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<section id="introduction-to-snapshot-testing-in-r" class="slide level2" style="margin-top: 1em;"><h2>Introduction to Snapshot Testing in R</h2>
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<div style="margin-top: 0.5em; margin-bottom: 0.5em; font-size: 1em">
<p>Indrajeet Patil</p>
</div>
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<p><img data-src="media/logos_combined.jpeg" class="absolute" style="width: 750px; height: 300px; "></p>
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</section><section id="unit-testing" class="slide level2 smaller"><h2>Unit testing</h2>
<p>The goal of a unit test is to capture the <em>expected</em> output of a function using <em>code</em> and making sure that <em>actual</em> output after any changes matches the expected output.</p>
<p><a href="https://testthat.r-lib.org/"><code>{testthat}</code></a> is a popular framework for writing unit tests in R.</p>
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<p><strong>Benefits of unit testing</strong></p>
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<ul>
<li>insures against unintentionally changing function behaviour</li>
<li>prevents re-introducing already fixed bugs</li>
<li>acts as the most basic form of developer-focused documentation</li>
<li>catches breaking changes coming from upstream dependencies</li>
<li>etc.</li>
</ul>
</div>
</div>
</div>
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<p><strong>Test output</strong></p>
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<p>Test pass only when actual function behaviour matches expected.</p>
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<th style="text-align: left;">actual</th>
<th style="text-align: left;">expected</th>
<th style="text-align: left;">tests</th>
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<td style="text-align: left;"><p><i class="fa-regular fa-file-lines fa-2xl" aria-label="file-lines"></i></p></td>
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</section><section id="unit-testing-with-testthat-a-recap" class="slide level2 smaller"><h2>Unit testing with <code>{testthat}</code>: A recap</h2>
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<p><strong>Test organization</strong></p>
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<p>Testing infrastructure for R package has the following hierarchy:</p>
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<br><i class="fa-regular fa-file-code fa-2xl" aria-label="file-code"></i> <strong>Test file</strong>
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<td style="text-align: left;">Tests for <code>R/foo.R</code> will typically be in <code>tests/testthat/test-foo.R</code>.</td>
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<i class="fa-solid fa-flask fa-2xl" aria-label="flask"></i> <strong>Tests</strong>
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<td style="text-align: left;">A single file can contain multiple tests.</td>
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<i class="fa-solid fa-equals fa-2xl" aria-label="equals"></i> <strong>Expectations</strong>
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<td style="text-align: left;">A single test can have multiple expectations.</td>
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<p><strong>Example test file</strong></p>
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<ul>
<li><p>Every test is a call to <code><a href="https://testthat.r-lib.org/reference/test_that.html">testthat::test_that()</a></code> function.</p></li>
<li><p>Every expectation is represented by <code>testthat::expect_*()</code> function.</p></li>
<li><p>You can generate a test file using <code>usethis::use_test()</code> function.</p></li>
</ul>
<div class="sourceCode" id="cb1"><pre class="sourceCode numberSource r number-lines code-with-copy"><code class="sourceCode r"><span id="cb1-1"><a href=""></a><span class="co"># File: tests/testthat/test-op.R</span></span>
<span id="cb1-2"><a href=""></a></span>
<span id="cb1-3"><a href=""></a><span class="co"># test-1</span></span>
<span id="cb1-4"><a href=""></a><span class="fu">test_that</span>(<span class="st">"multiplication works"</span>, {</span>
<span id="cb1-5"><a href=""></a> <span class="fu">expect_equal</span>(<span class="dv">2</span> <span class="sc">*</span> <span class="dv">2</span>, <span class="dv">4</span>) <span class="co"># expectation-1</span></span>
<span id="cb1-6"><a href=""></a> <span class="fu">expect_equal</span>(<span class="sc">-</span><span class="dv">2</span> <span class="sc">*</span> <span class="dv">2</span>, <span class="sc">-</span><span class="dv">4</span>) <span class="co"># expectation-2</span></span>
<span id="cb1-7"><a href=""></a>})</span>
<span id="cb1-8"><a href=""></a></span>
<span id="cb1-9"><a href=""></a><span class="co"># test-2</span></span>
<span id="cb1-10"><a href=""></a><span class="fu">test_that</span>(<span class="st">"addition works"</span>, {</span>
<span id="cb1-11"><a href=""></a> <span class="fu">expect_equal</span>(<span class="dv">2</span> <span class="sc">+</span> <span class="dv">2</span>, <span class="dv">4</span>) <span class="co"># expectation-1</span></span>
<span id="cb1-12"><a href=""></a> <span class="fu">expect_equal</span>(<span class="sc">-</span><span class="dv">2</span> <span class="sc">+</span> <span class="dv">2</span>, <span class="dv">0</span>) <span class="co"># expectation-2</span></span>
<span id="cb1-13"><a href=""></a>})</span>
<span id="cb1-14"><a href=""></a></span>
<span id="cb1-15"><a href=""></a>...</span></code><button title="Copy to Clipboard" class="code-copy-button"><i class="bi"></i></button></pre></div>
</div>
