Unit testing is an essential tool to consistently verify your code works correctly. Discover techniques for designing the code of your app so that it can be easily tested. Find out the best practices for developing a test suite that evolves with your app and scales as your app grows.
Welcome to Engineering for
My name is Brian Croom, and I
work on the Xcode Team.
My colleague Greg and I, want to
share some things that we've
been learning about testability
and what it means for the
process of developing great
I'll start by talking about what
it means for an app's code to be
And we'll see some examples of
techniques that can be applied
to improving testability of an
Then Gregg will come up, and
talk about some ways of working
with your test suite to help
ensure that it helps support the
development of your app, even as
it grows in size and complexity.
So, let's dive in, and talk
I remember a while back, when I
was first learning about writing
I'd been hearing a lot about how
a test suite could help during
the development of my app.
How it could provide confidence,
that the code I was writing was
working the way it was supposed
How it could help patch
regressions in my code as my app
grew and changed over time.
And how it could serve as
executable documentation for my
But, I would start writing a
test, and get stuck before I'd
gotten very far.
It felt as if my app's code were
actively trying to prevent me
from writing the test.
It took a while, but eventually
I discovered that I was
structuring my code in a way
that interfered with effective
To explore what this means,
let's first take a look at a
unit test for some code that is
readily testable, sorting an
This is a test that exercises
the Sorted Method from the Swift
It begins by setting up an array
of unsorted values.
It calls the sorted method on
And then asserts that the
returned array has the values in
the expected order.
Generalizing this, we can see
that the test structure consists
of first, setting up any
required input state or values,
then, calling the code being
tested, and finally, asserting
that the returned output is
This is sometimes referred to as
the Arrange Act Assert Pattern.
So, we've seen that sorted is a
nicely testable piece of code.
But I know what you're thinking,
"My app's code doesn't have a
single sorting algorithm in it."
In my experience, most of the
code in apps, looks pretty
different than the sorting
Still, there are some
characteristics of the sorted
method that we can strive to
achieve in our app's code, and
to make them more testable.
Specifically, testable code
provides a way for its clients
to have control over all of the
inputs that it operates on.
It provides a way for its
clients to inspect any output
And it avoids relying on
internal state that could affect
the code's behavior later on.
I want to use code examples to
demonstrate a couple of
techniques that can be used to
help application code take on
these characteristics, and thus,
improve its testability.
The first technique is how to
introduce protocols and
parameterization into a piece of
For this example, imagine a
document browser app, that is
capable of previewing documents
of various types, and has the
ability to switch to a different
app for viewing it in more
detail or editing it.
The previous screen we see here,
includes an open button for
this, along with a segmented
control for choosing [inaudible]
open for viewing or editing.
So, let's have a look at the
first try at some code for this
So, the event handler in the
view controller that gets
invoked when that Open button is
It starts with some business
logic for constructing a URL to
be used to request that iOS
switch to the other app.
Then, it uses the shared UI
application instance, provided
by UIKit, to determine whether
the system is able to handle
this Open request, and perform
the open URL action if it is.
And if not, it calls a helper
method to show some UI to direct
the user to install the other
Now, I want to write some tests
to make sure that this Open
button is working the way it's
There are a couple of different
ways to approach testing this.
One option would be to write a
We ask to launch the app,
navigate to this screen, tap on
the [inaudible] control, to peck
and open mode, tap the Open
button, and then verify that the
phone switched to show the other
While this would work, there are
some drawbacks to this approach.
For one, the task would probably
take a while to run, especially
if I wanted to expand it to test
with several different documents
types for the different open
The bigger problem, however, is
that a UI test would have no way
to inspect that URL that was
being generated, to request that
iOS switch apps.
And that URL is really something
that I want to be able to
inspect more precisely.
So, it really feels like a unit
test is more appropriate for the
So, let's see what it would take
to write a unit test for this
First, you need an instance of
the view controller to work
My view controller uses a
storyboard for its UI, so I'll
ask UI Storyboard to give me an
instance of the view controller.
