AnimatePinning.swift

/*
 Experiment that animates a red square left to right in a container
 by swapping the priority of the left and right layout constraints
*/

import UIKit
import XCPlayground

let container = UIView(frame: CGRectZero)
container.backgroundColor = .whiteColor()
container.translatesAutoresizingMaskIntoConstraints = false

let inner = UIView(frame: CGRectZero)
inner.translatesAutoresizingMaskIntoConstraints = false
inner.backgroundColor = .redColor()
container.addSubview(inner)

let innerWidth = NSLayoutConstraint(
    item: inner,
    attribute: .Width,
    relatedBy: .Equal,
    toItem: nil,
    attribute: .NotAnAttribute,
    multiplier: 1.0,
    constant: 200)

let innerHeight = NSLayoutConstraint(
    item: inner,
    attribute: .Height,
    relatedBy: .Equal,
    toItem: nil,
    attribute: .NotAnAttribute,
    multiplier: 1.0,
    constant: 200)

let containerWidth = NSLayoutConstraint(
    item: container,
    attribute: .Width,
    relatedBy: .Equal,
    toItem: nil,
    attribute: .NotAnAttribute,
    multiplier: 1.0,
    constant: 500)

let containerHeight = NSLayoutConstraint(
    item: container,
    attribute: .Height,
    relatedBy: .Equal,
    toItem: nil,
    attribute: .NotAnAttribute,
    multiplier: 1.0,
    constant: 500)

let top = NSLayoutConstraint(
    item: inner,
    attribute: .Top,
    relatedBy: .Equal,
    toItem: container,
    attribute: .Top,
    multiplier: 1.0,
    constant: 50
)

let left = NSLayoutConstraint(
    item: inner,
    attribute: .Leading,
    relatedBy: .Equal,
    toItem: container,
    attribute: .Leading,
    multiplier: 1.0,
    constant: 0
)
left.priority = 999

let right = NSLayoutConstraint(
    item: inner,
    attribute: .Trailing,
    relatedBy: .Equal,
    toItem: container,
    attribute: .Trailing,
    multiplier: 1.0,
    constant: 0
)
right.priority = 998


NSLayoutConstraint.activateConstraints([
    innerWidth,
    innerHeight,
    containerWidth,
    containerHeight,
    top,
    left,
    right
])


// animate the inner square left -> right -> left
// by swapping the left and right constraint priorities
func animate() {
    UIView.animateWithDuration(1.0, animations: {
        let prevRight = right.priority
        let prevLeft = left.priority
        left.priority = prevRight
        right.priority = prevLeft
        container.layoutIfNeeded()
        }, completion: { if $0 { animate() }})
}

animate()

XCPlaygroundPage.currentPage.needsIndefiniteExecution = true
XCPlaygroundPage.currentPage.liveView = container