Code Reuse with MVVM

My Xamarin Evolve 2013 talk.

Video available at: http://xamarin.com/evolve/2013#session-zm59b5yptf

Transcript

1. ( And Halp clients ) Code Reuse with MVVM 1 Dienstag, 16. April 13 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. 2 Dienstag, 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 3 Dienstag, 16. April 13 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 4 Dienstag, 16. April 13 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 5 Dienstag, 16. April 13 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  ) 6 Dienstag, 16. April 13 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 ) 7 Dienstag, 16. April 13 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? 8 Dienstag, 16. April 13 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) 9 Dienstag, 16. April 13 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! 10 Dienstag, 16. April 13 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 11 Dienstag, 16. April 13 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 12 Dienstag, 16. April 13
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 13 Dienstag, 16. April 13 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) 14 Dienstag, 16. April 13 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 15 Dienstag, 16. April 13 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 16 Dienstag, 16. April 13 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! 17 Dienstag, 16. April 13 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.
18. 18 Dienstag, 16. April 13 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.
19. 19 Dienstag, 16. April 13 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) 20 Dienstag, 16. April 13 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 21 Dienstag, 16. April 13 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) 22 Dienstag, 16. April 13
23. Shared Behaviors Showing inboxes and messages Requesting and caching data Showing loading indicators 23 Dienstag, 16. April 13 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) 24 Dienstag, 16. April 13
25. Let’s get real. 25 Dienstag, 16. April 13 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! 26 Dienstag, 16. April 13 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 27 Dienstag, 16. April 13 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 28 Dienstag, 16. April 13 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 29 Dienstag, 16. April 13 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. 30 Dienstag, 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 31 Dienstag, 16. April 13