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<!DOCTYPE html>
<html lang="en">
<head>
<meta charset="utf-8"/>
<title>MVVM on iOS</title>
<meta name="author" content="(Josh Lehman)"/>
<link rel="stylesheet" href="https://cdn.jsdelivr.net/reveal.js/3.0.0/css/reveal.css"/>
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<link rel="stylesheet" href=""/>
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</head>
<body>
<div class="reveal">
<div class="slides">
<section id="sec-title-slide">
<h1>MVVM on iOS</h1>
<h2>Josh Lehman</h2>
<h2><a href="mailto:josh@savvy.ai">josh@savvy.ai</a></h2>
<h2></h2>
</section>
<section>
<section id="slide-orgheadline2">
<h2 id="orgheadline2">MVC: Model-View-Controller</h2>
<div class="figure">
<p><img src="./img/mvc.png" alt="mvc.png" />
</p>
</div>
<ul>
<li>Architectural design pattern</li>
<li>Developed by Xerox PARC in the 1970s</li>
<li>Expressed as a general concept in 1988</li>
</ul>
</section>
<section id="slide-orgheadline1">
<h3 id="orgheadline1">It's an old concept</h3>
<ul>
<li>Which is not bad</li>
<li>Good ideas are improved upon over time</li>
</ul>
</section>
</section>
<section>
<section id="slide-orgheadline14">
<h2 id="orgheadline14">MVVM: Model-View-ViewModel</h2>
<div class="figure">
<p><img src="./img/mvvm.png" alt="mvvm.png" />
</p>
</div>
<ul>
<li>Developed by Microsoft, announced in 2005</li>
<li>A slight addition to MVC</li>
<li>We'll discuss <b>Data Binding</b> later</li>
</ul>
</section>
<section id="slide-orgheadline3">
<h3 id="orgheadline3">Adds the <i>ViewModel</i></h3>
<blockquote nil>
<p>
"The central component of MVC, the model, captures the behavior of the
application in terms of its problem domain, independent of the user interface."
</p>
<p>
– <a href="https://en.wikipedia.org/wiki/Model%E2%80%93view%E2%80%93controller#Components">Wikipedia</a>
</p>
</blockquote>
<p>
<i>(where <b>user interface</b> is the View and Controller)</i>
</p>
</section>
<section>
<blockquote nil>
<p>
The <code>ViewModel</code> captures the <i>behaviors</i> of an user interface in terms of
general user interactions, independent of the view itself.
</p>
</blockquote>
</section>
<section id="slide-orgheadline4">
<h3 id="orgheadline4">Why is this a good thing?</h3>
<div class="figure">
<p><img src="./img/massive-view-controller.png" alt="massive-view-controller.png" />
</p>
</div>
<ul>
<li>Smaller view controllers!</li>
<li>Lower coupling
<ul>
<li>Decouples GUI code from presentation logic and state</li>
</ul></li>
<li>Headless testing</li>
</ul>
</section>
<section id="slide-orgheadline5">
<h3 id="orgheadline5">View and ViewModel Relationship</h3>
<ul>
<li><i>Generally</i>, one <code>ViewModel</code> per controller or <code>UIView</code> subclass:
<ul>
<li><code>UIViewController</code></li>
<li><code>UITableViewCell</code></li>
<li><code>UICollectionViewCell</code></li>
<li>etc.</li>
</ul></li>
</ul>
</section>
<section id="slide-orgheadline6">
<h3 id="orgheadline6">Ownership</h3>
<div class="figure">
<p><img src="./img/mvvm.png" alt="mvvm.png" />
</p>
</div>
<ul>
<li>The <b>View</b> <i>owns</i> the <b>ViewModel</b>
<ul>
<li>ViewModels know <i>nothing</i> about Views</li>
</ul></li>
<li>The <b>ViewModel</b> <i>owns</i> the <b>Model</b>
<ul>
<li>Models know <i>nothing</i> about ViewModels</li>
</ul></li>
<li>The <b>View</b> knows <i>nothing</i> about the <b>Model</b></li>
</ul>
</section>
<section id="slide-orgheadline7">
<h3 id="orgheadline7">Views do not communicate!</h3>
<p>
Views communicate with their ViewModels, which communicate with each other.