Then we need to load the view to
populate that [inaudible]
control property, so that we --
it's populated by the view data.
We can then configure that for
our open mode.
Provide a document to work with.
And with the setup finally
complete, we're now ready to
call the method being tested.
But what do we do now?
It's not so clear what kind of
assertion we could write here.
Let's go back to the code, and
look more closely at what makes
this challenging to test.
For one thing, just being in a
view controller, made the
methods test more complicated.
You're going to jump through
some hoops to get the view
controller instance to work
Then here, we're pulling input
state directly from the view.
[Inaudible] of the test had to
force the view to load, and then
indirectly provide the input by
setting a property on some
The biggest problem though was
this usage of a UI application
The return value from this call
to can open URL, is effectively
another input for the method.
But since this relies on global
system state, there's no
programmatic way for the test to
control the result of this
Nor is there a good way for a
unit test to observe the side
effects of opening a URL.
In fact, after calling this, the
test render app would actually
be sent to the background, and
there'd be no way to bring it
back to the foreground
So, let's see what we can do to
improve the testability of this
We can start by getting it out
of the view controller.
Let's introduce a new document
opener class, to encapsulate
this logic and behavior.
The open mode and document
inputs, should now be provided a
simple method arguments that the
test can pass indirectly.
But we still have to fix the
problems caused by that shared
UI application incidence.
What can we do about that?
Well, to start, we can stop
using that [inaudible] accessor
directly into method.
Let's add an initializer to the
class that lets us pass in a
particular application instance.
We can provide a default value
for the argument, so the
[inaudible] in the view
controller, doesn't have to
worry about this detail.
Back in the open method, we then
switch over to use the
application instance that was
Let's see how far this
refactoring gets us.
If we try to rewrite our test
now, with the document open or
in its current state, we'll
still find ourselves getting
You want to pass in an
application instance that we
have control over.
So, you can imagine sub-classing
UI application, overriding the
can open URL and open methods,
to get the control that we need.
However, it turns out that UI
application strictly enforces
its singleton nature.
And throws an exception to try
to make a second instance, even
if it's a subclass.
So, instead of using
sub-classing, to get the control
that we need, let's instead add
a protocol, URL Opening.
We go to protocol, two methods,
with precisely the same
signatures as the application
methods that we've been using up
to this point.
We still want UI application to
be the primary implementation of
So, we'll throw an extension on
UI application to give it the
URL opening conformance.
Since the method signatures line
up exactly, you don't have to
add any additional code in the
extension to get the
With a protocol in place, let's
update the document opener to
use the protocol instead of
requiring UI application itself.
First, we change the property
and initialize a parameter to
accept any implementation of the
URL opening protocol.
Note that we're still able to
keep the shared UI application
instance with the default
argument as a convenience when
we use it in the view
A final change requires the
document opener simply to adopt
the URL opener property name.
With that, let's return to the
test and see how the pieces fit
Since UI Application doesn't
give us the control that we need
in our test, we want to create a
secondary mock implementation of
the protocol, to use in its
Here we add a sub-implementation
of the two methods.
The can open URL method acts as
an input from the document
So, the test needs to have
control over this input.
We can get that by having the
implementation return the value
of a property that the test can
The open method, act as an
output from the document opener.
The test wants to be able to
access any URL that was passed
into this method.
We can achieve that by stashing
the URL into a property for the
test to -- to read afterwards.
So, let's go ahead and write the
First, we create an instance of
our mock URL opener that we just
wrote, and configure the input
using the can open property.
And create a document opener.
And we pass in that mock URL
opener as the argument.
With that setup finished, we can
call the open method, passing in
document and open mode values,
to act as the rest of the input.
And we can then assert that the
opened URL property of the mock
URL opener, has been set to the
This one assertion, is testing
both at the open method was
called at all, as well as the
URL passed in contains the
With that test under our belt,
you could continue to write
tests for other variations of
input data, such as when the can
open property is set to false.
