iOS8 Day-by-Day :: Day 10 :: Xcode 6 Playgrounds

Written by Sam Davies

Updated 9 Apr 2015: This post has been updated to Swift 1.2

This post is part of a daily series of posts introducing the most exciting new parts of iOS8 for developers – #iOS8DayByDay. To see the posts you’ve missed check out the index page, but have a read through the rest of this post first!

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Playgrounds are a completely new concept in Xcode 6 – bringing the world of a completely interactive code development environment with the powerful new language that is Swift. Using a playground feels very much like a powered-up REPL – combining the persistence of a source code file with the immediate response of a REPL.

Playgrounds are really easy to get started with – in fact when you open Xcode 6 there is an option to create a new playground. You can choose either an iOS playground or an OSX playground, and they only support writing in Swift.

Today’s article won’t cover many of the basics of playgrounds – it’s not difficult to get started, but will instead cover some of the more advanced features available through the XCPlayground framework.

The accompanying project is itself a playground, and is available in the ShinobiControls github repo at You’ll notice that there are two projects within this repo – one for iOS, one for OSX.

Note: At the time of publication, not all the features are available for iOS (beta 4 of Xcode 6). Therefore, the OSX playground is provided for reference. The code is very similar – with like-for-like exchanges where possible. The code snippets in this article will refer to the iOS version.

The aim of the playground project is to create a view which can draw a cycloid. A cycloid is the name of the curve which is traced by a point on the edge of a wheel as it rotates. This development process is very representative of what you could use a playground for. You can read more about cyloids on Wikipedia, and the following image (also from Wikipedia) gives you an idea of what you’re going to try and create:


Interactive Coding & Timelines

As has already been mentioned, playgrounds allow for instantaneous feedback from code as you write it. This makes it ideal for algorithm development. In the centroid example, we can use high-school geometry to calculate the x and y positions of the point on the rim of the wheel for a given angle:

let radius = 10.0
for circleProp in 1...100 {
  let alpha = Double(circleProp) / 100.0 * 2.0 * M_PI
  let y = radius * (1 - cos(alpha))
  let x = radius * (alpha - sin(alpha))

This code loops through 100 samples of the angle of rotation of the circle and calculates the location of the point for each of them. If you look at this code in the playground you’ll see that over on the right-hand side it tells you that each line was executed 100 times (as you would expect):

Executed 100 times

That in itself is not particularly ground-breaking, but if you mouse-over the quicklook gutter then you’ll see a couple of icons appearing – a quicklook eye and a plus for Value History:

Value History

If you click this button then the assistant editor will open with a timeline view, containing a graph of these values over time:

Timeline for x and y

The timeline will automatically add reassignments to variables of the same name. If you want to specify that a particular value should be added to a given timeline chart you can use the XCPCaptureValue(identifier:, value:) method. This is part of the XCPlayground framework, so you’ll need to import that:

import XCPlayground

Any time that you call XCPCaptureValue(), it’ll be added to the timeline representation for the given string identifier. i.e. it’ll be added to the end of a chart if one exists for the given name (and the type can be plotted). If the identifier has not yet been seen then a new view will appear in the timeline assistant editor and will be populated appropriately for the given type.

You can add other types to the timeline as well, including views, bezier paths, colors, strings, arrays, dictionaries etc. In fact pretty much anything which has an associated quick look. However, how do you add something which doesn’t have a quick look representation? In the next section you’ll learn how to do this.

Custom QuickLook

Now that you’re happy with the algorithm itself, you’ll want to build a class to contain the functionality. You can do this within a playground as well, and they offer some additional tools for making the experience great.

With the cycloid example, the initializer requires a radius and number of rotations:

init(radius: Double, numberOfRotations: Double = 2.5) {
  self.radius = radius
  self.numberOfRotations = numberOfRotations

And the previously developed algorithm is used in the generateDatapoint() method:

func generateDatapoint(angle: Double) -> CGPoint {
  let y = radius * (1 - cos(angle))
  let x =  radius * (angle - sin(angle))
  return CGPoint(x: CGFloat(x), y: CGFloat(y))

So to actually create the points which make up the cycloid path you create the following method:

func pointsForCycloid(numberSamples: UInt) -> [CGPoint] {
  var dataPoints = [CGPoint]()
  for sampleIndex in 0..<numberSamples {
    let angle = Double(sampleIndex) / Double(numberSamples) * 2.0 * M_PI * numberOfRotations
  return dataPoints

OK, so you think that you’ve got this class working correctly, but how can you check? Well, if your class inherits from NSObject then you can implement the debugQuickLookObject() method to return something that the playground can visualize. The objects you can return include bezier paths, strings, colors and images.