</p>
</section>
<section id="slide-orgheadline8">
<h3 id="orgheadline8">Problem</h3>
<p>
If the view's state is stored in a <code>ViewModel</code> class, how do we keep the two in
sync?
</p>
</section>
<section id="slide-orgheadline13">
<h3 id="orgheadline13">Data Binding</h3>
<ul>
<li>Not strictly <i>necessary</i>, but really helpful
<ul>
<li>Delegates can work here, but are more verbose</li>
</ul></li>
<li>Helps keep <code>ViewModel</code> in sync with its <code>View</code></li>
</ul>
</section>
<section id="slide-orgheadline9">
<h4 id="orgheadline9">1-way > 2-way</h4>
<ul>
<li>2-way binding is really hard (it's cyclical)
<ul>
<li>If 2-way binding seems like the only solution, find a better solution</li>
</ul></li>
</ul>
</section>
<section id="slide-orgheadline10">
<h4 id="orgheadline10"><a href="https://github.com/ReactiveCocoa/ReactiveCocoa">ReactiveCocoa</a> (RAC)</h4>
<ul>
<li>Mostly based on <a href="https://en.wikipedia.org/wiki/Functional_reactive_programming">Functional Reactive Programming (FRP)</a>
<ul>
<li>Represent data as "streams of values over time"</li>
</ul></li>
<li>Used at: <a href="http://savvy.ai/">Savvy</a>, Snapchat, GitHub, probably some other places too</li>
</ul>
</section>
<section id="slide-orgheadline11">
<h4 id="orgheadline11">Popular and well-maintained</h4>
<dl>
<dt>First released</dt><dd>2/26/2012</dd>
<dt>Last commit to master</dt><dd>11/3/2015 (at time of writing)</dd>
<dt>Stars</dt><dd>11,081</dd>
<dt>Contributors</dt><dd>129</dd>
</dl>
</section>
<section id="slide-orgheadline12">
<h4 id="orgheadline12"><a href="https://github.com/SwiftBond/Bond">Bond</a></h4>
<ul>
<li>A data binding framework</li>
<li>Less concept-heavy</li>
<li>Also well maintained</li>
<li>I am less familiar with it – examples will use RAC</li>
</ul>
</section>
</section>
<section>
<section id="slide-orgheadline22">
<h2 id="orgheadline22">Interlude: ReactiveCocoa</h2>
<div class="outline-text-2" id="text-orgheadline22">
</div></section>
<section id="slide-orgheadline15">
<h3 id="orgheadline15">What is "Functional Reactive Programming"?</h3>
<blockquote nil>
<p>
Functional reactive programming (FRP) is a programming paradigm for reactive
programming (asynchronous dataflow programming) using the building blocks of
functional programming (e.g. map, reduce, filter).
</p>
</blockquote>
</section>
<section id="slide-orgheadline16">
<h3 id="orgheadline16">Signals</h3>
<ul>
<li>Represent streams of values (data) as they change</li>
<li>Signals can be <i>observed</i></li>
<li>Two varieties in RAC: <code>SignalProducer</code> and <code>Signal</code></li>
<li>Send <i>events</i>:
<ul>
<li><code>next</code>: The data that the signal carries – can happen many times</li>
<li><code>error</code>: An error occurred – terminates</li>
<li><code>interrupted</code>: The signal was interrupted – terminates</li>
<li><code>completed</code>: Successful completion – terminates</li>
</ul></li>
</ul>
</section>
<section id="slide-orgheadline17">
<h3 id="orgheadline17">Signal Producers</h3>
<div class="org-src-container">
<pre class="src src-swift"><span style="color: #859900; font-weight: bold;">func</span> <span style="color: #268bd2;">doNetworkStuff</span><span style="color: #2aa198;">()</span> -> <span style="color: #b58900;">SignalProducer</span><<span style="color: #b58900;">JSON</span>, <span style="color: #b58900;">NoError</span>>
<span style="color: #859900; font-weight: bold;">let</span> <span style="color: #268bd2;">producer </span>= doNetworkStuff<span style="color: #2aa198;">()</span>
producer.startWithNext <span style="color: #2aa198;">{</span> json <span style="color: #859900; font-weight: bold;">in</span> print<span style="color: #b58900;">(</span>json<span style="color: #b58900;">)</span> <span style="color: #2aa198;">}</span>