But we have a lot more to cover,
so let's just leave it there.
So, in this example, you
performed a few refactorings to
allow us to write unit tests for
We pulled out explicit
references to a shared singleton
instance, and replaced them with
parameterized input to offer
This is sometimes referred to as
a dependency injection.
In this example, we use an
initializer parameter to achieve
We could also have used a
property setter, or a parameter
of the method being tested.
We created a protocol to
decouple the code from the
concrete class that it
previously depended on.
And we created a test
implementation to use in its
And it gave us the control that
we needed over the inputs, and
the visibility into the outputs.
Next, I want to look at how
separating out logic from
effects, can also be used to
The example here is an on-disk
cache class, which might be used
by an app for faster retrieval
of assets that have been
previously downloaded from a
This cache defines a script for
representing the items that it
It defines the path to the item
on the file system, how long
it's been in the cache, and its
size on disk.
And it provides a way to get the
set of all the items currently
stored in the cache.
The method that you want to look
at now, though, is a cleanup
It's meant to be called
periodically, to ensure that the
cache doesn't grow to take up
too much space in the file
So, let's have a look at the
starting implementation for this
First, it asks for the set of
all the current items in the
cache, sorts them from newest to
But then it [inaudible] over all
of these items, keeping track of
the total size of the items
Once you've seen enough items
then we reached our maximum
size, we begin removing the rest
of them from the file system.
So, let's think about how to
test this method.
What are the inputs?
What are the outputs?
Well, one input is the parameter
specifying how large you want
the cache to be able to grow.
This is a simple integer, and a
method parameter, so the test
already has full control over
The other input, is the list of
items that are currently stored
in the cache.
We didn't take the time to look
at how this is implemented, but
the key point is that it uses a
file manager to retrieve a list
of files from the disc.
This means that the input is
actually derived from the file
system, which is the dependency
that the tests would need to
The clean cache method has no
So, its output can't be data.
Rather, it's the side effect of
a certain set of files having
been removed from the disc.
Because of this dependency on
the file system, tests for this
method would need to deal with a
file manager in the file system.
For setup, they might need to
create a temporary directory and
populate it with a bunch of
files of certain sizes and give
them particular timestamps to
provide the input.
To validate the output, you
would need to then return to the
file system to see which files
are still there.
One way to approach this could
be to use the protocols and
parameterization techniques that
we've already seen.
You could introduce a file
manager protocol that has the
methods that we need for getting
a list of files, and for
removing a file.
And then create a test
implementation that would allow
specifying the list of files
that would be returned, and a
query of which ones have been
If we do this though, we're
still interacting indirectly
with the code that we're trying
to test mediated by the file
Instead, let's try something a
little bit different.
We could take our clean cache
method and factor out the logic
responsible for deciding while
files should be removed, the
clean-up policy, which you can
then interact with more
Let's see how that might work.
To clearly define the APIs we're
going to work with, let's first
define clean-up policy as a
It just needs a single method --
items to remove.
Notice how the type signature
that we've given it, looks a bit
different than from the clean
cache method that we started
As input, the new method takes a
set of cache item values.
And for output, it returns
another set of them, but that is
the ones to be removed.
So, let's see how we can
implement this protocol using
the algorithm that we previously
saw from the cache class.
Well, first define a property
for that max size input.
That'll let us specify this max
size, and we construct the
Then we add the items to remove
method that the protocol
We want to inspect each of the
items passed into the method,
and build up a set of items to
remove, and return that to the
method when we're done.
To populate the set, we'll loop
over all of the items from
newest to oldest, summing up the
size of all the items we've seen
Once we've reached the maximum
size, we start adding the rest
to the set of items to remove.
Looking at this code, we can see
that the side effects that were
prone in the earlier version
have been removed.
