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<!doctype html>
<html lang="en">
<head>
<meta charset="utf-8">
<title>JavaScript - Asynchronous Programming</title>
<meta name="description" content="JavaScript - Asynchronous Programming. Promise API in AngularJS.">
<meta name="author" content="Marek Matczak">
<meta name="apple-mobile-web-app-capable" content="yes" />
<meta name="apple-mobile-web-app-status-bar-style" content="black-translucent" />
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<body>
<div class="reveal">
<!-- Any section element inside of this container is displayed as a slide -->
<div class="slides">
<section>
<p class="title_image"><img height="200" src="img/256px-JavaScript-logo.png">
</p>
<h2 class="highlight">Asynchronous programming model</h2>
<h3 class="highlight">Promise API in AngularJS</h3>
<p>Marek Matczak</p>
</section>
<section>
<div id="js-engine">
<p>JavaScript Engine (simplified)</p>
<pre><code data-trim class="javascript">
var bar = function () {
console.log('bar...');
},
foo = function () {
setTimeout(bar, 5000);
console.log('foo...');
};
foo();
</code></pre>
<div id="js-engine-core">
<div id="call-stack">
<p>Call Stack</p>
</div>
<div id="event-table">
<p>Event Table</p>
</div>
<div id="event-queue">
<p>Event Queue</p>
</div>
<div id="console">
<p>Console</p>
<!--<pre class="code-element"><code data-trim class="javascript">"I'm second!"</code></pre>-->
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<a href="javascript:jsEngineSteps.backward()"><<</a>
<a href="javascript:jsEngineSteps.forward()">>></a>
</div>
</section>
<section>
<h3 class="highlight">Asynchronous programming</h3>
<ul>
<li>Browsers are full of asynchronous events: <span class="highlight">XHR responses, DOM events, timeouts</span>, etc.
</li>
<li>Blocking operations are implemented using callbacks.
</li>
<li>Synchronous vs. asynchronous model:
<pre><code data-trim class="javascript">
// synchronous style
var currentOrder = orders.get(1234);
console.log(currentOrder.referenceNumber);
// asynchronous style
orders.get(1234,
function (currentOrder) { // anonymous callback function
console.log(currentOrder.referenceNumber);
});
</code></pre>
</li>
</ul>
</section>
<section>
<h3 class="highlight">Challenges:</h3>
<ul>
<li>synchronizing multiple asynchronous events</li>
<li>exception handling</li>
</ul>
<pre><code data-trim class="javascript">
var orderId = 1234,
orderDetails;
try {
orders.get(orderId, function (currentOrder) {
orderDetails.order = currentOrder;
orders.getPositions(orderId, function (currentPositions) {
orderDetails.positions = currentPositions;
// open a dialog and show the order details
});
});
} catch (err) {
// this catches errors occurred while calling orders.get()
// but not while executing its callback!
}
</code></pre>
</section>
<section>
<h3 class="highlight">Promise / Deferred API</h3>
<p class="justify_text">Promise is an <span class="highlight">interface for interacting </span>with an object that represents the <span class="highlight">result of an action</span> that is performed asynchronously, and <span class="highlight">may or may not be finished</span> at any given point in time</p>
<div class="margin_top_30">
<p class="align_right"><small><a href="http://wiki.commonjs.org/wiki/Promises">The CommonJS Promise proposal</a></small>
</p>
</div>
<p class="justify_text">From the perspective of dealing with <span class="highlight">error handling</span>, deferred and promise APIs are to asynchronous programming what <span class="highlight"><code>try</code>, <code>catch</code> and <code>throw</code> keywords are to synchronous programming</span>
</p>
<div class="margin_top_30">
<p class="align_right"><small><a href="https://docs.angularjs.org/api/ng/service/$q">AngularJS Documentation</a></small>
</p>
</div>
</section>
<section>
<h3 class="highlight">Promise API</h3>
<pre><code data-trim class="javascript">
var promise = tables.get(1234);
promise.then(
function (table) {
console.log(table.id);
}, function (reason) {
console.log('Could not get a table because of: ' + reason);
}, function (progress) {
console.log(progress + '/100 %');
});
</code></pre>
</section>
<section>
<h3 class="highlight">Deferred API</h3>
<pre><code data-trim class="javascript">
var tables = function () {
return {
get: function (id) {
var deferred = $q.defer(); // represents a deferred action
$timeout(function () {
deferred.notify(50); // notify about progress
$timeout(function () {
deferred.notify(100); // notify about progress
deferred.resolve({id: 1}); // return a table with id = 1
}, 1000);
}, 1000);
return deferred.promise; // return a promise for the caller
}
}
}();
</code></pre>
</section>
<section>
<h3 class="highlight">Chaining promises (success callbacks)</h3>
<pre><code data-trim class="javascript">
$q.when('Pizza with') // wraps a value in a promise and resolves it
.then(function (pizza) {
return pizza + ' cheese,';
})
.then(function (pizza) {
return pizza + ' onion,'
})
.then(function (pizza) {
return pizza + ' sausage'
})
.then(function (pizza) {
$log.info(pizza); // Pizza with cheese, onion, sausage
});
</code></pre>
</section>
<section>
<h3 class="highlight">Chaining promises (failure callbacks)</h3>
<pre><code data-trim class="javascript">
$q.reject('Out of cheese...') // wraps a value in a promise and rejects it
.catch(function (reason) {
return $q.reject(reason + ' Oh no!'); // failure CAN'T be handled
})
.catch(function (reason) {
$log.error('Easy, I bought some cheese'); // failure CAN be handled
return 'Pizza with cheese';
})
.then(function (pizza) {
$log.info(pizza); // Pizza with cheese
}, function (reason) {
$log.info('Hi!'); // unreachable - has already been handled
});
</code></pre>
</section>
<section>
<h3 class="highlight">Chaining vs. Aggregation</h3>
<pre><code data-trim class="javascript">
var promise =
$q.when('Pizza with cheese'); // wraps a value in a promise and rejects it
promise.then(function (pizza) {
$log.info('Let us add something to ' + pizza); // 1
return pizza + ' and sausage';
}).then(function (pizza) {
$log.info('Andrew is eating ' + pizza); // 3
});
promise.then(function (pizza) {
$log.info('John is eating ' + pizza); // 2
});
</code></pre>
</section>
<section>
<h3 class="highlight">Returning a promise from a promise callback</h3>
<pre><code data-trim class="javascript">
$q.when('Pizza with')
.then(function (pizza) {
return pizza + ' cheese,';
})
.then(function (pizza) {
return $q.when(pizza + ' (add-ons')
.then(function (addOn) {
return addOn + ' mushrooms,';
})
.then(function (addOn) {
return addOn + ' oregano)';
});
})
.then(function (pizza) {
$log.info(pizza); // Pizza with cheese, (add-ons mushrooms, oregano)
});
</code></pre>
</section>
</div>
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