</pre>
</div>
<ul>
<li>Has to be "started" to do anything</li>
<li>Kind of like promises</li>
<li>Network requests are a good example</li>
</ul>
</section>
<section id="slide-orgheadline18">
<h3 id="orgheadline18">Signals</h3>
<ul>
<li>Send values regardless of whether or not anything is observing</li>
<li>"Always On" semantics</li>
</ul>
</section>
<section id="slide-orgheadline19">
<h3 id="orgheadline19">Mutable Properties</h3>
<div class="org-src-container">
<pre class="src src-swift"><span style="color: #859900; font-weight: bold;">let</span> <span style="color: #268bd2;">text </span>= <span style="color: #b58900;">MutableProperty</span><<span style="color: #b58900;">String</span>><span style="color: #2aa198;">(</span><span style="color: #2aa198;">"Hello, World!"</span><span style="color: #2aa198;">)</span>
text.value <span style="color: #586e75;">// </span><span style="color: #586e75;">=> "Hello, World!"</span>
text.producer <span style="color: #586e75;">// </span><span style="color: #586e75;">=> SignalProducer<String, NoError></span>
text.producer.startWithNext <span style="color: #2aa198;">{</span> s <span style="color: #859900; font-weight: bold;">in</span> print<span style="color: #b58900;">(</span>s<span style="color: #b58900;">)</span> <span style="color: #2aa198;">}</span> <span style="color: #586e75;">// </span><span style="color: #586e75;">prints "Hello, World!"</span>
text.value = <span style="color: #2aa198;">"Yo."</span> <span style="color: #586e75;">// </span><span style="color: #586e75;">prints "Yo"</span>
</pre>
</div>
<ul>
<li>Exposes a <code>SignalProducer</code> of the values in the property</li>
</ul>
</section>
<section id="slide-orgheadline20">
<h3 id="orgheadline20">Binding</h3>
<div class="org-src-container">
<pre class="src src-swift"><span style="color: #859900; font-weight: bold;">let</span> <span style="color: #2aa198;">(</span><span style="color: #268bd2;">observer, producer</span><span style="color: #2aa198;">)</span> = <span style="color: #b58900;">SignalProducer</span><<span style="color: #b58900;">String</span>, <span style="color: #b58900;">NoError</span>>.buffer<span style="color: #2aa198;">()</span>
<span style="color: #859900; font-weight: bold;">let</span> <span style="color: #268bd2;">text </span>= <span style="color: #b58900;">MutableProperty</span><<span style="color: #b58900;">String</span>><span style="color: #2aa198;">(</span><span style="color: #2aa198;">""</span><span style="color: #2aa198;">)</span>
text <~ producer
observer.sendNext<span style="color: #2aa198;">(</span><span style="color: #2aa198;">"a"</span><span style="color: #2aa198;">)</span>
text.value <span style="color: #586e75;">// </span><span style="color: #586e75;">"a"</span>
observer.sendNext<span style="color: #2aa198;">(</span><span style="color: #2aa198;">"b"</span><span style="color: #2aa198;">)</span>
text.value <span style="color: #586e75;">// </span><span style="color: #586e75;">"b"</span>
</pre>
</div>
<ul>
<li>We can <i>bind</i> the result of a <code>SignalProducer</code> to a <code>MutableProperty</code></li>
<li>The binding operator: <code><~</code></li>
<li>No <code>KVO</code>!</li>
</ul>
</section>
<section id="slide-orgheadline21">
<h3 id="orgheadline21">Actions</h3>
<div class="org-src-container">
<pre class="src src-swift"><span style="color: #859900; font-weight: bold;">func</span> <span style="color: #268bd2;">saveTodoOnServer</span><span style="color: #2aa198;">(</span>todo: <span style="color: #b58900;">Todo</span><span style="color: #2aa198;">)</span> -> <span style="color: #b58900;">SignalProducer</span><<span style="color: #b58900;">Bool</span>, <span style="color: #b58900;">NSError</span>> <span style="color: #2aa198;">{</span>