</div>
</div>
</div></div>
</section><section id="what-is-different-about-snapshot-testing" class="slide level2"><h2>What is different about snapshot testing?</h2>
<div class="fragment">
<p>A <strong>unit test</strong> records the code to describe expected output.</p>
<p><br></p>
<p>(actual) <i class="fa-regular fa-file-code fa-2xl" aria-label="file-code"></i> <i class="fa-solid fa-arrows-left-right fa-2xl" aria-label="arrows-left-right"></i> <i class="fa-solid fa-file-code fa-2xl" aria-label="file-code"></i> (expected)</p>
<p><br></p>
</div>
<div class="fragment">
<p>A <strong>snapshot test</strong> records expected output in a separate, human-readable file.</p>
<p><br></p>
<p>(actual) <i class="fa-regular fa-file-code fa-2xl" aria-label="file-code"></i> <i class="fa-solid fa-arrows-left-right fa-2xl" aria-label="arrows-left-right"></i> <i class="fa-solid fa-file-lines fa-2xl" aria-label="file-lines"></i> (expected)</p>
</div>
</section><section id="why-do-you-need-snapshot-testing" class="slide level2"><h2>Why do you need snapshot testing?</h2>
<p>If you develop R packages and have struggled to</p>
<div>
<ul>
<li class="fragment">test that text output <em>prints</em> as expected</li>
<li class="fragment">test that an entire file <em>is</em> as expected</li>
<li class="fragment">test that generated graphical output <em>looks</em> as expected</li>
<li class="fragment">update such tests <em>en masse</em>
</li>
</ul>
</div>
<div class="fragment">
<p>then you should be excited to know more about <em>snapshot tests</em> (aka <em>golden tests</em>)! 🤩</p>
</div>
</section><section id="prerequisites" class="title-slide slide level1 center"><h1>Prerequisites</h1>
<p>Familiarity with writing unit tests using <a href="https://testthat.r-lib.org/index.html" target="_blank"><code>{testthat}</code></a>.</p>
<p>If not, have a look at <a href="https://r-pkgs.org/testing-basics.html" target="_blank">this</a> chapter from <em>R Packages</em> book.</p>
</section><section><section id="testing-text-outputs" class="title-slide slide level1 center"><h1>Testing text outputs</h1>
<p>Snapshot tests can be used to test that text output <em>prints</em> as expected.</p>
<p>Important for testing functions that pretty-print R objects to the console, create elegant and informative exceptions, etc.</p>
</section><section id="example-function" class="slide level2 smaller"><h2>Example function</h2>
<p>Let’s say you want to write a unit test for the following function:</p>
<div class="columns">
<div class="column" style="width:60%;">
<p><strong>Source code</strong></p>
<div class="cell">
<div class="sourceCode cell-code" id="cb2"><pre class="sourceCode numberSource r number-lines code-with-copy"><code class="sourceCode r"><span id="cb2-1"><a href=""></a>print_movies <span class="ot"><-</span> <span class="cf">function</span>(keys, values) {</span>
<span id="cb2-2"><a href=""></a> <span class="fu">paste0</span>(</span>
<span id="cb2-3"><a href=""></a> <span class="st">"Movie: </span><span class="sc">\n</span><span class="st">"</span>,</span>
<span id="cb2-4"><a href=""></a> <span class="fu">paste0</span>(<span class="st">" "</span>, keys, <span class="st">": "</span>, values, <span class="at">collapse =</span> <span class="st">"</span><span class="sc">\n</span><span class="st">"</span>)</span>
<span id="cb2-5"><a href=""></a> )</span>
<span id="cb2-6"><a href=""></a>}</span></code><button title="Copy to Clipboard" class="code-copy-button"><i class="bi"></i></button></pre></div>
</div>
</div><div class="column" style="width:40%;">
<p><strong>Output</strong></p>
<div class="cell">
<div class="sourceCode cell-code" id="cb3"><pre class="sourceCode numberSource r number-lines code-with-copy"><code class="sourceCode r"><span id="cb3-1"><a href=""></a><span class="fu">cat</span>(<span class="fu">print_movies</span>(</span>
<span id="cb3-2"><a href=""></a> <span class="fu">c</span>(<span class="st">"Title"</span>, <span class="st">"Director"</span>),</span>
<span id="cb3-3"><a href=""></a> <span class="fu">c</span>(<span class="st">"Salaam Bombay!"</span>, <span class="st">"Mira Nair"</span>)</span>
<span id="cb3-4"><a href=""></a>))</span></code><button title="Copy to Clipboard" class="code-copy-button"><i class="bi"></i></button></pre></div>
<div class="cell-output cell-output-stdout">
<pre><code>Movie:
Title: Salaam Bombay!
Director: Mira Nair</code></pre>
</div>
</div>
</div></div>
<div class="fragment">
<p><br></p>
<p>Note that you want to test that the printed output <em>looks</em> as expected.</p>
<p>Therefore, you need to check for all the little bells and whistles in the printed output.</p>
</div>
</section><section id="example-test" class="slide level2 smaller"><h2>Example test</h2>
<p>Even testing this simple function is a bit painful because you need to keep track of every escape character, every space, etc.</p>
<div class="cell">
<div class="sourceCode cell-code" id="cb5"><pre class="sourceCode numberSource r number-lines code-with-copy"><code class="sourceCode r"><span id="cb5-1"><a href=""></a><span class="fu">test_that</span>(<span class="st">"`print_movies()` prints as expected"</span>, {</span>
<span id="cb5-2"><a href=""></a> <span class="fu">expect_equal</span>(</span>
<span id="cb5-3"><a href=""></a> <span class="fu">print_movies</span>(</span>
<span id="cb5-4"><a href=""></a> <span class="fu">c</span>(<span class="st">"Title"</span>, <span class="st">"Director"</span>),</span>
<span id="cb5-5"><a href=""></a> <span class="fu">c</span>(<span class="st">"Salaam Bombay!"</span>, <span class="st">"Mira Nair"</span>)</span>
<span id="cb5-6"><a href=""></a> ),</span>
