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Lecture 6 - CS50's Web Programming with Python and JavaScript.htm
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<!DOCTYPE html>
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<meta name="viewport" content="initial-scale=1, width=device-width"><meta property="og:description" content="This course picks up where Harvard University's CS50 leaves off, diving more deeply into the design and implementation of web apps with Python, JavaScript, and SQL using frameworks like Django, React, and Bootstrap. Topics include database design, scalability, security, and user experience. Through hands-on projects, students learn to write and use APIs, create interactive UIs, and leverage cloud services like GitHub and Heroku. By semester’s end, students emerge with knowledge and experience in principles, languages, and tools that empower them to design and deploy applications on the Internet.">
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<li><a href="#infinite-scroll" id="markdown-toc-infinite-scroll">Infinite Scroll</a></li>
</ul>
</li>
<li><a href="#animation" id="markdown-toc-animation">Animation</a></li>
<li><a href="#react" id="markdown-toc-react">React</a> <ul>
<li><a href="#addition" id="markdown-toc-addition">Addition</a></li>
</ul>
</li>
</ul>
<a data-id="" id="introduction" style="top: -81.3333px;"></a><h2><a data-id="" href="#introduction">Introduction</a></h2>
<ul class="fa-ul">
<li data-marker="*"><span class="fa-li"><i class="fas fa-square"></i></span>So
far, we’ve discussed how to build simple web pages using HTML and CSS,
and how to use Git and GitHub in order to keep track of changes to our
code and collaborate with others. We also familiarized ourselves with
the Python programming language, started using Django to create web
applications, and learned how to use Django models to store information
in our sites. We then introduced JavaScript and learned how to use it to
make web pages more interactive.</li>
<li data-marker="*"><span class="fa-li"><i class="fas fa-square"></i></span>Today,
we’ll discuss common paradigms in User Interface design, using
JavaScript and CSS to make our sites even more user friendly.</li>
</ul>
<a data-id="" id="user-interfaces" style="top: -81.3333px;"></a><h2><a data-id="" href="#user-interfaces">User Interfaces</a></h2>
<p>A User Interface is how visitors to a web page interact with that
page. Our goal as web developers is to make these interactions as
pleasant as possible for the user, and there are many methods we can use
to do this.</p>
<a data-id="" id="single-page-applications" style="top: -81.3333px;"></a><h2><a data-id="" href="#single-page-applications">Single Page Applications</a></h2>
<p>Previously, if we wanted a website with multiple pages, we would
accomplish that using different routes in our Django application. Now,
we have the ability to load just a single page and then use JavaScript
to manipulate the DOM. One major advantage of doing this is that we only
need to modify the part of the page that is actually changing. For
example, if we have a Nav Bar that doesn’t change based on your current
page, we wouldn’t want to have to re-render that Nav Bar every time we
switch to a new part of the page.</p>
<p>Let’s look at an example of how we could simulate page switching in JavaScript:</p>
<div class="language-html highlighter-rouge"><div class="highlight"><pre class="highlight"><code><span class="cp"><!DOCTYPE html></span>
<span class="nt"><html</span> <span class="na">lang=</span><span class="s">"en"</span><span class="nt">></span>
<span class="nt"><head></span>
<span class="nt"><title></span>Single Page<span class="nt"></title></span>
<span class="nt"><style></span>
<span class="nt">div</span> <span class="p">{</span>
<span class="nl">display</span><span class="p">:</span> <span class="nb">none</span><span class="p">;</span>
<span class="p">}</span>
<span class="nt"></style></span>
<span class="nt"><script </span><span class="na">src=</span><span class="s">"singlepage.js"</span><span class="nt">></script></span>
<span class="nt"></head></span>
<span class="nt"><body></span>
<span class="nt"><button</span> <span class="na">data-page=</span><span class="s">"page1"</span><span class="nt">></span>Page 1<span class="nt"></button></span>
<span class="nt"><button</span> <span class="na">data-page=</span><span class="s">"page2"</span><span class="nt">></span>Page 2<span class="nt"></button></span>
<span class="nt"><button</span> <span class="na">data-page=</span><span class="s">"page3"</span><span class="nt">></span>Page 3<span class="nt"></button></span>
<span class="nt"><div</span> <span class="na">id=</span><span class="s">"page1"</span><span class="nt">></span>
<span class="nt"><h1></span>This is page 1<span class="nt"></h1></span>
<span class="nt"></div></span>
<span class="nt"><div</span> <span class="na">id=</span><span class="s">"page2"</span><span class="nt">></span>
<span class="nt"><h1></span>This is page 2<span class="nt"></h1></span>
<span class="nt"></div></span>
<span class="nt"><div</span> <span class="na">id=</span><span class="s">"page3"</span><span class="nt">></span>
<span class="nt"><h1></span>This is page 3<span class="nt"></h1></span>
<span class="nt"></div></span>
<span class="nt"></body></span>
<span class="nt"></html></span>
</code></pre></div></div>
<p>Notice in the HTML above that we have three buttons and three divs.