<span style="color: #859900; font-weight: bold;">return</span> <span style="color: #b58900;">SignalProducer</span><span style="color: #b58900;">(</span>value: <span style="color: #268bd2; font-weight: bold;">true</span><span style="color: #b58900;">)</span>
<span style="color: #2aa198;">}</span>
<span style="color: #859900; font-weight: bold;">let</span> <span style="color: #268bd2;">createTodo </span>= <span style="color: #b58900;">Action</span> <span style="color: #2aa198;">{</span> <span style="color: #b58900;">(</span>t: <span style="color: #b58900;">Todo</span><span style="color: #b58900;">)</span> -> <span style="color: #b58900;">SignalProducer</span><<span style="color: #b58900;">Bool</span>, <span style="color: #b58900;">NSError</span>> <span style="color: #859900; font-weight: bold;">in</span>
<span style="color: #859900; font-weight: bold;">return</span> saveTodoOnServer<span style="color: #b58900;">(</span>t<span style="color: #b58900;">)</span>
<span style="color: #2aa198;">}</span>
<span style="color: #859900; font-weight: bold;">let</span> <span style="color: #268bd2;">todo </span>= <span style="color: #b58900;">Todo</span><span style="color: #2aa198;">()</span>
createTodo.values.observeNext <span style="color: #2aa198;">{</span> success <span style="color: #859900; font-weight: bold;">in</span> print<span style="color: #b58900;">(</span>success<span style="color: #b58900;">)</span> <span style="color: #2aa198;">}</span>
createTodo.apply<span style="color: #2aa198;">(</span>todo<span style="color: #2aa198;">)</span> <span style="color: #586e75;">// </span><span style="color: #586e75;">=> SignalProducer<Bool, NSError></span>
createTodo.apply<span style="color: #2aa198;">(</span>todo<span style="color: #2aa198;">)</span>.start<span style="color: #2aa198;">()</span> <span style="color: #586e75;">// </span><span style="color: #586e75;">prints "true"</span>
createTodo.apply<span style="color: #2aa198;">(</span>todo<span style="color: #2aa198;">)</span>.start<span style="color: #2aa198;">()</span> <span style="color: #586e75;">// </span><span style="color: #586e75;">prints "true"</span>
</pre>
</div>
<ul>
<li>Like a function, but where the result of invocation is <i>observed</i> rather than
<i>returned</i>
<ul>
<li>Can have many observers!</li>
</ul></li>
<li>Take parameters, return a <code>SignalProducer</code>
<ul>
<li>We <i>apply</i> parameters, and then <i>start</i> the resulting producer</li>
<li>Expose <code>values</code> property: A <code>Signal</code> of the values of the <code>SignalProducer</code></li>
</ul></li>
</ul>
</section>
</section>
<section>
<section id="slide-orgheadline28">
<h2 id="orgheadline28">A Sample Application: Todo List</h2>
<div class="outline-text-2" id="text-orgheadline28">
</div></section>
<section id="slide-orgheadline25">
<h3 id="orgheadline25">ViewModels Drive the Application</h3>
<div class="org-src-container">
<pre class="src src-swift"><span style="color: #859900; font-weight: bold;">protocol</span> <span style="color: #b58900;">ViewModelServicesProtocol</span> <span style="color: #2aa198;">{</span>
<span style="color: #859900; font-weight: bold;">var</span> <span style="color: #268bd2;">todo</span>: <span style="color: #b58900;">TodoServiceProtocol</span> <span style="color: #b58900;">{</span> <span style="color: #859900; font-weight: bold;">get</span> <span style="color: #b58900;">}</span>
<span style="color: #859900; font-weight: bold;">var</span> <span style="color: #268bd2;">date</span>: <span style="color: #b58900;">DateServiceProtocol</span> <span style="color: #b58900;">{</span> <span style="color: #859900; font-weight: bold;">get</span> <span style="color: #b58900;">}</span>
<span style="color: #859900; font-weight: bold;">func</span> <span style="color: #268bd2;">push</span><span style="color: #b58900;">(</span>viewModel: <span style="color: #b58900;">ViewModelProtocol</span><span style="color: #b58900;">)</span>