<span id="cb5-7"><a href=""></a> <span class="st">"Movie: </span><span class="sc">\n</span><span class="st"> Title: Salaam Bombay!</span><span class="sc">\n</span><span class="st"> Director: Mira Nair"</span></span>
<span id="cb5-8"><a href=""></a> )</span>
<span id="cb5-9"><a href=""></a>})</span></code><button title="Copy to Clipboard" class="code-copy-button"><i class="bi"></i></button></pre></div>
<div class="cell-output cell-output-stdout">
<pre><code>Test passed 🥇</code></pre>
</div>
</div>
<div class="fragment">
<p>With a more complex code, it’d be impossible for a human to reason about what the output is supposed to look like.</p>
</div>
<div class="fragment">
<div class="callout callout-important callout-titled callout-style-default">
<div class="callout-body">
<div class="callout-title">
<div class="callout-icon-container">
<i class="callout-icon"></i>
</div>
<p><strong>Important</strong></p>
</div>
<div class="callout-content">
<p>If this is a utility function used by many other functions, changing its behaviour would entail <em>manually</em> changing expected outputs for many tests.</p>
<p>This is not maintainable! 😩</p>
</div>
</div>
</div>
</div>
</section><section id="alternative-snapshot-test" class="slide level2 smaller"><h2>Alternative: Snapshot test</h2>
<p>Instead, you can use <code>expect_snapshot()</code>, which, when run for the first time, generates a Markdown file with expected/reference output.</p>
<div class="cell">
<div class="sourceCode cell-code" id="cb7"><pre class="sourceCode numberSource r number-lines code-with-copy"><code class="sourceCode r"><span id="cb7-1"><a href=""></a><span class="fu">test_that</span>(<span class="st">"`print_movies()` prints as expected"</span>, {</span>
<span id="cb7-2"><a href=""></a> <span class="fu">expect_snapshot</span>(<span class="fu">cat</span>(<span class="fu">print_movies</span>(</span>
<span id="cb7-3"><a href=""></a> <span class="fu">c</span>(<span class="st">"Title"</span>, <span class="st">"Director"</span>),</span>
<span id="cb7-4"><a href=""></a> <span class="fu">c</span>(<span class="st">"Salaam Bombay!"</span>, <span class="st">"Mira Nair"</span>)</span>
<span id="cb7-5"><a href=""></a> )))</span>
<span id="cb7-6"><a href=""></a>})</span></code><button title="Copy to Clipboard" class="code-copy-button"><i class="bi"></i></button></pre></div>
<div class="cell-output cell-output-stdout">
<pre><code>── Warning: `print_movies()` prints as expected ────────────────────────────────
Adding new snapshot:
Code
cat(print_movies(c("Title", "Director"), c("Salaam Bombay!", "Mira Nair")))
Output
Movie:
Title: Salaam Bombay!
Director: Mira Nair</code></pre>
</div>
</div>
<div class="fragment">
<div class="callout callout-warning callout-titled callout-style-default">
<div class="callout-body">
<div class="callout-title">
<div class="callout-icon-container">
<i class="callout-icon"></i>
</div>
<p><strong>Warning</strong></p>
</div>
<div class="callout-content">
<p>The first time a snapshot is created, it becomes <em>the truth</em> against which future function behaviour will be compared.</p>
<p>Thus, it is <strong>crucial</strong> that you carefully check that the output is indeed as expected. 🔎</p>
</div>
</div>
</div>
</div>
</section><section id="human-readable-markdown-file" class="slide level2 smaller"><h2>Human-readable Markdown file</h2>
<p>Compared to your unit test code representing the expected output</p>
<div class="sourceCode" id="cb9"><pre class="sourceCode numberSource r number-lines code-with-copy"><code class="sourceCode r"><span id="cb9-1"><a href=""></a><span class="st">"Movie: </span><span class="sc">\n</span><span class="st"> Title: Salaam Bombay!</span><span class="sc">\n</span><span class="st"> Director: Mira Nair"</span></span></code><button title="Copy to Clipboard" class="code-copy-button"><i class="bi"></i></button></pre></div>
<p>notice how much more human-friendly the Markdown output is!</p>
<div class="sourceCode" id="cb10"><pre class="sourceCode numberSource md number-lines code-with-copy"><code class="sourceCode markdown"><span id="cb10-1"><a href=""></a>Code</span>
<span id="cb10-2"><a href=""></a> cat(print_movies(c("Title", "Director"), c("Salaam Bombay!", "Mira Nair")))</span>
<span id="cb10-3"><a href=""></a>Output</span>
<span id="cb10-4"><a href=""></a> Movie: </span>
<span id="cb10-5"><a href=""></a> Title: Salaam Bombay!</span>
<span id="cb10-6"><a href=""></a> Director: Mira Nair</span></code><button title="Copy to Clipboard" class="code-copy-button"><i class="bi"></i></button></pre></div>
<p>It is easy to <em>see</em> what the printed text output is <em>supposed</em> to look like. In other words, snapshot tests are useful when the <em>intent</em> of the code can only be verified by a human.</p>
<div class="fragment">
<div class="callout callout-note callout-titled callout-style-default">
<div class="callout-body">
<div class="callout-title">
<div class="callout-icon-container">
<i class="callout-icon"></i>
</div>
<p><strong>More about snapshot Markdown files</strong></p>
</div>
<div class="callout-content">
<ul>
<li><p>If test file is called <code>test-foo.R</code>, the snapshot will be saved to <code>test/testthat/_snaps/foo.md</code>.</p></li>
<li><p>If there are multiple snapshot tests in a single file, corresponding snapshots will also share the same <code>.md</code> file.</p></li>
<li><p>By default, <code>expect_snapshot()</code> will capture the code, the object values, and any side-effects.</p></li>
</ul>
</div>
</div>
</div>
</div>
</section><section id="what-test-success-looks-like" class="slide level2 smaller"><h2>What test success looks like</h2>
<p>If you run the test again, it’ll succeed:</p>
<div class="cell">
