I have hopefully tried here to gather some useful stuff about MVVM which is the "proper" way to build a highly decoupled app in iOS today, a model that makes use of ReactiveCocoa as binding solution.

1. Modelsin iOS programming
describing MVVM as MCV’s natural evolution step
Andrei Popa

2. Agenda1) Separation of Concerns
2) MVC
3) Problems with MVC
4) MVVM - describing MVVM as MCV’s natural evolution step
5) ReactiveCocoa as binding solution in iOS
6) Android correspondence, MVP
Andrei Popa

4. In computer science, separation of concerns (SoC) is a design
principle for separating a computer program into distinct sections,
such that each section addresses a separate concern.
When concerns are well-separated, individual sections can be reused,
as well as developed and updated independently.
Of special value is the ability to later improve or modify one section of code
without having to know the details of other sections, and without having to make
corresponding changes to those sections.
separation of concerns

5. Separation of concerns is an important design principle in many other areas
as well, such as urban planning, architecture and information design.
The goal is to more effectively understand, design, and manage complex
interdependent systems, so that functions can be reused, optimized
independently of other functions, and insulated from the potential failure of
other functions.
separation of concerns

6. MVC or MVP can separate content from presentation
MVVM can separate data gathering / logic / transforming from the
complexities of a view controller.
separation of concerns

7. M-V-C (Model View Controller)
• a proven pattern for organizing your code in complex
application design, also the Apple recommended way of
programming an app
• defines the roles objects play in the application
• It’s also proven to have a second meaning in iOS
development: Massive View Controller
• Apple hasn’t provided any real guidance on how to solve
the Massive View Controller problem

8. M-V-C

10. • God object
• Responsible for the actual UI (whether that means
UIView code, storyboards, or xibs), any view specific
logic, and reaction to user input
• This includes a lot of the responsibilities handled by
the UIViewController in iOS, not just UIView code and
files
Massive View Controller

12. M-V-VM
• MVVM stands for Model View View-Model, and it’s here to
help us create more manageable, better designed code.
• The testability and separation of concerns are some of the
good things that come out of this particular pattern.
• MVVM is a pattern to separate the concern of
presentation from domain.
• Developed by Microsoft's for WPF and Silverlight architecture, and
currently the pattern of choice for Windows Phone Development

13. M-V-VM

14. M-V-VM

15. • This leaves the view (controller) with a more clearly defined task of
presenting the data supplied by the view-model.
• The view-model exposes the minimum information necessary to our
view controller, and the view controller really doesn’t care how the
view-model got that information.
M-V-VM

16. M-V-VM

17. • View independent representation of business entities
• Represents the data layer
• Responsible for fetching / storing the data
• CoreData / Realm / Parse / …
• JSON
• Encryption
M-V-VM (Model)

18. M-V-VM (viewModel)
• In some of the WWDC videos this year, viewModels were actually
spotted in the sample code apple engineers had on screen.
• One of its responsibilities is that of a static model representing the
data necessary for the view to display itself; but it’s also
responsible for gathering, interpreting, and transforming that data.
• Kicking off network or database requests
• Determining when information should be hidden or shown
• Localization
• Date and number formatting
• It is NOT acting directly on the view controller in any form or
notifying it directly of changes

19. M-V-VM (viewModel)
• MVVM and MVC share a common weakness: neither
defines where the network logic of an app should go.
• Add the networking logic in the viewModel

20. • View models are (mostly) platform-agnostic.
• It applies logic and massages the data from the model into
presentation data for the view (controller).
• It exposes (usually via properties) only the information the
view controller needs to know about to do its job of
displaying the view (ideally you are not exposing your data-
model objects).
• It is NOT acting directly on the view controller in any form or
notifying it directly of changes
M-V-VM (viewModel)

21. • The view controller’s only concerns are:
• configuring and managing the various views with data from the
view-model
• letting the view-model know when relevant user input occurs
that needs to change data upstream
• Layout / Animations / Device rotations / View transitions
• Inject viewModel to viewController during init
• The view controller doesn’t need to know about anything
else
M-V-VM (view / UIViewController)

22. • ViewModel becomes extremely unit testable. If you have
methods that should generate the same output every time
they are supplied the same input (pure functions), that fits
extremely well into the world of unit tests.
• We now have our data gathering / logic / transforming
extracted away from the complexities of a view controller.
• When the view model needs to communicate something
to the view, it does so through a system of data bindings
M-V-VM (Advantages)

23. Data bindingsWhat about when data on the view-model changes?

24. Data bindings

25. Data bindings

26. A signal takes all those async methods for controlling the
flow of information through your app (delegates, callback
blocks, notifications, KVO, target/action event observers,
etc) and unifies them under one interface.
Stateless binding using
ReactiveCocoa

27. In some cases you can eliminate even more state on
your view-model by exposing RACSignals instead of
properties like strings and images.
Then your view controller wouldn’t have to create its own
with RACObserve, and could just leverage those signals
straightaway.
ReactiveCocoa

28. • By modeling changes as signals, the view model can
communicate to the view without actually needing to know
that it exists
• similarly for model → view model communication
• This decoupling is why view models can be tested without
a view in place — the test simply needs to connect to the
VM's signals and verify that the behavior is correct.
M-V-VM

29. • Framework inspired by Functional Reactive
Programming
• Fancy bindings
• Event streams unify all of Cocoa’s common patterns for
asynchrony and event handling
• Mature, production-ready framework
• Wide-adoption by top-class developers
ReactiveCocoa

30. • The Model-View-Presenter-pattern (MVP) has been the
dominating trend lately when it comes the UI-layer
architecture of Android applications.
• Data Binding, as announced on Google I/O 2015
changes everything.
• https://developer.android.com/tools/data-binding/
guide.html
• https://github.com/ReactiveX/RxAndroid
MVVM on Android

31. • Data Binding framework will take over the main
responsibilities of the Presenter (“acting upon the
model and the view”), while the remainder is left to the
enhanced Model – the ViewModel (“retrieving data from
repositories and formatting”).
• The ViewModel is a standard Java class whose sole
responsibility is to represent the data behind a single
View. It can merge data from multiple sources (Models)
and prepare that data for presentation.
MVVM on Android

34. Thank you!Q&A