At the moment, the divs contain only a small bit of text, but we could
imagine each div containing the contents of one page on our site. Now,
we’ll add some JavaScript that allows us to use the buttons to toggle
between pages.</p>
<div class="language-js highlighter-rouge"><div class="highlight"><pre class="highlight"><code><span class="c1">// Shows one page and hides the other two</span>
<span class="kd">function</span> <span class="nx">showPage</span><span class="p">(</span><span class="nx">page</span><span class="p">)</span> <span class="p">{</span>
<span class="c1">// Hide all of the divs:</span>
<span class="nb">document</span><span class="p">.</span><span class="nx">querySelectorAll</span><span class="p">(</span><span class="dl">'</span><span class="s1">div</span><span class="dl">'</span><span class="p">).</span><span class="nx">forEach</span><span class="p">(</span><span class="nx">div</span> <span class="o">=></span> <span class="p">{</span>
<span class="nx">div</span><span class="p">.</span><span class="nx">style</span><span class="p">.</span><span class="nx">display</span> <span class="o">=</span> <span class="dl">'</span><span class="s1">none</span><span class="dl">'</span><span class="p">;</span>
<span class="p">});</span>
<span class="c1">// Show the div provided in the argument</span>
<span class="nb">document</span><span class="p">.</span><span class="nx">querySelector</span><span class="p">(</span><span class="s2">`#</span><span class="p">${</span><span class="nx">page</span><span class="p">}</span><span class="s2">`</span><span class="p">).</span><span class="nx">style</span><span class="p">.</span><span class="nx">display</span> <span class="o">=</span> <span class="dl">'</span><span class="s1">block</span><span class="dl">'</span><span class="p">;</span>
<span class="p">}</span>
<span class="c1">// Wait for page to loaded:</span>
<span class="nb">document</span><span class="p">.</span><span class="nx">addEventListener</span><span class="p">(</span><span class="dl">'</span><span class="s1">DOMContentLoaded</span><span class="dl">'</span><span class="p">,</span> <span class="kd">function</span><span class="p">()</span> <span class="p">{</span>
<span class="c1">// Select all buttons</span>
<span class="nb">document</span><span class="p">.</span><span class="nx">querySelectorAll</span><span class="p">(</span><span class="dl">'</span><span class="s1">button</span><span class="dl">'</span><span class="p">).</span><span class="nx">forEach</span><span class="p">(</span><span class="nx">button</span> <span class="o">=></span> <span class="p">{</span>
<span class="c1">// When a button is clicked, switch to that page</span>
<span class="nx">button</span><span class="p">.</span><span class="nx">onclick</span> <span class="o">=</span> <span class="kd">function</span><span class="p">()</span> <span class="p">{</span>
<span class="nx">showPage</span><span class="p">(</span><span class="k">this</span><span class="p">.</span><span class="nx">dataset</span><span class="p">.</span><span class="nx">page</span><span class="p">);</span>
<span class="p">}</span>
<span class="p">})</span>
<span class="p">});</span>
</code></pre></div></div>
<p><img src="Lecture%206%20-%20CS50's%20Web%20Programming%20with%20Python%20and%20JavaScript_files/singlepage1.gif" alt="single page 1"></p>
<p>In many cases, it will be inefficient to load the entire contents of
every page when we first visit a site, so we will need to use a server
to access new data. For example, when you visit a news site, it would
take far too long for the site to load if it had to load every single
article it has available when you first visit the page. We can avoid
this problem using a strategy similar to the one we used while loading
currency exchange rates in the previous lecture. This time, we’ll take a
look at using Django to send and receive information from our single
page application. To show how this works, let’s take a look at a simple
Django application. It has two URL patterns in <code class="language-plaintext highlighter-rouge">urls.py</code>:</p>
<div class="language-python highlighter-rouge"><div class="highlight"><pre class="highlight"><code><span class="n">urlpatterns</span> <span class="o">=</span> <span class="p">[</span>
<span class="n">path</span><span class="p">(</span><span class="s">""</span><span class="p">,</span> <span class="n">views</span><span class="p">.</span><span class="n">index</span><span class="p">,</span> <span class="n">name</span><span class="o">=</span><span class="s">"index"</span><span class="p">),</span>
<span class="n">path</span><span class="p">(</span><span class="s">"sections/<int:num>"</span><span class="p">,</span> <span class="n">views</span><span class="p">.</span><span class="n">section</span><span class="p">,</span> <span class="n">name</span><span class="o">=</span><span class="s">"section"</span><span class="p">)</span>
<span class="p">]</span>
</code></pre></div></div>
<p>And two corresponding routes in <code class="language-plaintext highlighter-rouge">views.py</code>. Notice that the <code class="language-plaintext highlighter-rouge">section</code> route takes in an integer, and then returns a string of text based on that integer as an HTTP Response.</p>
<div class="language-python highlighter-rouge"><div class="highlight"><pre class="highlight"><code><span class="kn">from</span> <span class="nn">django.http</span> <span class="kn">import</span> <span class="n">Http404</span><span class="p">,</span> <span class="n">HttpResponse</span>
<span class="kn">from</span> <span class="nn">django.shortcuts</span> <span class="kn">import</span> <span class="n">render</span>
<span class="c1"># Create your views here.
