Summary and Notes from Lecture 2, Developing iOS 11 Apps with Swift by Stanford (cs193p)


MVC Communication Methods

Views use:

  • Outlets – target action
  • Delegates – should, will, did
  • Data Source – data at, count

Models use:

  • Notification & KVO (Key Value Observing)

Additional Notes

The Card struct is UI independent. It’s tempting to allow the Card to also store the emoji symbol. However the emoji symbol is a view element. Therefore it should not be allowed in Card.

Classes get free initializers as long as all their instance variables are initialized.
Struct also get free initializer allowing all its instance variables to be initialized.

Lazy vars cannot have property observers (didSet).

State of the App after Implementing the Code from Lecture 2

Source code

The source code of the app Concentration is available here:

Summary of Lecture 1, Developing iOS 11 Apps with Swift by Stanford (cs193p)

Developing iOS11 Apps with Swfit - Lecture 1 Summary

What’s New

After completing the 1st lecture, here is what I found new from the previous courses I took a few years ago.

  • Uses Swift 4, Xcode 9
  • Rename (refactor) with cmd + click on variable name
  • Fuzzy match on intellisense (was available form Xcode 8)
  • Back to developing card game  (the previous few years focused on developing calculator)
  • Outlet collections


  • Property observers are used to keep the UI in sync with instance variables. For example: var flipCount = 0 { didSet { flipCountLabel.text = “Flips: \(flipCount)” } }

The following video shows the state of the app Concentration after completing lecture 1.


The course Developing iOS 11 Apps with Swift by Stanford (cs193p) is available for free from

Animation / Breakout: Assignment 5, CS193P, Stanford University (Winter 2015)

The Animation assignment also referred to as the Breakout game is the 5th and final assignment from the CS193P course Developing iOS 8 Apps with Swift from Stanford University. Really happy to have finally finished this assignment. In my opinion this was the most complex of all the five assignments in this course.

Platform: iOS 9
Swift: 2.1

Full source code available here at the Github repository

All the 9 required tasks mentioned in page 2 of the specifications document were completed. Though device rotation is not mentioned as a requirement it does appear as a hint. I didn’t allow device rotation because it seemed to me that it would drastically affect the game play negatively.

The main challenge that I faced in completing this assignment was the nuance of using UIKit Dynamics to make a game. There were numerous issues that I was not fully clear in the beginning on how to implement them. So slowly chipped away each issue one by one before I had a clear picture of how they all fit together.

I am summarizing some of the main issues I had problems with below and how I solved them. Please see the inline documentation in the source code for all the issues and their implementation details.

  1. Creating the ball and animating it: The shape of the rectangular view of the ball as the collision boundary didn’t seem right. Though prior to iOS 9 I don’t think it was possible to do anything about it. Please see Variation on Dropit Demo from Lecture 12 on how I solved this by setting UIDynamicItemCollisionBoundsType to be an .Ellipse.
  2. Animating the paddle on pan gesture and having the ball collide. Hints are given in the assignment specifications on how to implement this, but since I hadn’t done it before it wasn’t clear what the outcome would look like.
    The solution is to add the paddle as a subview of the game view and additionally create a bezier path that acts as a collision boundary in the UIDynamicBehavior subclass. Then update that boundary repeatedly whenever the paddle moves.
    Please see the movePaddle and syncPaddleBoundary methods for details.
  3. When the paddle was moved too quickly when hitting a ball, the ball would often end up trapped inside the paddle. The way I solved this is first check if the new paddle view’s frame intersected with the balls frame using CGRectIntersectsRect. If the frames don’t intersect then update / sync the boundary.
  4. Bricks are setup similar to the paddle, where they are added to the game view and their colliding boundaries added to the UIDynamicBehavior subclass.
    The ball collides with the boundary and when a collision is detected, the brick is removed from the game view. A bricks array [String:Brick] is used to track all the bricks in the game view and match them against the NSCopying identifier from the collision delegate.
    For details please see the methods createBricks and collisionBehavior.
  5. Special bricks and what behavior they would cause were open ended in the assignment specifications. I used 4 kinds of special bricks, one kind requires 3 hits before it disappears and the other three drops special powers that cause:
    • paddles to become larger
    • paddles to become smaller
    • adds additional balls
  6. Pausing the game when user taps on Settings. This wasn’t too hard to implement, the main issue here is the moving ball(s). I used an array to capture the balls and remove them from the screen. When user came back, I re-created them and added back their linear velocity. Please see method settingsDidUpdate.
  7. Instead of putting up an alert when the game ends, a view is used (where it’s state is toggled) to show that the game is over indicating if you won or lost. This view is also used during startup. The startGame and gameOver methods show how this is done.

