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<article class="md-content__inner md-typeset">
<a href="https://github.com/vapor/documentation/edit/master/3.0/docs/getting-started/async.md" title="Edit this page" class="md-icon md-content__icon">&#xE3C9;</a>
<h1 id="async">Async<a class="headerlink" href="#async" title="Permanent link">&para;</a></h1>
<p>You may have noticed some APIs in Vapor expect or return a generic <code>Future</code> type. If this is your first time hearing about futures, they might seem a little confusing at first. But don't worry, Vapor makes them easy to use.</p>
<p>This guide will give you a quick introduction to working with Async. Check out <a href="../../async/overview/">Async → Overview</a> for more information.</p>
<h2 id="futures">Futures<a class="headerlink" href="#futures" title="Permanent link">&para;</a></h2>
<p>Since <code>Future</code>s work asynchronously, we must use closures to interact with and transform their values. Just like optionals in Swift, futures can be mapped and flat-mapped. </p>
<h3 id="map">Map<a class="headerlink" href="#map" title="Permanent link">&para;</a></h3>
<p>The <code>.map(to:_:)</code> method allows you to transform the future's value to another value. The closure provided will be called once the <code>Future</code>'s data becomes available. </p>
<div class="codehilite"><pre><span></span><span class="c1">/// Assume we get a future string back from some API</span>
<span class="kd">let</span> <span class="nv">futureString</span><span class="p">:</span> <span class="n">Future</span><span class="p">&lt;</span><span class="nb">String</span><span class="p">&gt;</span> <span class="p">=</span> <span class="p">...</span>
<span class="c1">/// Map the future string to an integer</span>
<span class="kd">let</span> <span class="nv">futureInt</span> <span class="p">=</span> <span class="n">futureString</span><span class="p">.</span><span class="bp">map</span><span class="p">(</span><span class="n">to</span><span class="p">:</span> <span class="nb">Int</span><span class="p">.</span><span class="kc">self</span><span class="p">)</span> <span class="p">{</span> <span class="n">string</span> <span class="k">in</span>
<span class="bp">print</span><span class="p">(</span><span class="n">string</span><span class="p">)</span> <span class="c1">// The actual String</span>
<span class="k">return</span> <span class="nb">Int</span><span class="p">(</span><span class="n">string</span><span class="p">)</span> <span class="p">??</span> <span class="mi">0</span>
<span class="p">}</span>
<span class="c1">/// We now have a future integer</span>
<span class="bp">print</span><span class="p">(</span><span class="n">futureInt</span><span class="p">)</span> <span class="c1">// Future&lt;Int&gt;</span>
</pre></div>
<h3 id="flat-map">Flat Map<a class="headerlink" href="#flat-map" title="Permanent link">&para;</a></h3>
<p>The <code>.flatMap(to:_:)</code> method allows you to transform the future's value to another future value. It gets the name "flat" map because it is what allows you to avoid creating nested futures (e.g., <code>Future&lt;Future&lt;T&gt;&gt;</code>). In other words, it helps you keep your futures flat.</p>
<div class="codehilite"><pre><span></span><span class="c1">/// Assume we get a future string back from some API</span>
<span class="kd">let</span> <span class="nv">futureString</span><span class="p">:</span> <span class="n">Future</span><span class="p">&lt;</span><span class="nb">String</span><span class="p">&gt;</span> <span class="p">=</span> <span class="p">...</span>
<span class="c1">/// Assume we have created an HTTP client</span>
<span class="kd">let</span> <span class="nv">client</span><span class="p">:</span> <span class="n">Client</span> <span class="p">=</span> <span class="p">...</span>
<span class="c1">/// Flat-map the future string to a future response</span>
<span class="kd">let</span> <span class="nv">futureResponse</span> <span class="p">=</span> <span class="n">futureString</span><span class="p">.</span><span class="n">flatMap</span><span class="p">(</span><span class="n">to</span><span class="p">:</span> <span class="n">Response</span><span class="p">.</span><span class="kc">self</span><span class="p">)</span> <span class="p">{</span> <span class="n">string</span> <span class="k">in</span>
<span class="k">return</span> <span class="n">client</span><span class="p">.</span><span class="kr">get</span><span class="p">(</span><span class="n">string</span><span class="p">)</span> <span class="c1">// Future&lt;Response&gt;</span>
<span class="p">}</span>
<span class="c1">/// We now have a future response</span>
<span class="bp">print</span><span class="p">(</span><span class="n">futureResponse</span><span class="p">)</span> <span class="c1">// Future&lt;Response&gt;</span>
</pre></div>
<div class="admonition info">
<p class="admonition-title">Info</p>
<p>If we instead used <code>.map(to:_:)</code> in the above example, we would have ended up with a <code>Future&lt;Future&lt;Response&gt;&gt;</code>. Yikes!</p>