Since you’re dealing with CGPoint objects here then a bezier path would be ideal. The following method will create a bezier path from using the result of the pointsForCycloid() method:

func bezierPath(numberSamples: UInt) -> UIBezierPath {
  let bezierPath = UIBezierPath()
  for point in pointsForCycloid(numberSamples) {
  return bezierPath

This means that the debugQuickLookObject() method is as simple as:

func debugQuickLookObject() -> AnyObject? {
  return bezierPath(100)

To see this in action, simple instantiate one of these Cycloid objects:

let c = Cycloid(radius: 50, numberOfRotations: 3)

You can either click the quicklook icon to see the result in a popover:

Quicklook Cycloid

Or, like you did with the values of x and y, you can add it to the timeline so that it will stay there and get updated each time the playground re-evaluates :

Timeline Cycloid

This is starting to look pretty cool now – you’ve got a class which will create the cycloid path for you. In the next section you’re going to see how to use this to create a view which animated a wheel drawing this path.

Custom View Development

Creating custom views can be a really slow process in Xcode – although this has improved with the ability to do live rendering in interface builder. Even so, you are often required to build your project and run it up on a simulator before you can see what it actually looks like. This makes for a slow iterative loop – speeding this up could offer massive gains in productivity. Even just the ability to quicklook a view in playgrounds really can help here, but that’s just scratching the surface of what’s possible.

Note: Remember throughout this section that at the time of publication, animation of UIView in playgrounds was not supported. Therefore, in order to see it in action, use the OSX version instead. It is very similar code, with a few platform differences (including coordinate system differences).

The sample playground includes a UIView subclass called AnimatingCycloidView. This uses the previously created Cycloid class to determine the path of the appropriate cycloid and draws it on screen using a CAShapeLayer.

func createCycloidLayer() -> CAShapeLayer {
  let layer = CAShapeLayer()
  layer.bounds = self.bounds
  layer.position = CGPoint(x: self.bounds.width / 2.0, y: self.bounds.height / 2.0)
  layer.path = self.cycloid.bezierPath(100).CGPath
  layer.fillColor = UIColor.clearColor().CGColor
  layer.strokeColor = UIColor.blueColor().CGColor
  layer.lineWidth = 3.0

  return layer

A similar method is used to create the wheel. Since this is is a UIView you get a quicklook for free:

let view = AnimatingCycloidView(frame: CGRect(x: 0, y: 0, width: 700, height: 100))

Quicklook Cycloid View

However, you want to be able to animate the wheel to trace the cycloid path. The animation method is fairly standard, and just uses CABasicAnimation to rotate and translate the wheel simultaneously:

func beginAnimation() {
  self.wheelLayer.setValue(-2 * M_PI * self.cycloid.numberOfRotations, forKeyPath: "transform.rotation.z")
  self.wheelLayer.setValue(self.bounds.width, forKeyPath: "position.x")
  self.cycloidLayer.strokeEnd = 1.0


  let animation = CABasicAnimation(keyPath: "transform.rotation.z")
  animation.fromValue = 0
  animation.toValue   = -2 * M_PI * cycloid.numberOfRotations

  let translation = CABasicAnimation(keyPath: "position.x")
  translation.fromValue = 0
  translation.toValue   = self.bounds.width

  let animationGroup = CAAnimationGroup()
  animationGroup.animations = [animation, translation]
  animationGroup.timingFunction = CAMediaTimingFunction(name: kCAMediaTimingFunctionLinear)
  animationGroup.removedOnCompletion = false

  wheelLayer.addAnimation(animationGroup, forKey: "wheelSpin")


Try adding the view to the timeline with the Value History button and then watching what happens. You’ll see the cycloid view in the timeline, but only in its completed state:

Cycloid after animation

It would be really helpful if you could see the animation happening. Well, the XCPlayground has a function which can help you out – in the form of XCPShowView(). This takes an identifier and a view object:

XCPShowView("CycloidView", view)

This function automatically adds a view to the timeline, and then delays the end of execution within the timeline, until the timeout value specified in the lower right hand corner of the timeline:

Timeline Timeout

You will now see the wheel animating across the view. You can also then use the slider at the bottom of the timeline to track backwards and forwards in time. The frames of the view animation are captured so you can see exactly how the animation is behaving.

Cycloid Animation 1 Cycloid Animation 2 Cycloid Animation 3 Cycloid Animation 4 Cycloid Animation 5

The XCPShowView() method is using a method called XCPSetExecutionShouldContinueIndefinitely under the hood. This method allows the playground process to continue until either the timeline timeout is reached, or the playground source has changed – at which point the code will be re-run.


Playgrounds offer the potential to be really very useful. They are certainly great for learning Swift – but in that respect they don’t offer a lot more than the REPL does. Once you add the full force of the Cocoa and CocoaTouch APIs then there really is a lot to play with. Even in the the short sample project, it was possible to go from building an algorithm from simple mathematical principles through to designing a class to encompass that functionality, right up to building a view containing fairly complex animations. At no point do you have to consider any of the mechanics of interacting with an app, which is of huge importance when iterating quickly.

I encourage you to jump in and muck around in a playground if you haven’t already. Even without some of the more advanced topics covered here today, they offer huge value.

Don’t forget that the source code which accompanies today’s article is on the ShinobiControls github at Feel free to grab it and gimme a shout on twitter if you have any questions or complaints – I’m @iwantmyrealname.