What remains is the underlying
algorithm, taking data as input,
and returning some data as
We can also visualize the data
flow that we've achieved by
Notice that the code has taken
on a very functional style.
Data in; data out.
With the logic factored out in
this way, we've enabled
ourselves to write clear,
concise tests that put the
algorithm through its paces.
All we have to do is define an
input set with some cache items,
create an instance of the type,
calling the method, directly
passing in the values that it
needs, and then asserting that
the returned items match the
With a code in this form, we now
have easy control over the
inputs, visibility into the
outputs, and there's no hidden
state to contend with at all.
It's very reminiscent of the
test for the sorted method that
we saw at the beginning.
This allows the test to be easy
to read, with minimal
distraction from the essentials
of what is being tested to run
very quickly because it has no
dependency on slow resources
like the file system.
And to be deterministic because
all of the data in use is fully
Taking a peek back at the
original clean cache method,
after the extraction, we see
that there's very little left.
All we're doing is asking the
clean-up policy for the list of
items to remove, and iterating
over all of them, removing them
from the file system.
To test what remains, we could
decide to introduce that file
manager protocol and testable
implementation to allow writing
a very isolated unit tests for
Or we might decide that a couple
of integration tests are
sufficient to give the
confidence that we need that the
code is doing the right thing.
This thin layer of [inaudible]
code that's left.
So, in this example, we looked
at how to extract business logic
and algorithms into separate
types, away from the code, using
When doing this, the algorithms
tend to take on a rather
functional style, using value
types to describe the inputs and
This allows for straightforward
unit tests that exercise the
algorithm in as much detail as
We're left with a small amount
of code that perform side
effects based on the computer
This bit is often a good
candidate for testing with
integration tests in order to
track that its interaction with
the rest of the system is
So, to wrap up, we saw examples
that allowed us to explore two
sets of techniques that allow us
to structure or ask code, so
that tests have control over the
code's inputs, and visibility
into its outputs, thereby
allowing us to write effective
unit tests for it.
And now, I want to call up my
colleague, Greg Tracy, to talk
about how to create a test suite
that scales with your app as it
My name is Greg.
And I also work on Xcode.
Earlier, Brian showed you some
techniques about how to make app
code more testable.
Now, I want to show you how to
make the accompanying test code
To do this, we'll look at a few
methods to make tests faster,
more readable, and more
I also want to mention that many
of the techniques that Brian
described earlier, can also be
applied to test code.
Here, we're going to go through
some additional tips.
First, I'll talk about having a
balance between UI and unit
Then, dive into code that helps
test scale with the focus on UI
And then, I'll talk about the
importance of having quality in
Striking the right balance
between UI and unit tests.
I sometimes like to view my
distribution of tests as a
At the top, you have your UI
tests, and at the bottom, you
have your unit tests.
These [inaudible] pyramid
structure we usually have more
unit tests than we will UI
This is usually because unit
tests can run much more quickly
than UI tests.
Now between UI and unit tests,
we also have integration tests.
However, today, we're going to
focus more on just UI and unit
Aside from the distribution, we
might also look at the pyramid
as a way to do maintenance
Generally, UI tests tend to have
a higher maintenance cost
because of the number of things
that can happen.
Unit tests on the other hand,
have a lower maintenance cost.
So, if a unit test fails, it's
usually immediately obvious
what's gone wrong.
With UI tests, it's like casting
a wide net where you can get
failures that are difficult to
understand, or might not be
relevant to the test at hand.
So, it can be a bit more tricky.
While the testing pyramid is a
great way to represent
distributions of -- our
distribution of tests, it
doesn't represent every
In fact, you might think of
testing as a spectrum, rather
than a pyramid.
It's often the case that some UI
and unit tests, exist on
opposite ends of the spectrum,
or opposite ends of the pyramid.
Some UI tests might be more like
unit tests, and a unit test
might interact with several
different modules of code and
not just single, isolated bits.
The pyramid is just a good
It's not written in stone.
