Category Archives: Swift

Tests help catch simple mistakes

This morning I ran into yet another example of where tests can help you catch simple mistakes. I’m working on creating a custom control that simulates an automotive style fuel gauge.


This involves rotating a needle layer to indicate the fuel level. However, I don’t want to needle to rotate across the bottom 1/4th of the circle (over the gas pump icons), only across the top 3/4ths.

This became a problem when I added animations. UIView.animateWithDuration always takes the shortest distance. So animating the needle from pointing to empty to having it point to full would result in the needle moving counter-clockwise across the bottom of the gauge. A real gauge would never do that.

So to fix this problem, I split the animation into 2 sections. When the distance to move the needle is equal-to-or-greater than 180 degrees, then I use 2 animations, each of which are less than 180 degrees. So I created a method to perform the 180 degree test as shown:

func angleInRadiansIsEqualToOrGreaterThan180Degrees(angle: CGFloat) -> Bool {
let isGreaterThan180degrees: Bool = angle >= CGFloat(M_PI_2) || angle <= CGFloat(-M_PI_2)
return isGreaterThan180degrees

Unfortunately, angles are expressed in radians, not degrees, and that’s where I made the mistake. My quick, mental conversion of degrees-to-radians was wrong; 180 degrees equals pi, not pi/2. This mistake didn’t cause the code to fail, it just didn’t work as desired (using 2-part animations for all angles greater than 90 degrees instead of 180).

This morning I was going back and adding unit tests to the code (blush: not using TDD since this is a spike to figure out how to do the animations). In doing so, I correctly calculated the number of radians, causing the new tests to fail:

func testAngleInRadiansIsEqualToOrGreaterThan180DegreesTrueFor180() {
let angle = convertDegreesToRadians(180)
XCTAssertTrue(fuelGaugeView.angleInRadiansIsEqualToOrGreaterThan180Degrees(CGFloat(angle)), "Angle is equal to 180, but reported false")

func testAngleInRadiansIsEqualToOrGreaterThan180DegreesFalseFor90() {
let angle = convertDegreesToRadians(97)
XCTAssertFalse(fuelGaugeView.angleInRadiansIsEqualToOrGreaterThan180Degrees(CGFloat(angle)), “Angle (97) is less than 180, but reported true”)

func convertDegreesToRadians(degrees: Float) -> Float {
let radians = Float(M_PI / 180) * degrees
return radians

This raises a couple interesting points:

  1. Beware of mental math; for example, converting degrees to radians in my head in this case. I was lucky, and the 2nd time got it right, so my tests exposed the problem. However, I could very easily have made the exact same mistake in my tests.
  2. Where possible, use explicit calculations in your tests (if not also in your code). In this example, I created a convertDegreesToRadians() method in my tests. Had there been a similar function available in math.h, I would have used it.

Note: I will be releasing this code to GitHub shortly. It will include all of the code for a framework containing a set of animated, automotive style gauges including Fuel gauge, Speedometer, and Tachometer. I’ll update this post with a link once it is available.

Using Overridable Properties Instead of “Tell Don’t Ask”

In order for objects to be easily testable, it is often good practice to use “Tell, don’t ask”. What this phrase means is that methods should not directly “ask” for (create) system objects that they need. Instead, they should be “told” which system object to use.

This really runs contrary to common iOS practice though. It is very common, even in Apple example code, to see objects creating system objects whenever and wherever needed.

For example, a class needing access to CoreLocation’s CLLocationManager object will typically insert the following statement wherever it is needed:

let locationManager = LocationManager()

This is quick and easy, but makes testing difficult. Unit test would probably want to ensure that calls to locationManager occur, either by testing for side effects, or else using a test double (mock) in place of a real LocationManager instance.

“Tell, Don’t Ask” would suggest that the class or method needing an instance of LocationManager be passed that instance. A negative effect of doing this though, is that the creator of the object or caller of the method needs to know about, and/or create the other object to pass. In this case, it would need to import CoreLocation.