<div class="sourceCode cell-code" id="cb11"><pre class="sourceCode numberSource r number-lines code-with-copy"><code class="sourceCode r"><span id="cb11-1"><a href=""></a><span class="fu">test_that</span>(<span class="st">"`print_movies()` prints as expected"</span>, {</span>
<span id="cb11-2"><a href=""></a> <span class="fu">expect_snapshot</span>(<span class="fu">cat</span>(<span class="fu">print_movies</span>(</span>
<span id="cb11-3"><a href=""></a> <span class="fu">c</span>(<span class="st">"Title"</span>, <span class="st">"Director"</span>),</span>
<span id="cb11-4"><a href=""></a> <span class="fu">c</span>(<span class="st">"Salaam Bombay!"</span>, <span class="st">"Mira Nair"</span>)</span>
<span id="cb11-5"><a href=""></a> )))</span>
<span id="cb11-6"><a href=""></a>})</span></code><button title="Copy to Clipboard" class="code-copy-button"><i class="bi"></i></button></pre></div>
<div class="cell-output cell-output-stdout">
<pre><code>Test passed 🥳</code></pre>
</div>
</div>
<div class="fragment">
<p><br></p>
<div class="callout callout-note callout-titled callout-style-default">
<div class="callout-body">
<div class="callout-title">
<div class="callout-icon-container">
<i class="callout-icon"></i>
</div>
<p><strong>Why does my test fail on a re-run?</strong></p>
</div>
<div class="callout-content">
<p>If testing a snapshot you just generated fails on re-running the test, this is most likely because your test is not deterministic. For example, if your function deals with random number generation.</p>
<p>In such cases, setting a seed (e.g. <code>set.seed(42)</code>) should help.</p>
</div>
</div>
</div>
</div>
</section><section id="what-test-failure-looks-like" class="slide level2 smaller"><h2>What test failure looks like</h2>
<p>When function changes, snapshot doesn’t match the reference, and the test fails:</p>
<div class="columns">
<div class="column" style="width:35%;">
<p><strong>Changes to function</strong></p>
<div class="sourceCode" id="cb13" data-code-line-numbers="5"><pre class="sourceCode numberSource r number-lines code-with-copy"><code class="sourceCode r"><span id="cb13-1"><a href=""></a>print_movies <span class="ot"><-</span> <span class="cf">function</span>(keys, values) {</span>
<span id="cb13-2"><a href=""></a> <span class="fu">paste0</span>(</span>
<span id="cb13-3"><a href=""></a> <span class="st">"Movie: </span><span class="sc">\n</span><span class="st">"</span>,</span>
<span id="cb13-4"><a href=""></a> <span class="fu">paste0</span>(</span>
<span id="cb13-5"><a href=""></a> <span class="st">" "</span>, keys, <span class="st">"- "</span>, values,</span>
<span id="cb13-6"><a href=""></a> <span class="at">collapse =</span> <span class="st">"</span><span class="sc">\n</span><span class="st">"</span></span>
<span id="cb13-7"><a href=""></a> )</span>
<span id="cb13-8"><a href=""></a> )</span>
<span id="cb13-9"><a href=""></a>}</span></code><button title="Copy to Clipboard" class="code-copy-button"><i class="bi"></i></button></pre></div>
<p><br></p>
<p>Failure message provides expected (<code>-</code>) vs observed (<code>+</code>) diff.</p>
</div><div class="column" style="width:65%;">
<p><strong>Test failure</strong></p>
<div class="sourceCode" id="cb14"><pre class="sourceCode numberSource r number-lines code-with-copy"><code class="sourceCode r"><span id="cb14-1"><a href=""></a><span class="fu">test_that</span>(<span class="st">"`print_movies()` prints as expected"</span>, {</span>
<span id="cb14-2"><a href=""></a> <span class="fu">expect_snapshot</span>(<span class="fu">cat</span>(<span class="fu">print_movies</span>(</span>
<span id="cb14-3"><a href=""></a> <span class="fu">c</span>(<span class="st">"Title"</span>, <span class="st">"Director"</span>),</span>
<span id="cb14-4"><a href=""></a> <span class="fu">c</span>(<span class="st">"Salaam Bombay!"</span>, <span class="st">"Mira Nair"</span>)</span>
<span id="cb14-5"><a href=""></a> )))</span>
<span id="cb14-6"><a href=""></a>})</span></code><button title="Copy to Clipboard" class="code-copy-button"><i class="bi"></i></button></pre></div>
<div class="cell">
<div class="cell-output cell-output-stdout">
<pre><code>── Failure: `print_movies()` prints as expected ────────────────────────────────
Snapshot of code has changed:
old[2:6] vs new[2:6]
cat(print_movies(c("Title", "Director"), c("Salaam Bombay!", "Mira Nair")))
Output
Movie:
- Title: Salaam Bombay!
+ Title- Salaam Bombay!
- Director: Mira Nair
+ Director- Mira Nair
* Run `testthat::snapshot_accept('slides.qmd')` to accept the change.
* Run `testthat::snapshot_review('slides.qmd')` to interactively review the change.</code></pre>
</div>
<div class="cell-output cell-output-error">
<pre><code>Error:
! Test failed</code></pre>
</div>
</div>
</div></div>
</section><section id="fixing-tests" class="slide level2 smaller"><h2>Fixing tests</h2>
<p>Message accompanying failed tests make it explicit how to fix them.</p>
<div class="fragment">
<ul>
<li>If the change was <em>deliberate</em>, you can accept the new snapshot as the current <em>truth</em>.</li>
</ul>
<div class="sourceCode" id="cb17"><pre class="sourceCode numberSource r number-lines code-with-copy"><code class="sourceCode r"><span id="cb17-1"><a href=""></a><span class="sc">*</span> Run <span class="st">`</span><span class="at">snapshot_accept('foo.md')</span><span class="st">`</span> to accept the change</span></code><button title="Copy to Clipboard" class="code-copy-button"><i class="bi"></i></button></pre></div>
<ul>
<li>If this was <em>unexpected</em>, you can review the changes, and decide whether to change the snapshot or to correct the function behaviour instead.</li>
</ul>