When thinking about these two
kinds of tests, we need to
consider each of their
Unit tests are great at testing
small bits of code that might be
hard to reach without access to
all of our app source code.
UI tests on the other hand, are
great when you need to test
large chunks of code, working
Of course, we need to keep in
mind that unit tests do have
access to all of our app's
source, whereas UI tests do not.
Focusing more on UI tests, let's
look at a few things you could
to do improve the quality of
your test code.
By making some of the changes
I'm about to suggest, we can
make it easier to create tests
that scale alongside our app
We'll look at abstracting UI
element queries, creating
objects in utility functions,
which can then be placed in a
library for later use, and
utilizing keyboard shortcuts.
So, first we'll look at
abstracting UI element queries.
Say I have an app that has
several buttons in a view
And each button is at the same
level in the view hierarchy.
The only difference is the name
of each button.
Instead of writing out this
query seven times, let's wrap
this up in a method.
We can now modify each of our
queries to use the new method we
However, I might even go
Since each method calls the
same, except for the name, let's
put all those names in array and
just loop through them.
This adds some benefit of
maintainability for this code.
If I add an extra button in the
future, I don't have to add a
new line of code.
I just have to add an extra
button name to the array.
By nature of what a UI test is,
we're issuing a lot of these
So, if you're using the same
query multiple times, store it
as a variable.
Even if it's only part of a
query, store it somewhere.
Also, if you have queries that
are very similar, consider
creating a helper method around
The code will look a lot cleaner
and become much more readable.
In terms of scaling our test
suite, the use of shorter lines
of code -- of shorter lines of
test code and thoughtfully named
helper methods, will make it
faster and easier to implement
new tests when the time comes.
So, that was abstracting UI
Now, let's move on to creating
objects in utility functions.
I have this game I've been
working on, and for each test, I
want to change some settings.
Now, this is not a great example
of scalable code.
Because I've been recently
working with this app, I'm
familiar with how everything is
I understand exactly what's
However, later, if I was to come
back to this code after a few
weeks, or better yet, somebody
not familiar with my code has to
sit down and read what I wrote,
it might not make all that much
I first have to realize that I
have a Settings page that I need
to get in and out of.
And I then have to realize that
between those two lines, I'm
going through a difficulty page,
setting the difficulty, then I'm
going through a sound page, and
setting the sound.
Coming after an extended period
of time, I might not understand
why I have two back tabs at the
I would have to run through this
test to actually see this
And if the test were broken
because of some change in the
actual UI, I wouldn't be able to
run my test, and I would be able
to see what I wanted to see.
I'd be clueless as to why the
code was written the way it was.
To fix this, let's try to
abstract away some of this logic
into helper methods.
We can create a method to set
And then similarly, we can
create a method to set the
But can we do a little better?
Sure. We can instead, of instead
of passing stream typed
arguments, let's utilize enums.
That way, Xcode can -- helps
determine if the arguments we're
passing are even valid, before
we even compile.
Now, looking at the code from
before, if we replace some of
the code with our new helper
methods, we reduce what we had
And this is already starting to
look a lot better.
What about the initial jump in
and out of the Settings page?
Can we improve this as well?
I think we can.
Let's make a game app class.
And in this class, I'll include
the enums I defined earlier for
difficulty and sound.
I'll also include the helper
methods from before that set
We'll create another method
called Configure Settings, that
takes the two settings as
inputs, and we'll migrate the
setup logic from before, into
the configure method.
Back to where we were before,
now that we've created this game
app class, we can take away all
the code we wrote before, and
just use a single call, the
This looks a lot more readable
to me than what we had before.
Now, if I wrote more tests than
needed to set the settings, I
would just call our configure
And if I needed to -- or if I
decided that I wanted to add
more settings to my app, I would
just have to update our
configure settings method to
handle these additional
From the example, one of the
most important things to do when
trying to scale your tests, is
to create abstraction that you
can later put into a library
By doing this, we're
encapsulating common workflows
that can be applied to more than
This also means that we're able
to share test code across
And, of course, by sharing code,
we improve maintainability.
