1. ( And Halp clients )
Code Reuse
with MVVM
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High-level talk, mostly focused on Apple (because that’s my forté), but most of the concepts here are generally applicable to all
platforms (and that’s exactly the point).

2. 2Dienstag, 16. April 13
But first, let me introduce myself and the other guys who worked on this app.

3. Who
Justin Spahr-Summers - @jspahrsummers
Paul Betts - @xpaulbettsx
Josh Vera - @joshvera
Josh Abernathy - @joshaber
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I work on GitHub for Mac. I primarily write Objective-C code, but I also regularly code in Haskell as well. Paul works on GitHub for
Windows (written in C#), Josh Vera works on an internal talks app written in Objective-C, and Josh Abernathy also works on
GitHub for Mac.
There’s quite a large diversity of experience between us, and I’ve included these guys on here because they all had something do
with this presentation today.

4. What
Write unit tests for UI behavior
Example: our native Halp apps
Maximize code reuse across platforms,
but keep 100% native UI
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Halp is an internal app that we use at GitHub for user support (but more on that later).

5. Why
“Build software better, together.”
At GitHub, we ask: “What is the best way to build software?”
Philosophies, tools, practices
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One of the questions _we_ work on every day is, “What’s the best way to build native user interfaces?”

6. View Model
Controller
How
( According to  )
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As recommended by Apple, Cocoa applications are typically designed using Model-View-Controller, shown here. The solid lines
represent direct references; the dashed lines represent indirect references (like observation).

7. View ViewModel Model
How
( According to us )
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At GitHub, we much prefer to use Model-View-ViewModel, shown here. If you haven’t been introduced to MVVM, here’s a quick
explanation:
The ViewModel replaces the role of the (View) Controller, but the VM doesn’t have a direct reference to the view like a controller
would. Instead, the VM communicates to the V with a system of bindings. … For example, If you want to show a loading spinner,
the view model might have a boolean property which indicates whether to show it. The view would observe that property for
changes, and hide/show the spinner in response.

8. Meh. So what?
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This might just seem like a way to restate the MVC pattern, but the reversed relationship between the View and the ViewModel
offers huge benefits.

9. Benefits of MVVM
✓ View models are testable, no UI
automation required
✓ View models can do model-like
things (e.g., serialization)
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Traditionally, view controllers rarely get unit tested in Cocoa, simply because it’s such a pain to write a controller that doesn’t
depend on having a view (or, alternatively, to set up a valid view in unit tests). Since the VM doesn’t even know about view
objects, they can be tested without a GUI at all!
Serialization: for example, to save and restore the state of your UI, you can just save and restore your VM hierarchy. Doing this in
MVC would require a separate set of “view state” objects – which are basically view models anyways!

10. Also, portability!
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I mentioned code sharing between platforms, so let’s take a look at how that works in MVC and MVVM. Naturally, we’ll assume
the use of Xamarin for both.

11. Model-View-Controller
View Model
Controller
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The blue circle here is the code we can share across platforms.

12. What’s Shared?
Xamarin means we only have to write
our models once, in .NET
Any networking and domain logic
is trivially cross-platform
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13. What’s Unique?
We want 100% native UI on each
platform – no Qt, GTK+, or Java
To do this, we need to create views
specific to each platform
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This makes sense and is perfectly appropriate. Sharing view code leads to lower-quality apps which cater to the lowest common
denominator and ignore each platform’s individual UI conventions.

14. What’s Unshared?
Logic for when to fetch
resources from the API
UI behaviors
(e.g., how to populate lists, or when to show spinners)
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If we follow MVC, we’re also rewriting this logic for each platform (as part of our controller layer), even though it’s not platform-
specific. This is code that _should_ be shared, but isn’t.
Now, let’s contrast that with MVVM.

15. Model-View-ViewModel
View ViewModel Model
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Interestingly, because the VM doesn’t reference the view (or any UI) directly, it becomes reusable across platforms. The VM
describes only how the UI should update and respond to user actions – not how it should look. Multiple types of view can be
created for one view model, and each can look completely different, but most of the underlying logic will remain the same.
If we’re using Xamarin, we can now write most of our model _and view model_ code just once. The VM implements most of our
UI behavior, like…

16. View Models Handle…
Loading content the UI needs
Hiding and showing content
Date, number, and string formatting
Responding to the user
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There are just some typical use cases, not a complete list.
Loading: note that the view model is not actually responsible for the details of persistence, networking, etc. It’s only responsible
for communicating with whatever that layer is, _based on_ what the UI needs to show at any point in time.