<span style="color: #859900; font-weight: bold;">func</span> <span style="color: #268bd2;">pop</span><span style="color: #b58900;">(</span>viewModel: <span style="color: #b58900;">ViewModelProtocol</span><span style="color: #b58900;">)</span>
<span style="color: #2aa198;">}</span>
<span style="color: #859900; font-weight: bold;">protocol</span> <span style="color: #b58900;">ViewModelProtocol</span> <span style="color: #2aa198;">{</span>
<span style="color: #859900; font-weight: bold;">var</span> <span style="color: #268bd2;">services</span>: <span style="color: #b58900;">ViewModelServicesProtocol</span> <span style="color: #b58900;">{</span> <span style="color: #859900; font-weight: bold;">get</span> <span style="color: #b58900;">}</span>
<span style="color: #2aa198;">}</span>
</pre>
</div>
</section>
<section id="slide-orgheadline23">
<h4 id="orgheadline23">Navigation</h4>
<div class="org-src-container">
<pre class="src src-swift"><span style="color: #859900; font-weight: bold;">func</span> <span style="color: #268bd2;">push</span><span style="color: #2aa198;">(</span>viewModel: <span style="color: #b58900;">ViewModelProtocol</span><span style="color: #2aa198;">)</span>
<span style="color: #859900; font-weight: bold;">func</span> <span style="color: #268bd2;">pop</span><span style="color: #2aa198;">(</span>viewModel: <span style="color: #b58900;">ViewModelProtocol</span><span style="color: #2aa198;">)</span>
</pre>
</div>
<ul>
<li><b>ViewModels</b> will instantiate and <code>push</code> other <b>ViewModels</b>.</li>
<li>Services are responsible for instantiating the proper Views.</li>
</ul>
</section>
<section id="slide-orgheadline24">
<h4 id="orgheadline24">Model Services</h4>
<div class="org-src-container">
<pre class="src src-swift"><span style="color: #859900; font-weight: bold;">protocol</span> <span style="color: #b58900;">TodoServiceProtocol</span> <span style="color: #2aa198;">{</span>
<span style="color: #859900; font-weight: bold;">func</span> <span style="color: #268bd2;">update</span><span style="color: #b58900;">(</span>todo: <span style="color: #b58900;">Todo</span><span style="color: #b58900;">)</span> -> <span style="color: #b58900;">SignalProducer</span><<span style="color: #b58900;">Todo</span>, <span style="color: #b58900;">NoError</span>>
<span style="color: #859900; font-weight: bold;">func</span> <span style="color: #268bd2;">delete</span><span style="color: #b58900;">(</span>todo: <span style="color: #b58900;">Todo</span><span style="color: #b58900;">)</span> -> <span style="color: #b58900;">SignalProducer</span><<span style="color: #b58900;">Bool</span>, <span style="color: #b58900;">NoError</span>>
<span style="color: #859900; font-weight: bold;">func</span> <span style="color: #268bd2;">create</span><span style="color: #b58900;">(</span>note: <span style="color: #b58900;">String</span>, dueDate: <span style="color: #b58900;">NSDate</span><span style="color: #b58900;">)</span> -> <span style="color: #b58900;">SignalProducer</span><<span style="color: #b58900;">Todo</span>, <span style="color: #b58900;">NoError</span>>
<span style="color: #2aa198;">}</span>
</pre>
</div>
<ul>
<li>Model services deal with stateful resources, e.g. network operations</li>
<li>Only <b>ViewModels</b> have access to services</li>
</ul>
</section>
<section id="slide-orgheadline26">
<h3 id="orgheadline26">Views Observe ViewModels and <i>React</i></h3>
<div class="org-src-container">
<pre class="src src-swift"><span style="color: #859900; font-weight: bold;">class</span> <span style="color: #b58900;">TodoTableViewModel</span>: <span style="color: #b58900;">ViewModel</span>, <span style="color: #b58900;">CreateTodoViewModelDelegate</span> <span style="color: #2aa198;">{</span>