If something related to an
abstracted workflow changes, we
only have to update our code in
a single place, as opposed to
One other improvement that I
want to mention, in our
configure method, and new in
Xcode this year, we can add an
XCTContent.runActivity block to
This makes it so that when we
run our test, instead of having
a log that contains all the
actions that we made at the top
level, we can nest our logging,
This helps organize our logging
to make things look a little bit
For more information regarding
the test activity feature, check
out the earlier talk about
What's New in Testing?
Now, let's move on to Utilizing
Keyboard Shortcuts for macOS UI
Let's say I have an app where
the user can pick a color for
their text, using the standard
macOS color picker.
And I'm writing a test to verify
that the color is set correctly.
The typical way to bring up the
color picker in my app is by
opening the Format menu, and
navigating to the Font sub-menu,
and finally choosing Show
I can write this in my UI test
But there's a faster way to do
this that'll scale better as my
test [inaudible] grows.
Notice that the Show Colors menu
item has an associated keyboard
Rather than using multiple lines
of code to bring up the color
picker, we can just use one line
of code using the shortcut.
And for the sake of readability,
I might use a wrapper to make
this call for me.
So, not only is this less code
to maintain in my tests, it's
less code that has to be run
that isn't directly relevant to
the actual test I'm trying to
So, in an example test method, I
was previously going through the
menu to get the color picker to
Using the new wrapper method, we
remove all those extra lines and
reduce to a single method.
Not only does this make my test
faster, it makes my code look a
lot more readable.
Looking at what we just saw, if
you're writing a UI test for a
macOS application, you can offer
a keyboard shortcut instead of
going through the menu bar.
I'm still going to have at least
one test that ensures that
bringing up the color picker via
the menu bar, works properly,
but that doesn't need to be
repeated across every single
test that involves the color
In the process of using keyboard
shortcuts, we make our test code
more compact by skipping extra
steps needed to work through the
UI, sometimes reducing multiple
lines of code to a single line,
which can help for readability.
Finally, I want to stress to all
of you that writing good tests
is about writing good code.
It's easy to treat tests as an
Usually, we focus on trying to
make our app the best it can
possibly be, by investing all
this time in writing beautiful
app code that adheres to all
these principles of good design.
We might have this additional
requirement of writing test
It might be something tacked on
at the end or done in a hurry
just to check off a checkbox.
But we can't allow this to
Without the same attention to
detail, test code isn't going to
scale the same way our app code
So, to that end, test code is
important even though it isn't
Also note that the test suite
should support the evolution of
your app, and not hinder change.
With low quality test code, it
becomes a burden to have to
update your test whenever you
make a change to your app.
But by consciously designing
test code with quality in mind,
our ability to scale won't be
inhibited by poorly designed
And of course, coding principles
that apply to app code, also
apply to test code.
Test code and app code should be
And here's an idea for you.
We should have code reviews for
test code, not just code reviews
with test code.
Having code reviews that are
exclusively for test code,
ensures that somebody else is
checking your work, or
[inaudible] with a test cover,
and it's a chance to further
improve the tests themselves.
Now, I want to leave you with a
App code and the tests that
verify it, are really two halves
of a whole.
When you update your app code,
you'll need to update your test
We need to think of app code and
the test code, as part of the
same thing, our code.
By making our code more
testable, as Brian discussed
earlier, and by treating test
code with the same care as your
app code, you improve the
quality of the whole app.
We ought to be proud of both
halves, and treat each with the
care and attention that they
For more information and
resources regarding this
session, you can visit the link
listed on the screen.
Here are a few related sessions.
One that happened yesterday, and
a few that happened in previous
You can check those out online,
or through the WWDC app.
And with that, I'd like to thank
you for your attention, and I
hope you enjoy the rest of the
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