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122 lines
5.2 KiB
Markdown
122 lines
5.2 KiB
Markdown
# ReactiveViewModel
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ReactiveViewModel is a combination code/documentation project for building Cocoa
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applications using [Model-View-ViewModel](#model-view-viewmodel) and
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[ReactiveCocoa](#reactivecocoa).
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By explaining rationale, documenting best practices, and providing reusable
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library components, we want to make MVVM in Objective-C appealing and easy.
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## Model-View-ViewModel
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Most Cocoa developers are familiar with the
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[Model-View-Controller](http://en.wikipedia.org/wiki/Model-View-Controller)
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(MVC) pattern:
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**[Model-View-ViewModel](http://en.wikipedia.org/wiki/Model-View-ViewModel)
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(MVVM)** is another architectural paradigm for GUI applications:
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Although it seems similar to MVC (except with a "view model" object in place of
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the controller), there's one major difference — **the view owns the view
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model**. Unlike a controller, a view model has no knowledge of the specific view
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that's using it.
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This seemingly minor change offers huge benefits:
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1. **View models are testable.** Since they don't need a view to do their work,
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presentation behavior can be tested without any UI automation or stubbing.
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1. **View models can be used like models.** If desired, view models can be
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copied or serialized just like a domain model. This can be used to quickly
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implement UI restoration and similar behaviors.
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1. **View models are (mostly) platform-agnostic.** Since the actual UI code
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lives in the view, well-designed view models can be used on the iPhone,
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iPad, and Mac, with only minor tweaking for each platform.
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1. **Views and view controllers are simpler.** Once the important logic is
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moved elsewhere, views and VCs become dumb UI objects. This makes them
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easier to understand and redesign.
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In short, replacing MVC with MVVM can lead to more versatile and rigorous UI
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code.
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### What's in a view model?
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A view model is like an [adapter](http://en.wikipedia.org/wiki/Adapter_pattern)
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for the model that makes it suitable for presentation. The view model is also
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where presentation _behavior_ goes.
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For example, a view model might handle:
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* Kicking off network or database requests
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* Determining when information should be hidden or shown
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* Date and number formatting
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* Localization
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However, the view model is not responsible for actually presenting
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information or handling input — that's the sole domain of the view layer. When
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the view model needs to communicate something to the view, it does so through
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a system of [data binding](#reactivecocoa).
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### What about view controllers?
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OS X and iOS both have view (or window) controllers, which may be confusing at
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first glance, since MVVM only refers to a view.
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But upon closer inspection, it becomes apparent that view controllers are
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_actually just part of the view layer_, since they handle things like:
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* Layout
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* Animations
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* Device rotation
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* View and window transitions
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* Presenting loaded UI
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So, "the view" actually means the view _layer_, which includes view controllers.
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There's no need to have a view and a view controller for the same section of the
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screen, though — just pick whichever class is easier for the use case.
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No matter whether you decide to use a view or a view controller, you'll still
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have a view model.
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## ReactiveCocoa
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MVVM is most successful with a powerful system of [data
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binding](http://en.wikipedia.org/wiki/UI_data_binding).
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[ReactiveCocoa](https://github.com/ReactiveCocoa/ReactiveCocoa) is one such
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system.
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By modeling changes as
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[signals](https://github.com/ReactiveCocoa/ReactiveCocoa#introduction), the view
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model can communicate to the view without actually needing to know that it
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exists (similarly for model → view model communication). This decoupling is
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why view models can be tested without a view in place — the test simply needs to
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connect to the VM's signals and verify that the behavior is correct.
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ReactiveCocoa also includes other conveniences that are hugely beneficial for
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MVVM, like
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[commands](https://github.com/ReactiveCocoa/ReactiveCocoa/blob/master/Documentation/FrameworkOverview.md#commands),
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and built-in bindings for AppKit and UIKit.
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## Getting Started
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To build ReactiveViewModel in isolation, open `ReactiveViewModel.xcworkspace`. To integrate it into your project, include `ReactiveViewModel.xcodeproj` and `ReactiveCocoa.xcodeproj` and link your target against the ReactiveViewModel and ReactiveCocoa targets for your platform.
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## More Resources
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Model-View-ViewModel was originally developed by
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[Microsoft](http://bit.ly/gQY00r), so many of the examples are specific to WPF
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or Silverlight, but there are still a few resources that may be useful:
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**Blog posts:**
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* [Basic MVVM with ReactiveCocoa](http://cocoasamurai.blogspot.com/2013/03/basic-mvvm-with-reactivecocoa.html)
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* [Model-View-ViewModel for iOS](http://www.teehanlax.com/blog/model-view-viewmodel-for-ios/)
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* [Presentation Model](http://martinfowler.com/eaaDev/PresentationModel.html)
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**Presentations:**
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* [Code Reuse with MVVM](https://speakerdeck.com/jspahrsummers/code-reuse-with-mvvm)
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