<div class="sourceCode" id="cb18"><pre class="sourceCode numberSource r number-lines code-with-copy"><code class="sourceCode r"><span id="cb18-1"><a href=""></a><span class="sc">*</span> Run <span class="st">`</span><span class="at">snapshot_review('foo.md')</span><span class="st">`</span> to interactively review the change</span></code><button title="Copy to Clipboard" class="code-copy-button"><i class="bi"></i></button></pre></div>
</div>
<div class="fragment">
<p><br></p>
<div class="callout callout-tip callout-titled callout-style-default">
<div class="callout-body">
<div class="callout-title">
<div class="callout-icon-container">
<i class="callout-icon"></i>
</div>
<p><strong>Fixing multiple snapshot tests</strong></p>
</div>
<div class="callout-content">
<p>If this is a utility function used by many other functions, changing its behaviour would lead to failure of many tests.</p>
<p>You can update <em>all</em> new snapshots with <code>snapshot_accept()</code>. And, of course, check the diffs to make sure that the changes are expected.</p>
</div>
</div>
</div>
</div>
</section><section id="capturing-messages-and-warnings" class="slide level2 smaller"><h2>Capturing messages and warnings</h2>
<p>So far you have tested text output printed to the console, but you can also use snapshots to capture messages, warnings, and errors.</p>
<div class="columns">
<div class="column" style="width:50%;">
<p><strong>message</strong></p>
<div class="cell">
<div class="sourceCode cell-code" id="cb19"><pre class="sourceCode numberSource r number-lines code-with-copy"><code class="sourceCode r"><span id="cb19-1"><a href=""></a>f <span class="ot"><-</span> <span class="cf">function</span>() <span class="fu">message</span>(<span class="st">"Some info for you."</span>)</span>
<span id="cb19-2"><a href=""></a></span>
<span id="cb19-3"><a href=""></a><span class="fu">test_that</span>(<span class="st">"f() messages"</span>, {</span>
<span id="cb19-4"><a href=""></a> <span class="fu">expect_snapshot</span>(<span class="fu">f</span>())</span>
<span id="cb19-5"><a href=""></a>})</span></code><button title="Copy to Clipboard" class="code-copy-button"><i class="bi"></i></button></pre></div>
<div class="cell-output cell-output-stdout">
<pre><code>── Warning: f() messages ───────────────────────────────────────────────────────
Adding new snapshot:
Code
f()
Message
Some info for you.</code></pre>
</div>
</div>
</div><div class="column" style="width:50%;">
<p><strong>warning</strong></p>
<div class="cell">
<div class="sourceCode cell-code" id="cb21"><pre class="sourceCode numberSource r number-lines code-with-copy"><code class="sourceCode r"><span id="cb21-1"><a href=""></a>g <span class="ot"><-</span> <span class="cf">function</span>() <span class="fu">warning</span>(<span class="st">"Managed to recover."</span>)</span>
<span id="cb21-2"><a href=""></a></span>
<span id="cb21-3"><a href=""></a><span class="fu">test_that</span>(<span class="st">"g() warns"</span>, {</span>
<span id="cb21-4"><a href=""></a> <span class="fu">expect_snapshot</span>(<span class="fu">g</span>())</span>
<span id="cb21-5"><a href=""></a>})</span></code><button title="Copy to Clipboard" class="code-copy-button"><i class="bi"></i></button></pre></div>
<div class="cell-output cell-output-stdout">
<pre><code>── Warning: g() warns ──────────────────────────────────────────────────────────
Adding new snapshot:
Code
g()
Condition
Warning in `g()`:
Managed to recover.</code></pre>
</div>
</div>
</div></div>
<div class="callout callout-tip callout-titled callout-style-default">
<div class="callout-body">
<div class="callout-title">
<div class="callout-icon-container">
<i class="callout-icon"></i>
</div>
<p><strong>Tip</strong></p>
</div>
<div class="callout-content">
<p>Snapshot records both the <em>condition</em> and the corresponding <em>message</em>.</p>
<p>You can now rest assured that the users are getting informed the way you want! 😌</p>
</div>
</div>
</div>
</section><section id="capturing-errors" class="slide level2 smaller"><h2>Capturing errors</h2>
<p>In case of an error, the function <code>expect_snapshot()</code> itself will produce an error.</p>
<p>You can capture it:</p>
<div class="sourceCode" id="cb23" data-code-line-numbers="3"><pre class="sourceCode numberSource r number-lines code-with-copy"><code class="sourceCode r"><span id="cb23-1"><a href=""></a><span class="fu">test_that</span>(<span class="st">"`log()` errors"</span>, {</span>
<span id="cb23-2"><a href=""></a> <span class="fu">expect_snapshot</span>(<span class="fu">log</span>(<span class="st">"x"</span>), <span class="at">error =</span> <span class="cn">TRUE</span>)</span>
<span id="cb23-3"><a href=""></a>})</span></code><button title="Copy to Clipboard" class="code-copy-button"><i class="bi"></i></button></pre></div>
<div class="cell">
<div class="cell-output cell-output-stdout">
<pre><code>── Warning: `log()` errors ─────────────────────────────────────────────────────
Adding new snapshot:
Code
log("x")
Condition
Error in `log()`:
! non-numeric argument to mathematical function</code></pre>
</div>
</div>
</section><section id="further-reading" class="slide level2 smaller"><h2>Further reading</h2>
<ul>
<li><p><a href="https://testthat.r-lib.org">testthat</a> article on <a href="https://testthat.r-lib.org/articles/snapshotting.html" target="_blank">snapshot testing</a></p></li>
<li><p>Introduction to <a href="https://ro-che.info/articles/2017-12-04-golden-tests" target="_blank">golden testing</a></p></li>
<li><p>Docs for <a href="https://jestjs.io/docs/snapshot-testing" target="_blank">Jest</a> library in JavaScript, which inspired snapshot testing implementation in <a href="https://testthat.r-lib.org">testthat</a></p></li>
</ul></section></section><section><section id="testing-graphical-outputs" class="title-slide slide level1 center"><h1>Testing graphical outputs</h1>