</span><span class="k">def</span> <span class="nf">index</span><span class="p">(</span><span class="n">request</span><span class="p">):</span>
<span class="k">return</span> <span class="n">render</span><span class="p">(</span><span class="n">request</span><span class="p">,</span> <span class="s">"singlepage/index.html"</span><span class="p">)</span>
<span class="c1"># The texts are much longer in reality, but have
# been shortened here to save space
</span><span class="n">texts</span> <span class="o">=</span> <span class="p">[</span><span class="s">"Text 1"</span><span class="p">,</span> <span class="s">"Text 2"</span><span class="p">,</span> <span class="s">"Text 3"</span><span class="p">]</span>
<span class="k">def</span> <span class="nf">section</span><span class="p">(</span><span class="n">request</span><span class="p">,</span> <span class="n">num</span><span class="p">):</span>
<span class="k">if</span> <span class="mi">1</span> <span class="o"><=</span> <span class="n">num</span> <span class="o"><=</span> <span class="mi">3</span><span class="p">:</span>
<span class="k">return</span> <span class="n">HttpResponse</span><span class="p">(</span><span class="n">texts</span><span class="p">[</span><span class="n">num</span> <span class="o">-</span> <span class="mi">1</span><span class="p">])</span>
<span class="k">else</span><span class="p">:</span>
<span class="k">raise</span> <span class="n">Http404</span><span class="p">(</span><span class="s">"No such section"</span><span class="p">)</span>
</code></pre></div></div>
<p>Now, within our <code class="language-plaintext highlighter-rouge">index.html</code>
file, we’ll take advantage of AJAX, which we learned about last
lecture, to make a request to the server to gain the text of a
particular section and display it on the screen:</p>
<div class="language-html highlighter-rouge"><div class="highlight"><pre class="highlight"><code><span class="cp"><!DOCTYPE html></span>
<span class="nt"><html</span> <span class="na">lang=</span><span class="s">"en"</span><span class="nt">></span>
<span class="nt"><head></span>
<span class="nt"><title></span>Single Page<span class="nt"></title></span>
<span class="nt"><style></span>
<span class="nt"></style></span>
<span class="nt"><script></span>
<span class="c1">// Shows given section</span>
<span class="kd">function</span> <span class="nx">showSection</span><span class="p">(</span><span class="nx">section</span><span class="p">)</span> <span class="p">{</span>
<span class="c1">// Find section text from server</span>
<span class="nx">fetch</span><span class="p">(</span><span class="s2">`/sections/</span><span class="p">${</span><span class="nx">section</span><span class="p">}</span><span class="s2">`</span><span class="p">)</span>
<span class="p">.</span><span class="nx">then</span><span class="p">(</span><span class="nx">response</span> <span class="o">=></span> <span class="nx">response</span><span class="p">.</span><span class="nx">text</span><span class="p">())</span>
<span class="p">.</span><span class="nx">then</span><span class="p">(</span><span class="nx">text</span> <span class="o">=></span> <span class="p">{</span>
<span class="c1">// Log text and display on page</span>
<span class="nx">console</span><span class="p">.</span><span class="nx">log</span><span class="p">(</span><span class="nx">text</span><span class="p">);</span>
<span class="nb">document</span><span class="p">.</span><span class="nx">querySelector</span><span class="p">(</span><span class="dl">'</span><span class="s1">#content</span><span class="dl">'</span><span class="p">).</span><span class="nx">innerHTML</span> <span class="o">=</span> <span class="nx">text</span><span class="p">;</span>
<span class="p">});</span>
<span class="p">}</span>
<span class="nb">document</span><span class="p">.</span><span class="nx">addEventListener</span><span class="p">(</span><span class="dl">'</span><span class="s1">DOMContentLoaded</span><span class="dl">'</span><span class="p">,</span> <span class="kd">function</span><span class="p">()</span> <span class="p">{</span>
<span class="c1">// Add button functionality</span>
<span class="nb">document</span><span class="p">.</span><span class="nx">querySelectorAll</span><span class="p">(</span><span class="dl">'</span><span class="s1">button</span><span class="dl">'</span><span class="p">).</span><span class="nx">forEach</span><span class="p">(</span><span class="nx">button</span> <span class="o">=></span> <span class="p">{</span>
