NSLayoutAnchor, UILayoutGuide and UIStackView

For more convenient Auto Layout, check How to make Auto Layout more convenient in iOS and Anchors

NSLayoutAnchor

The NSLayoutAnchor class is a factory class for creating NSLayoutConstraint objects using a fluent API. Use these constraints to programmatically define your layout using Auto Layout.

It has 3 subclasses

NSLayoutDimension

• func constraintEqualToConstant(_ c: CGFloat) -> NSLayoutConstraint!

NSLayoutXAxisAnchor

• Allows working with horizontal constraints
• Prevent these

// This constraint generates an incompatible pointer type warning
cancelButton.leadingAnchor.constraintEqualToAnchor(saveButton.topAnchor, constant: 8.0).active = true

NSLayoutYAxisAnchor

• Allows working with vertical constraints
• Prevent these

// This constraint generates an incompatible pointer type warning
cancelButton.topAnchor.constraintEqualToAnchor(saveButton.trailingAnchor, constant: 8.0).active = true

UILayoutGuide

Previously, we used dummy views to aid constraints. Now we use UILayoutGuide

Define an equal spacing between a series of views

See full gist

let space1 = UILayoutGuide()
view.addLayoutGuide(space1)

let space2 = UILayoutGuide()
view.addLayoutGuide(space2)

space1.widthAnchor.constraintEqualToAnchor(space2.widthAnchor).active = true

saveButton.trailingAnchor.constraintEqualToAnchor(space1.leadingAnchor).active = true

cancelButton.leadingAnchor.constraintEqualToAnchor(space1.trailingAnchor).active = true
cancelButton.trailingAnchor.constraintEqualToAnchor(space2.leadingAnchor).active = true

clearButton.leadingAnchor.constraintEqualToAnchor(space2.trailingAnchor).active = true

Layout guides can also act as a black box, containing a number of other views and controls

See the full gist

let container = UILayoutGuide()
view.addLayoutGuide(container)

// Set interior constraints
label.lastBaselineAnchor.constraintEqualToAnchor(textField.lastBaselineAnchor).active = true
label.leadingAnchor.constraintEqualToAnchor(container.leadingAnchor).active = true

textField.leadingAnchor.constraintEqualToAnchor(label.trailingAnchor, constant: 8.0).active = true
textField.trailingAnchor.constraintEqualToAnchor(container.trailingAnchor).active = true

textField.topAnchor.constraintEqualToAnchor(container.topAnchor).active = true
textField.bottomAnchor.constraintEqualToAnchor(container.bottomAnchor).active = true

// Set exterior constraints
// The contents of the container can be treated as a black box
let margins = view.layoutMarginsGuide

container.leadingAnchor.constraintEqualToAnchor(margins.leadingAnchor).active = true
container.trailingAnchor.constraintEqualToAnchor(margins.trailingAnchor).active = true

// Must use NSLayoutConstraint with the scene's top and bottom layout guides.
NSLayoutConstraint(item: container,
    attribute: .Top,
    relatedBy: .Equal,
    toItem: topLayoutGuide,
    attribute: .Bottom,
    multiplier: 1.0,
    constant: 20.0).active = true

layoutMarginsGuide

Margins are now represented as layoutMarginsGuide, a subclass of UILayoutGuide

topLayoutGuide and bottomLayoutGuide

In the container example, we saw how we must use NSLayoutConstraint with the topLayoutGuide. topLayoutGuide and bottomLayoutGuide are object conforming to UILayoutSupport protocol

layoutFrame

The layout guide defines a rectangular space in its owning view’s coordinate system. This property contains a valid CGRect value by the time its owning view’s layoutSubviews method is called.

In the above container example, the container layout guide frame is

(16.0, 40.0, 343.0, 21.0)

Piano

See Piano on Github on how to create a Piano using UILayoutGuide, NSLayoutAnchor and UIStackView