<span style="color: #859900; font-weight: bold;">let</span> <span style="color: #268bd2;">todos </span>= <span style="color: #b58900;">MutableProperty</span><<span style="color: #b58900;">[</span><span style="color: #b58900;">TodoCellViewModel</span><span style="color: #b58900;">]</span>><span style="color: #b58900;">(</span><span style="color: #268bd2;">[]</span><span style="color: #b58900;">)</span>
<span style="color: #859900; font-weight: bold;">let</span> <span style="color: #268bd2;">deleteTodo</span>: <span style="color: #b58900;">Action</span><<span style="color: #b58900;">(</span>todos: <span style="color: #268bd2;">[</span><span style="color: #b58900;">TodoCellViewModel</span><span style="color: #268bd2;">]</span>, cell: <span style="color: #b58900;">TodoCellViewModel</span><span style="color: #b58900;">)</span>, <span style="color: #b58900;">NSIndexPath</span>?, <span style="color: #b58900;">NoError</span>>
<span style="color: #2aa198;">}</span>
<span style="color: #859900; font-weight: bold;">class</span> <span style="color: #b58900;">TodoTableViewController</span>: <span style="color: #b58900;">ReactiveViewController</span><<span style="color: #b58900;">TodoTableViewModel</span>> <span style="color: #2aa198;">{</span>
<span style="color: #859900; font-weight: bold;">override</span> <span style="color: #859900; font-weight: bold;">func</span> <span style="color: #268bd2;">viewDidLoad</span><span style="color: #b58900;">()</span> <span style="color: #b58900;">{</span>
<span style="color: #859900; font-weight: bold;">super</span>.viewDidLoad<span style="color: #268bd2;">()</span>
<span style="color: #859900; font-weight: bold;">func</span> <span style="color: #268bd2;">removeRow</span><span style="color: #268bd2;">(</span>indexPath: <span style="color: #b58900;">NSIndexPath</span>?<span style="color: #268bd2;">)</span> <span style="color: #268bd2;">{</span>
todoTableView.deleteRowsAtIndexPaths<span style="color: #6c71c4;">(</span><span style="color: #859900;">[</span>indexPath!<span style="color: #859900;">]</span>, withRowAnimation: .<span style="color: #b58900;">Left</span><span style="color: #6c71c4;">)</span>
<span style="color: #268bd2;">}</span>
<span style="color: #586e75;">// </span><span style="color: #586e75;">Remove a row whenever a Todo is deleted</span>
viewModel.deleteTodo.values
.filter <span style="color: #268bd2;">{</span> $0 != <span style="color: #268bd2; font-weight: bold;">nil</span> <span style="color: #268bd2;">}</span>
.observeOn<span style="color: #268bd2;">(</span><span style="color: #b58900;">UIScheduler</span><span style="color: #6c71c4;">()</span><span style="color: #268bd2;">)</span>
.observeNext<span style="color: #268bd2;">(</span>removeRow<span style="color: #268bd2;">)</span>
<span style="color: #b58900;">}</span>
<span style="color: #2aa198;">}</span>
</pre>
</div>
</section>
<section id="slide-orgheadline27">
<h3 id="orgheadline27">The Code</h3>
<p>
<a href="https://github.com/jalehman/todolist-mvvm">https://github.com/jalehman/todolist-mvvm</a>
</p>
</section>
</section>
<section>
<section id="slide-orgheadline30">
<h2 id="orgheadline30">Inspiration & Credits</h2>
<ul>
<li><a href="http://blog.scottlogic.com/ceberhardt/">Colin Eberhardt</a>'s series of tutorials on MVVM and RAC</li>
<li><a href="https://realm.io/news/andy-matuschak-controlling-complexity">Controlling Complexity in Swift</a> by Andy Matuschak</li>
<li><a href="https://www.youtube.com/watch?v=7AqXBuJOJkY">Enemy of the State</a> by Justin Spahr-Summers</li>
<li>Wikipedia</li>
</ul>
</section>
<section id="slide-orgheadline29">
<h3 id="orgheadline29">Mobile Makers</h3>
<p>
Thanks for letting me talk!
</p>
</section>
</section>
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