<p>To create graphical expectations, you will use <a href="https://testthat.r-lib.org">testthat</a> extension package: <a href="https://vdiffr.r-lib.org/" target="_blank"><code>{vdiffr}</code></a>.</p>
</section><section id="how-does-vdiffr-work" class="slide level2 smaller"><h2>How does <code>{vdiffr}</code> work?</h2>
<p><a href="https://vdiffr.r-lib.org/">vdiffr</a> introduces <code>expect_doppelganger()</code> to generate <a href="https://testthat.r-lib.org">testthat</a> expectations for graphics. It does this by writing SVG snapshot files for outputs!</p>
<div class="fragment">
<p>The figure to test can be:</p>
<ul>
<li>a <code>ggplot</code> object (from <code><a href="https://ggplot2.tidyverse.org/reference/ggplot.html">ggplot2::ggplot()</a></code>)</li>
<li>a <code>recordedplot</code> object (from <code><a href="https://rdrr.io/r/grDevices/recordplot.html">grDevices::recordPlot()</a></code>)</li>
<li>any object with a <code><a href="https://rdrr.io/r/base/print.html">print()</a></code> method</li>
</ul>
</div>
<div class="fragment">
<div class="callout callout-note callout-titled callout-style-default">
<div class="callout-body">
<div class="callout-title">
<div class="callout-icon-container">
<i class="callout-icon"></i>
</div>
<p><strong>Note</strong></p>
</div>
<div class="callout-content">
<ul>
<li><p>If test file is called <code>test-foo.R</code>, the snapshot will be saved to <code>test/testthat/_snaps/foo</code> folder.</p></li>
<li><p>In this folder, there will be one <code>.svg</code> file for every test in <code>test-foo.R</code>.</p></li>
<li><p>The name for the <code>.svg</code> file will be sanitized version of <code>title</code> argument to <code>expect_doppelganger()</code>.</p></li>
</ul>
</div>
</div>
</div>
</div>
</section><section id="example-function-1" class="slide level2 smaller"><h2>Example function</h2>
<p>Let’s say you want to write a unit test for the following function:</p>
<div class="columns">
<div class="column" style="width:50%;">
<p><strong>Source code</strong></p>
<div class="cell">
<div class="sourceCode cell-code" id="cb25"><pre class="sourceCode numberSource r number-lines code-with-copy"><code class="sourceCode r"><span id="cb25-1"><a href=""></a><span class="fu">library</span>(ggplot2)</span>
<span id="cb25-2"><a href=""></a></span>
<span id="cb25-3"><a href=""></a>create_scatter <span class="ot"><-</span> <span class="cf">function</span>() {</span>
<span id="cb25-4"><a href=""></a> <span class="fu">ggplot</span>(mtcars, <span class="fu">aes</span>(wt, mpg)) <span class="sc">+</span></span>
<span id="cb25-5"><a href=""></a> <span class="fu">geom_point</span>(<span class="at">size =</span> <span class="dv">3</span>, <span class="at">alpha =</span> <span class="fl">0.75</span>) <span class="sc">+</span></span>
<span id="cb25-6"><a href=""></a> <span class="fu">geom_smooth</span>(<span class="at">method =</span> <span class="st">"lm"</span>)</span>
<span id="cb25-7"><a href=""></a>}</span></code><button title="Copy to Clipboard" class="code-copy-button"><i class="bi"></i></button></pre></div>
</div>
</div><div class="column" style="width:50%;">
<p><strong>Output</strong></p>
<div class="cell">
<div class="sourceCode cell-code" id="cb26"><pre class="sourceCode numberSource r number-lines code-with-copy"><code class="sourceCode r"><span id="cb26-1"><a href=""></a><span class="fu">create_scatter</span>()</span></code><button title="Copy to Clipboard" class="code-copy-button"><i class="bi"></i></button></pre></div>
<div class="cell-output-display">
<div>
<figure><p><img data-src="slides_files/figure-revealjs/unnamed-chunk-16-1.png" style="width:100.0%"></p>
</figure>
</div>
</div>
</div>
</div></div>
<div class="fragment">
<p>Note that you want to test that the graphical output <em>looks</em> as expected, and this expectation is difficult to capture with a unit test.</p>
</div>
</section><section id="graphical-snapshot-test" class="slide level2 smaller"><h2>Graphical snapshot test</h2>
<p>You can use <code>expect_doppelganger()</code> from <a href="https://vdiffr.r-lib.org/">vdiffr</a> to test this!</p>
<div class="fragment">
<p>The <em>first time</em> you run the test, it’d generate an <code>.svg</code> file with expected output.</p>
<div class="cell">
<div class="sourceCode cell-code" id="cb27"><pre class="sourceCode numberSource r number-lines code-with-copy"><code class="sourceCode r"><span id="cb27-1"><a href=""></a><span class="fu">test_that</span>(<span class="st">"`create_scatter()` plots as expected"</span>, {</span>
<span id="cb27-2"><a href=""></a> <span class="fu">expect_doppelganger</span>(</span>
<span id="cb27-3"><a href=""></a> <span class="at">title =</span> <span class="st">"create scatter"</span>,</span>
<span id="cb27-4"><a href=""></a> <span class="at">fig =</span> <span class="fu">create_scatter</span>(),</span>
<span id="cb27-5"><a href=""></a> )</span>
<span id="cb27-6"><a href=""></a>})</span></code><button title="Copy to Clipboard" class="code-copy-button"><i class="bi"></i></button></pre></div>
<div class="cell-output cell-output-stdout">
<pre><code>── Warning: `create_scatter()` plots as expected ───────────────────────────────
Adding new file snapshot: 'tests/testthat/_snaps/create-scatter.svg'</code></pre>
</div>
</div>
</div>
<div class="fragment">
<div class="callout callout-warning callout-titled callout-style-default">
<div class="callout-body">
<div class="callout-title">
<div class="callout-icon-container">
<i class="callout-icon"></i>
</div>
<p><strong>Warning</strong></p>
</div>
<div class="callout-content">
<p>The first time a snapshot is created, it becomes <em>the truth</em> against which future function behaviour will be compared.</p>