The following extra credit tasks were also attempted:

  • Use sophisticated Dynamic Animation. For example, you might find a creative way to use the action method in a behavior or use something like linear velocity in your calculations.
  • As mentioned above, creativity will be rewarded, especially interesting game-play settings.
  • Do some cool artistic design in your user-interface (either by drawing or using images).
  • Pausing your game when you navigate away from it (to go to settings) is a bit of a challenge (because you basically have to freeze the ball where it is, but when you come back, you have to get the ball going with the same linear velocity it had). Give it a try. It’s all about controlling the linear velocity of the ball.

Video Demo

Variation on Dropit Demo from Lecture 12: Dynamic Animation, CS193P, Stanford University

Usually it doesn’t make much sense to repeat code from a lecture demo, but for the Dropit project I think it’s worth to post an article based on a slight variation using the new features of UIKit Dynamics available in iOS 9.

This is perhaps a good time to mention that all the CS193P assignment solutions so far created has been based on Swift 2.1 and iOS 9, whereas the Stanford U. lectures and demos are based on Swift 1.2 and iOS 8.

The motivation to create the variation was a result of attempting to implement assignment 5. Particularly the ball which is a UIView and ideally should be round, even though UIViews are obviously rectangles. The good news is that iOS 9 provides a collisionBoundsType property which can be easily overridden in UIView:

class SphereView: UIView {
    // iOS 9 specific
    override var collisionBoundsType: UIDynamicItemCollisionBoundsType {
        return .Ellipse

This sets the bounds of the view to be an ellipse and allows the right boundary collisions
as you might expect from a ball or sphere.

dropit-spaces-between-spheres dropit-no-spaces-between-spheres

The UIFieldBehavior new in iOS 9, also seems very interesting with properties like: dragField, springField, electricField, magneticField, noiseFieldWithSmoothness and more, which opens the door to new possibilities of animation in your apps. The Dropit variation also adds the noiseFieldWithSmoothness behavior for some cool
visual effects.


With debugEnabled for UIDynamicAnimator (by adding a Swift bridging header *), you can see the noise field in action as it shifts by adding random noise. To enable UIView debugging, the following code needs to be added in the bridging header file:

@import UIKit;

@interface UIDynamicAnimator (AAPLDebugInterfaceOnly)

// Used in DropitViewController.swift file:
// lazilyCreatedDynamicAnimator.debugEnabled = true
@property (nonatomic, getter=isDebugEnabled) BOOL debugEnabled;

Full source code of the demo is available here at Github:

Video Demo

For more on what’s new in UIKit Dynamics in iOS 9, please see the WWDC 2015 video: What’s New in UIKit Dynamics and Visual Effects

* Please see this article on how to add the Swift bridging header: Adding a Swift Bridging Header

GraphingCalculator: Assignment 3, Stanford University, CS193P, Winter 2015

The Graphing Calculator project is a continuation of the previous 2 assignments from the Stanford University CS193P course Developing iOS 8 Apps with Swift, available for free on iTunes. This is the third and final assignment in the Calculator series.

Full source code available here at Github repository

For the other 2 projects, please see:
CalculatorBrain: Assignment 2, Stanford University Winter 2015 (iOS)
Calculator: Assignment 1, Stanford University Winter 2015 (iOS)


This project provides graphing features in addition to the regular calculator functions. Graphs can be plotted by entering expressions where M is the independent variable. Please see examples below.

To plot M, enter:

M, Graph

To plot sin(M), enter:

M, Sin, Graph

To plot M x cos(M), enter:

M, M, cos, x, Graph

Some interesting notes

  • The CalculatorBrain is reused to calculate the values of M for graphing
  • Plot values are set in the GraphingViewController as the data source of GraphingView
  • To fix the sluggish performance when zoomed out and panned, only the number of plots required for the screen width are calculated
  • Graph scale and origin is stored in NSUserDefaults
  • Upon device rotation, the graph is centered and adjusted slightly with the width and height change ratio

Video demo