// Create beacon class and determine if we're near it
// caller code passes instance of CLLocationManager to init
 let myBeacon = MyBeaconClass(locationManager: CLLocationManager())
 let isNear = myBeacon.isNear()
// or passes it to specific methods that need it
 let myBeacon = MyBeaconClass()
 let isNear = myBeacon.isNear(locationManager:CLLocationManager())

Recently I have been working on cleaning up a reusable beacon class. I am trying to encapsulate it to the point that a user of the class would not need to know anything about CoreLocation. However, in the above example, the user would need to create an instance of LocationManager() to pass to the class’s initializer or to individual methods.

After pondering this a bit, I believe that in this case it works better to have locationManager defined as an instance variable (var instead of let) that is initialized by the beacon class initializers, but can then be overridden by the unit tests. This allows the unit tests to replace the default instance of LocationManager with a mock object, but does require the product code user to know anything about CLLocationManager. So this code would look like this:

// Create beacon class and determine if we're near it
  let myBeacon = MyBeaconClass()
  let isNear = myBeacon.isNear()

It is perfectly ok for the unit tests to have to know about CoreLocation, so the test code would look something like this:

  // Create beacon class and determine if we're near it.
  let beaconUnderTest = MyBeaconClass()
  // Substitute a mock object
  beaconUnderTest.locationManager = MyMockLocationManager()
  let isNear = beaconUnderTest.isNear()

One big consideration when doing this is to keep the amount of work being done in the initializer small. If the instance of LocationManager is being manipulated during initialization, this manipulation may be difficult to test. So it may be necessary to provide separate methods to be called after initialization. The Single Responsibility Principle probably suggests doing this anyways.

“Tell, Don’t Ask” is still a recommended best practice. But it appears that in some cases using overridable instance variables provides better separation of concerns.

Swift Test Coverage

Currently code coverage reporting for Swift code is broken. There is a radar open for this: rdr://15121260
This bug is preventing .gcov records from being generated.

If you are wanting to track test code coverage for your Swift code, please dup rdr://15121260 to help Apple assess the number of developers impacted by this issue.

Testing Swift Protocol Delegates

I ran into an interesting problem when trying to test that a delegate was set. Here’s a simplified example:

protocol MyProtocol {
  func myMethod()

class ClassWithDelegate {
  var delegate: MyProtocol?

class ClassUnderTest: MyProtocol {
  var otherClass: ClassWithDelegate

  init() {
    self.otherClass = ClassWithDelegate()
    self.otherClass.delegate = self

In order to test that our ClassUnderTest is instantiating the ClassWithDelegate and setting its delegate, we could do the following:

class OurTests: XCTestCase {
  var testClass: ClassUnderTest()


  func testThatOtherClassCreated {

  func testThatOtherClassDelegateSet {
    XCTAssertTrue(testClass.otherClass.delegate === testClass)

This looks pretty simple, right? For those of us still learning Swift, it isn’t simple. What happens is that the compiler refuses to build, with the error: ‘MyProtocol’ is not identical to ‘AnyObject’.

After tons of reading, and learning a lot about Swift protocols and pointers, I basically stumbled upon the solution: add the “class” keyword to the protocol. This tells the compiler that only objects will implement the protocol, not structs. It then is quite happy to perform tests for nil and/or identicalness (?). So the working protocol definition looks like this:

protocol MyProtocol : class {
  func myMethod()

Now all the other code works.

Testing Mixed Swift and Objective-C code

I’ve been working with a Swift project, and recently decided to add MMWormhole to it. This is the first Objective-C code to be added to this project, so I needed to review the Apple WWDC 2014 videos and developer documentation to figure out how to that.

It appears at first glance that the process is very simple. At least, it should be. What I ran into though appears to be bugs in Xcode.