<p>Thus, it is <strong>crucial</strong> that you carefully check that the output is indeed as expected. 🔎</p>
<p>You can open <code>.svg</code> snapshot files in a web browser for closer inspection.</p>
</div>
</div>
</div>
</div>
</section><section id="what-test-success-looks-like-1" class="slide level2 smaller"><h2>What test success looks like</h2>
<p>If you run the test again, it’ll succeed:</p>
<div class="cell">
<div class="sourceCode cell-code" id="cb29"><pre class="sourceCode numberSource r number-lines code-with-copy"><code class="sourceCode r"><span id="cb29-1"><a href=""></a><span class="fu">test_that</span>(<span class="st">"`create_scatter()` plots as expected"</span>, {</span>
<span id="cb29-2"><a href=""></a> <span class="fu">expect_doppelganger</span>(</span>
<span id="cb29-3"><a href=""></a> <span class="at">title =</span> <span class="st">"create scatter"</span>,</span>
<span id="cb29-4"><a href=""></a> <span class="at">fig =</span> <span class="fu">create_scatter</span>(),</span>
<span id="cb29-5"><a href=""></a> )</span>
<span id="cb29-6"><a href=""></a>})</span></code><button title="Copy to Clipboard" class="code-copy-button"><i class="bi"></i></button></pre></div>
<div class="cell-output cell-output-stdout">
<pre><code>Test passed 🥇</code></pre>
</div>
</div>
</section><section id="what-test-failure-looks-like-1" class="slide level2 smaller"><h2>What test failure looks like</h2>
<p>When function changes, snapshot doesn’t match the reference, and the test fails:</p>
<div class="columns">
<div class="column" style="width:40%;">
<p><strong>Changes to function</strong></p>
<div class="sourceCode" id="cb31" data-code-line-numbers="3"><pre class="sourceCode numberSource r number-lines code-with-copy"><code class="sourceCode r"><span id="cb31-1"><a href=""></a>create_scatter <span class="ot"><-</span> <span class="cf">function</span>() {</span>
<span id="cb31-2"><a href=""></a> <span class="fu">ggplot</span>(mtcars, <span class="fu">aes</span>(wt, mpg)) <span class="sc">+</span></span>
<span id="cb31-3"><a href=""></a> <span class="fu">geom_point</span>(<span class="at">size =</span> <span class="dv">2</span>, <span class="at">alpha =</span> <span class="fl">0.85</span>) <span class="sc">+</span></span>
<span id="cb31-4"><a href=""></a> <span class="fu">geom_smooth</span>(<span class="at">method =</span> <span class="st">"lm"</span>)</span>
<span id="cb31-5"><a href=""></a>}</span></code><button title="Copy to Clipboard" class="code-copy-button"><i class="bi"></i></button></pre></div>
</div><div class="column" style="width:60%;">
<p><strong>Test failure</strong></p>
<!-- Currently, doesn't work properly with on GHA workflow -->
<!-- The exact error is the following: -->
<!-- Error: Can't find `tests/testthat/` in current directory -->
<div class="sourceCode" id="cb32"><pre class="sourceCode numberSource r number-lines code-with-copy"><code class="sourceCode r"><span id="cb32-1"><a href=""></a><span class="fu">test_that</span>(<span class="st">"`create_scatter()` plots as expected"</span>, {</span>
<span id="cb32-2"><a href=""></a> <span class="fu">expect_doppelganger</span>(</span>
<span id="cb32-3"><a href=""></a> <span class="at">title =</span> <span class="st">"create scatter"</span>,</span>
<span id="cb32-4"><a href=""></a> <span class="at">fig =</span> <span class="fu">create_scatter</span>(),</span>
<span id="cb32-5"><a href=""></a> )</span>
<span id="cb32-6"><a href=""></a>})</span>
<span id="cb32-7"><a href=""></a></span>
<span id="cb32-8"><a href=""></a>── <span class="fu">Failure</span> (<span class="st">'<text>:3'</span>)<span class="sc">:</span> <span class="st">`</span><span class="at">create_scatter()</span><span class="st">`</span> plots as expected ──────────────────</span>
<span id="cb32-9"><a href=""></a>Snapshot of <span class="st">`</span><span class="at">testcase</span><span class="st">`</span> to <span class="st">'slides.qmd/create-scatter.svg'</span> has changed</span>
<span id="cb32-10"><a href=""></a>Run <span class="st">`</span><span class="at">testthat::snapshot_review('slides.qmd/')</span><span class="st">`</span> to review changes</span>
<span id="cb32-11"><a href=""></a>Backtrace<span class="sc">:</span></span>
<span id="cb32-12"><a href=""></a> <span class="fl">1.</span> vdiffr<span class="sc">::</span><span class="fu">expect_doppelganger</span>(...)</span>
<span id="cb32-13"><a href=""></a> <span class="fl">3.</span> testthat<span class="sc">::</span><span class="fu">expect_snapshot_file</span>(...)</span>
<span id="cb32-14"><a href=""></a>Error <span class="cf">in</span> <span class="st">`</span><span class="at">reporter$stop_if_needed()</span><span class="st">`</span><span class="sc">:</span></span>
<span id="cb32-15"><a href=""></a><span class="sc">!</span> Test failed</span></code><button title="Copy to Clipboard" class="code-copy-button"><i class="bi"></i></button></pre></div>
</div></div>
</section><section id="fixing-tests-1" class="slide level2 smaller"><h2>Fixing tests</h2>
<p>Running <code>snapshot_review()</code> launches a Shiny app which can be used to either accept or reject the new output(s).</p>
<div class="cell">
<div class="cell-output-display">
<div>
<figure><p><video data-src="media/shiny_app_graphics.mov" style="width:60.0%" controls=""><a href="media/shiny_app_graphics.mov">Video</a></video></p>
</figure>
</div>
</div>
</div>
</section><section id="section" class="slide level2"><h2></h2>
<div class="callout callout-tip callout-titled callout-style-default">
<div class="callout-body">
<div class="callout-title">
<div class="callout-icon-container">
<i class="callout-icon"></i>
</div>
<p><strong>Why are my snapshots for plots failing?! 😔</strong></p>
</div>