<span class="nx">button</span><span class="p">.</span><span class="nx">onclick</span> <span class="o">=</span> <span class="kd">function</span><span class="p">()</span> <span class="p">{</span>
<span class="nx">showSection</span><span class="p">(</span><span class="k">this</span><span class="p">.</span><span class="nx">dataset</span><span class="p">.</span><span class="nx">section</span><span class="p">);</span>
<span class="p">};</span>
<span class="p">});</span>
<span class="p">});</span>
<span class="nt"></script></span>
<span class="nt"></head></span>
<span class="nt"><body></span>
<span class="nt"><h1></span>Hello!<span class="nt"></h1></span>
<span class="nt"><button</span> <span class="na">data-section=</span><span class="s">"1"</span><span class="nt">></span>Section 1<span class="nt"></button></span>
<span class="nt"><button</span> <span class="na">data-section=</span><span class="s">"2"</span><span class="nt">></span>Section 2<span class="nt"></button></span>
<span class="nt"><button</span> <span class="na">data-section=</span><span class="s">"3"</span><span class="nt">></span>Section 3<span class="nt"></button></span>
<span class="nt"><div</span> <span class="na">id=</span><span class="s">"content"</span><span class="nt">></span>
<span class="nt"></div></span>
<span class="nt"></body></span>
<span class="nt"></html></span>
</code></pre></div></div>
<p><img src="Lecture%206%20-%20CS50's%20Web%20Programming%20with%20Python%20and%20JavaScript_files/singlepage2.gif" alt="Single page 2"></p>
<p>Now, we’ve created a site where we can load new data from a server without reloading our entire HTML page!</p>
<p>One disadvantage of our site though is that the URL is now less
informative. You’ll notice in the video above that the URL remains the
same even when we switch from section to section. We can solve this
problem using the <a href="https://developer.mozilla.org/en-US/docs/Web/API/History_API">JavaScript History API</a>.
This API allows us to push information to our browser history and
update the URL manually. Let’s take a look at how we can use this API.
Imagine we have a Django project identical to the previous one, but this
time we wish to alter our script to be employ the history API:</p>
<div class="language-js highlighter-rouge"><div class="highlight"><pre class="highlight"><code><span class="c1">// When back arrow is clicked, show previous section</span>
<span class="nb">window</span><span class="p">.</span><span class="nx">onpopstate</span> <span class="o">=</span> <span class="kd">function</span><span class="p">(</span><span class="nx">event</span><span class="p">)</span> <span class="p">{</span>
<span class="nx">console</span><span class="p">.</span><span class="nx">log</span><span class="p">(</span><span class="nx">event</span><span class="p">.</span><span class="nx">state</span><span class="p">.</span><span class="nx">section</span><span class="p">);</span>
<span class="nx">showSection</span><span class="p">(</span><span class="nx">event</span><span class="p">.</span><span class="nx">state</span><span class="p">.</span><span class="nx">section</span><span class="p">);</span>
<span class="p">}</span>
<span class="kd">function</span> <span class="nx">showSection</span><span class="p">(</span><span class="nx">section</span><span class="p">)</span> <span class="p">{</span>
<span class="nx">fetch</span><span class="p">(</span><span class="s2">`/sections/</span><span class="p">${</span><span class="nx">section</span><span class="p">}</span><span class="s2">`</span><span class="p">)</span>
<span class="p">.</span><span class="nx">then</span><span class="p">(</span><span class="nx">response</span> <span class="o">=></span> <span class="nx">response</span><span class="p">.</span><span class="nx">text</span><span class="p">())</span>
<span class="p">.</span><span class="nx">then</span><span class="p">(</span><span class="nx">text</span> <span class="o">=></span> <span class="p">{</span>
<span class="nx">console</span><span class="p">.</span><span class="nx">log</span><span class="p">(</span><span class="nx">text</span><span class="p">);</span>
<span class="nb">document</span><span class="p">.</span><span class="nx">querySelector</span><span class="p">(</span><span class="dl">'</span><span class="s1">#content</span><span class="dl">'</span><span class="p">).</span><span class="nx">innerHTML</span> <span class="o">=</span> <span class="nx">text</span><span class="p">;</span>
<span class="p">});</span>
<span class="p">}</span>