The first bug I ran into was a problem with the prompt to automatically create the bridging header. When an Objective-C file is added to an all-Swift project, Xcode is supposed to ask you if you’d like a bridging header to be created, and then do so. This didn’t happen, but it’s no big deal. The bridging header can be easily created manually. This is pretty simple, but may not work depending on whether you hit the second bug I ran into, described further on.

  1. Create a new header file. Name it -Bridging-Header.h (substituting with the name of your project)
  2. Search for the “Objective-C Bridging Header” setting, and set its value to point to the file just created
  3. Repeat step 2 for the test target if you’re going to do any testing that needs to reference the Objective-C code also. This shouldn’t be necessary in most cases, but I needed to do so in order to write a unit test to try to catch the second bug.

The second bug is much worse. It appears that once a project starts getting bigger or more complex, the bridging header just stops working. By that I mean that the objects included using the bridging header (MMWormhole in my case) stop being found, even if previously building ok.

I’m in the process of recreating my project to try to determine exactly what steps cause the bridging header to stop working. I’ll update this post once I’ve found it.

Update 3/11/15

A friend of mine ran into an issue when trying to test a Swift class that had been added to his otherwise Objective-C project. Since he already had a good suite of Objective-C tests, he simply wanted to add an Objective-C test. This caused various compile errors. The solution was to add the Swift file to the test target.

By default, tests are run as “application tests”. This means that the tests are injected into the application. Hence tests have access to everything in the application. However, Swift files are compiled into a separate module during the build process, and thus not visible to the test target. You can think of this as meaning that Swift files are in effect run as “library tests”, and thus must be added to the test target to be visible.

Manual Mocking in Swift

I’ve become a fan of creating one’s own test doubles (aka mock objects) instead of using a mock framework like OCMock or OCMockito. Uncle Bob describes this process in one of his Clean Code videos, and it seems to work very well in Swift.

Why create your own mock objects? There are a couple reasons for doing this.

  1. Tests can be more readable.
    I say “can” because its really up to you to create readable mocks.
  2. You have nearly unlimited faking capability.
    I say “nearly” because there are some things that may not be possible using class inheritance, for example overriding methods or properties that have been marked as “final“.

So let’s take a look at using manually created fake doubles for testing a simple Power class. This class has one purpose: to report whether or not the device is plugged-in. The way this is done on an iOS device is by using the batteryState property of the UIDevice currentDevice singleton.

Uh-oh, did I just say “singleton”? Yep. That’s typically bad news for unit testing. However, this won’t be a problem for us because we’re going to replace the UIDevice singleton with our own fake object. This means that we’ll have to follow the “tell, don’t ask” principle, and pass in the UIDevice object to our Power object instead of letting it “ask” for the singleton UIDevice currentDevice object.

class Power {

   var device: UIDevice

   init() {
      self.device = UIDevice.currentDevice()

   init(usingMockDevice mockDevice: UIDevice) {
      device = mockDevice

   func isPluggedIn() -&gt; Bool {
      let batteryState = self.device.batteryState
      let isPluggedIn = batteryState ==    UIDeviceBatteryState.Charging || batteryState ==  UIDeviceBatteryState.Full
      return isPluggedIn


Here we’re using separate initializer methods to allow passing in a fake UIDevice object, or default to using the singleton currentDevice. Normal production code usage would look something like this:

   var power = Power()
   if power.isPluggedIn() {

On the other hand, for testing this we’ll create a test double and use it with the usingMockDevice initializer as shown here:

class mockPluggedInDevice: UIDevice {
   override var batteryState: UIDeviceBatteryState {
      return UIDeviceBatteryState.Full
class PowerTests: XCTestCase {

   func testPluggedInWhenBatteryFull() {
      let mockDevice = mockPluggedInDevice()
      let power = Power(usingMockDevice: mockDevice)

So here’s a question for you. In which file do we put the mockPluggedInDevice code? I’m a bit conflicted on this. On one hand, I usually prefer to keep test code completely out of product code files. But on the other hand, mock objects are by nature going to be pretty dependent on the product code they are mocking, so I should probably keep it with the product code.