17. Halp!
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That’s most of the abstract stuff. I want to switch gears for a moment here and talk about our support tool, and the native clients
we’re implementing using MVVM and Xamarin.

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This is the web app that we use for user support. It lets us triage our users’ emails and get them to the right people as quickly as
possible. Supportocats and developers can reply to messages, bring other people into the discussion, cross-link to other internal
resources, etc.
Here, we’re looking at a discussion thread in the Technical inbox.

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GitHub is based in San Francisco, but about half of GitHub works remotely on a regular basis (I myself work from Seattle). Twice
every year, all of the company meets in SF for GitHub Summit.
Our last summit was earlier this year, and a few of us wanted to spend our Hack Day working on a native client for Halp. We
decided to use Xamarin to share code between our different desired platforms, and reduce the development and maintenance
effort that would otherwise be involved in each one. We started the iOS client that day, and a Mac client since.

20. Mac App Goals
Watch a specific inbox for new messages
Display a message count in the menu bar
View the messages in any inbox
(but especially the watched one)
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This is what we want to do for our Mac client.
We’ve started on a prototype. It’s still very premature, so it doesn’t do much yet.

21. iPhone App Goals
View the messages in any inbox
Read any message
Triage messages by moving to another inbox
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And this is what we want to do for our iPhone client. (An iPad client would be very similar as well.)
This one’s a bit further along, but still pretty rough around the edges. All the data here is loaded from the API and cached locally
by the app.

22. (Demo)
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23. Shared Behaviors
Showing inboxes and messages
Requesting and caching data
Showing loading indicators
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By no coincidence, these are the behaviors implemented by our cross-platform view models.
Let’s take a look at the code. (ViewModels, MenubarController?, PopoverController?, TableSources)

24. (Code)
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25. Let’s get real.
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Cocoa wasn’t really designed with MVVM in mind. Here are some minor obstacles you may encounter.

26. View Controllers
Layout, animations, device rotation,
view transitions
Seems like view controllers are
actually part of the view layer!
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OS X and iOS both have view (or window) controllers, which can make MVVM confusing at first glance. Once you look deeper,
though, it’s not much of a problem at all.

27. View Controllers
NSViewController doesn’t do much
UIViewController is quite powerful
Between views and view controllers,
use the easiest one
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Basically, use the class that will make implementing your view layer easiest. On OS X, you’ll probably just want NSView, since
NSViewController is relatively useless. On iOS, you’ll probably want UIViewController, so you can handle rotation, navigation, etc.
No matter what you decide to use for your UI, you’ll still have a ViewModel.

28. Data Binding
Notifications are too general,
and have global scope
Key-Value Observing is difficult
to use and comes with boilerplate
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It’s hard to write the indirect relationship from the ViewModel to the View without a powerful system of bindings. Cocoa (and, by
extension, Xamarin.Mac and Xamarin.iOS) offers a couple solutions, but they’re woefully inadequate.
In addition to these individual problems, neither supports automatic transformation or filtering of bound values. Worse, both are
specific to Cocoa, so our V <> VM bindings will look quite different from our VM <> M bindings (which should be cross-
platform).

29. Data Binding
In Objective-C, we wrote a
framework called ReactiveCocoa
In C#, we have Reactive Extensions
and our ReactiveUI framework
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Reactive Extensions (or Rx) is an implementation of Functional Reactive Programming, which is unfortunately beyond the scope
of this talk, but there are lots of great resources for learning more about it.
ReactiveUI is an MVVM framework for .NET. One of its major features is an API for declarative data bindings, built on top of Rx.

30. 30Dienstag, 16. April 13
GitHub for Mac uses ReactiveCocoa to implement MVVM at a large scale. The app itself is written in Objective-C, but the lessons
we’ve learned about MVVM are just as applicable to Xamarin.Mac and Xamarin.iOS.

31. Linkage
Rx – introtorx.com
ReactiveUI – reactiveui.net
github:mac – mac.github.com
github:windows – windows.github.com
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