</div>
<h3 id="chaining">Chaining<a class="headerlink" href="#chaining" title="Permanent link">&para;</a></h3>
<p>The great part about transformations on futures is that they can be chained. This allows you to express many conversions and subtasks easily.</p>
<p>Let's modify the examples from above to see how we can take advantage of chaining.</p>
<div class="codehilite"><pre><span></span><span class="c1">/// Assume we get a future string back from some API</span>
<span class="kd">let</span> <span class="nv">futureString</span><span class="p">:</span> <span class="n">Future</span><span class="p">&lt;</span><span class="nb">String</span><span class="p">&gt;</span> <span class="p">=</span> <span class="p">...</span>
<span class="c1">/// Assume we have created an HTTP client</span>
<span class="kd">let</span> <span class="nv">client</span><span class="p">:</span> <span class="n">Client</span> <span class="p">=</span> <span class="p">...</span>
<span class="c1">/// Transform the string to a url, then to a response</span>
<span class="kd">let</span> <span class="nv">futureResponse</span> <span class="p">=</span> <span class="n">futureString</span><span class="p">.</span><span class="bp">map</span><span class="p">(</span><span class="n">to</span><span class="p">:</span> <span class="n">URL</span><span class="p">.</span><span class="kc">self</span><span class="p">)</span> <span class="p">{</span> <span class="n">string</span> <span class="k">in</span>
<span class="k">guard</span> <span class="kd">let</span> <span class="nv">url</span> <span class="p">=</span> <span class="n">URL</span><span class="p">(</span><span class="n">string</span><span class="p">:</span> <span class="n">string</span><span class="p">)</span> <span class="k">else</span> <span class="p">{</span>
<span class="k">throw</span> <span class="n">Abort</span><span class="p">(.</span><span class="n">badRequest</span><span class="p">,</span> <span class="n">reason</span><span class="p">:</span> <span class="s">&quot;Invalid URL string: </span><span class="si">\(</span><span class="n">string</span><span class="si">)</span><span class="s">&quot;</span><span class="p">)</span>
<span class="p">}</span>
<span class="k">return</span> <span class="n">url</span>
<span class="p">}.</span><span class="n">flatMap</span><span class="p">(</span><span class="n">to</span><span class="p">:</span> <span class="n">Response</span><span class="p">.</span><span class="kc">self</span><span class="p">)</span> <span class="p">{</span> <span class="n">url</span> <span class="k">in</span>
<span class="k">return</span> <span class="n">client</span><span class="p">.</span><span class="kr">get</span><span class="p">(</span><span class="n">url</span><span class="p">)</span>
<span class="p">}</span>
<span class="bp">print</span><span class="p">(</span><span class="n">futureResponse</span><span class="p">)</span> <span class="c1">// Future&lt;Response&gt;</span>
</pre></div>
<p>After the initial call to map, there is a temporary <code>Future&lt;URL&gt;</code> created. This future is then immediately flat-mapped to a <code>Future&lt;Response&gt;</code></p>
<div class="admonition tip">
<p class="admonition-title">Tip</p>
<p>You can <code>throw</code> errors inside of map and flat-map closures. This will result in the future failing with the error thrown.</p>
</div>
<h2 id="worker">Worker<a class="headerlink" href="#worker" title="Permanent link">&para;</a></h2>
<p>You may see methods in Vapor that have an <code>on: Worker</code> parameter. These are usually methods that perform asynchronous work and require access to the <code>EventLoop</code>.</p>
<p>The most common <code>Worker</code>s you will interact with in Vapor are:</p>
<ul>
<li><code>Application</code></li>
<li><code>Request</code></li>
<li><code>Response</code></li>
</ul>
<div class="codehilite"><pre><span></span><span class="c1">/// Assume we have a Request and some ViewRenderer</span>
<span class="kd">let</span> <span class="nv">req</span><span class="p">:</span> <span class="n">Request</span> <span class="p">=</span> <span class="p">...</span>
<span class="kd">let</span> <span class="nv">view</span><span class="p">:</span> <span class="n">ViewRenderer</span> <span class="p">=</span> <span class="p">...</span>
<span class="c1">/// Render the view, using the Request as a worker. </span>
<span class="c1">/// This ensures the async work happens on the correct event loop.</span>
<span class="c1">///</span>
<span class="c1">/// This assumes the signature is:</span>
<span class="c1">/// func render(_: String, on: Worker)</span>
<span class="n">view</span><span class="p">.</span><span class="n">render</span><span class="p">(</span><span class="s">&quot;home.html&quot;</span><span class="p">,</span> <span class="n">on</span><span class="p">:</span> <span class="n">req</span><span class="p">)</span>
</pre></div>
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