Clarification: For simplicity, I am using the term “mock” object to describe what would more accurately be called a “stub”. Martin Fowler clearly defines the difference in this 2007 article.

Testing Storyboards in Swift?

Awhile back I blogged about how to test storyboards by loading the ViewController from a Storyboard. Recently I’ve been trying to figure out how to do the same thing in Swift.

Update: latest code appears to be working ok for testing view controllers as long as I don’t try to run them on an actual iPhone device. When I do that, the test hangs.

The code and instructions posted by Mike Cole appear to almost work in Swift with the latest Xcode (I’m using Xcode 6.2, but 6.1 works also). I had to also reference the ViewController’s view. Evidently the ViewController will lazy load the view, meaning that the outlets don’t become connected until after ViewController.view is referenced.

Also, be careful what you put into your view controller’s viewDidLoad method. Code that requires that the view is actually being displayed should be moved to viewWillDisplay or may cause the unit test to crash (since there isn’t really a display during unit testing).

Example XCTest setup code to load a viewcontroller named “MainVC” from main.storyboard.

override func setUp() {
  let storyboard = UIStoryboard(name: "Main", bundle: NSBundle(forClass: self.dynamicType))
  vc = storyboard.instantiateViewControllerWithIdentifier("MainVC") as ViewController
  let dummy = vc.view // force loading subviews and setting outlets

I’ve left the previous post below, but it’s basically outdated now. After running into various problems, I resorted to using Objective-C unit tests, and describe how to do so below. I wouldn’t recommend doing so anymore, since Swift tests appear to work ok now.
After fumbling around for awhile with the Swift test case file, and not making much progress, I decided to try something different, and tried adding an Objective-C test case file to my Swift project. At least in this case, I know how to load the ViewController from the Storyboard. And this actually worked.

This turned out to be fairly simple. Let me warn you though, that I consider this approach to be a temporary hack, albeit an interesting one. I believe the correct solution will be to figure out how to do this in a Swift unit test case file. At this point though, I’m not sure if I’m being prevented from doing so due to bugs in beta level Xcode 6, or just my lack of Swift knowledge.

But caveat emptor aside, here is how to create an Objective-C unit test case file in an otherwise entirely Swift project, and use it to load and test a ViewController from a storyboard:

  • Set the product module name so that you will know the swift header file name to import (eg. “#import ProductModuleName-swift.h”). Note that this should not be necessary, as the Product Module Name is supposed to default to the Product Name. This wasn’t working as of Beta 2.
  • Add an Objective-C unit test case file. Make sure to set the language to Objective-C. It will default to Swift.
  • I did not have to create an Objective-C Bridging  Header for the Swift ViewController, because I’m only going the other way, from Swift to Objective-C, but I did anyways when prompted. I don’t think having it will hurt anything, and you might want it later.
  • Import the swift header file into the Objective-C unit test case file:
    #import “ProductModuleName-swift.h”

That was it. This then gave me access to my ViewController definitions. Then use the previous described method of loading a storyboard and viewcontroller from it.

I will update this article once I figure out how to do it correctly in a Swift unit test case file.

Unit testing in Swift

The iOS folks around here are all abuzz about the new Swift language announced by Apple this week. Surprisingly, this information was released publicly, so no NDA required to talk about it.  You can find the information at

While there are still a lot of questions about the implications of testing in Swift, at first glance it looks pretty straight forward. One nice surprise, reported yesterday on, is that  Setup() isn’t needed to create fresh instances of the object-under-test. Simply creating a as a constant stored property of the test case class results in a fresh instance being created for each test.

I’ve run through some TDD katas using Swift, and after some initial adjustment to the new language, I have to say that things appear to work fairly well. I’m really warming up to Swift.

Now the next question is whether or not the existing mock frameworks still work, or how soon we can get updates. No comment from Jon Reid yet, but I’ll bet he will be speaking out soon 🙂