<div class="callout-content">
<p>If tests fail even if the function didn’t change, it can be due to any of the following reasons:</p>
<ul>
<li>R’s graphics engine changed</li>
<li>
<a href="https://ggplot2.tidyverse.org">ggplot2</a> itself changed</li>
<li>non-deterministic behaviour</li>
<li>changes in system libraries</li>
</ul>
<p>For these reasons, snapshot tests for plots tend to be fragile and are not run on CRAN machines by default.</p>
</div>
</div>
</div>
</section><section id="further-reading-1" class="slide level2"><h2>Further reading</h2>
<ul>
<li>
<a href="https://vdiffr.r-lib.org/">vdiffr</a> package <a href="https://vdiffr.r-lib.org/" target="_blank">website</a>
</li>
</ul></section></section><section><section id="testing-entire-files" class="title-slide slide level1 center"><h1>Testing entire files</h1>
<p>Whole file snapshot testing makes sure that media, data frames, text files, etc. are as expected.</p>
</section><section id="writing-test" class="slide level2 smaller"><h2>Writing test</h2>
<p>Let’s say you want to test JSON files generated by <code><a href="https://jeroen.r-universe.dev/jsonlite/reference/read_json.html">jsonlite::write_json()</a></code>.</p>
<div class="columns">
<div class="column" style="width:50%;">
<p><strong>Test</strong></p>
<div class="cell">
<div class="sourceCode cell-code" id="cb33"><pre class="sourceCode numberSource r number-lines code-with-copy"><code class="sourceCode r"><span id="cb33-1"><a href=""></a><span class="co"># File: tests/testthat/test-write-json.R</span></span>
<span id="cb33-2"><a href=""></a><span class="fu">test_that</span>(<span class="st">"json writer works"</span>, {</span>
<span id="cb33-3"><a href=""></a> r_to_json <span class="ot"><-</span> <span class="cf">function</span>(x) {</span>
<span id="cb33-4"><a href=""></a> path <span class="ot"><-</span> <span class="fu">tempfile</span>(<span class="at">fileext =</span> <span class="st">".json"</span>)</span>
<span id="cb33-5"><a href=""></a> jsonlite<span class="sc">::</span><span class="fu">write_json</span>(x, path)</span>
<span id="cb33-6"><a href=""></a> path</span>
<span id="cb33-7"><a href=""></a> }</span>
<span id="cb33-8"><a href=""></a></span>
<span id="cb33-9"><a href=""></a> x <span class="ot"><-</span> <span class="fu">list</span>(<span class="dv">1</span>, <span class="fu">list</span>(<span class="st">"x"</span> <span class="ot">=</span> <span class="st">"a"</span>))</span>
<span id="cb33-10"><a href=""></a> <span class="fu">expect_snapshot_file</span>(<span class="fu">r_to_json</span>(x), <span class="st">"demo.json"</span>)</span>
<span id="cb33-11"><a href=""></a>})</span></code><button title="Copy to Clipboard" class="code-copy-button"><i class="bi"></i></button></pre></div>
<div class="cell-output cell-output-stdout">
<pre><code>── Warning: json writer works ──────────────────────────────────────────────────
Adding new file snapshot: 'tests/testthat/_snaps/demo.json'</code></pre>
</div>
</div>
<p><strong>Snapshot</strong></p>
<div class="sourceCode" id="cb35"><pre class="sourceCode numberSource json number-lines code-with-copy"><code class="sourceCode json"><span id="cb35-1"><a href=""></a><span class="ot">[[</span><span class="dv">1</span><span class="ot">],</span><span class="fu">{</span><span class="dt">"x"</span><span class="fu">:</span><span class="ot">[</span><span class="st">"a"</span><span class="ot">]</span><span class="fu">}</span><span class="ot">]</span></span></code><button title="Copy to Clipboard" class="code-copy-button"><i class="bi"></i></button></pre></div>
</div><div class="column" style="width:50%;">
<div class="callout callout-note callout-titled callout-style-default">
<div class="callout-body">
<div class="callout-title">
<div class="callout-icon-container">
<i class="callout-icon"></i>
</div>
<p><strong>Note</strong></p>
</div>
<div class="callout-content">
<ul>
<li><p>To snapshot a file, you need to write a helper function that provides its path.</p></li>
<li><p>If a test file is called <code>test-foo.R</code>, the snapshot will be saved to <code>test/testthat/_snaps/foo</code> folder.</p></li>
<li><p>In this folder, there will be one file (e.g. <code>.json</code>) for every <code>expect_snapshot_file()</code> expectation in <code>test-foo.R</code>.</p></li>
<li><p>The name for snapshot file is taken from <code>name</code> argument to <code>expect_snapshot_file()</code>.</p></li>
</ul>
</div>
</div>
</div>
</div></div>
</section><section id="what-test-success-looks-like-2" class="slide level2 smaller"><h2>What test success looks like</h2>
<p>If you run the test again, it’ll succeed:</p>
<div class="cell">
<div class="sourceCode cell-code" id="cb36"><pre class="sourceCode numberSource r number-lines code-with-copy"><code class="sourceCode r"><span id="cb36-1"><a href=""></a><span class="co"># File: tests/testthat/test-write-json.R</span></span>
<span id="cb36-2"><a href=""></a><span class="fu">test_that</span>(<span class="st">"json writer works"</span>, {</span>
<span id="cb36-3"><a href=""></a> r_to_json <span class="ot"><-</span> <span class="cf">function</span>(x) {</span>
<span id="cb36-4"><a href=""></a> path <span class="ot"><-</span> <span class="fu">tempfile</span>(<span class="at">fileext =</span> <span class="st">".json"</span>)</span>
<span id="cb36-5"><a href=""></a> jsonlite<span class="sc">::</span><span class="fu">write_json</span>(x, path)</span>
<span id="cb36-6"><a href=""></a> path</span>
<span id="cb36-7"><a href=""></a> }</span>
<span id="cb36-8"><a href=""></a></span>
<span id="cb36-9"><a href=""></a> x <span class="ot"><-</span> <span class="fu">list</span>(<span class="dv">1</span>, <span class="fu">list</span>(<span class="st">"x"</span> <span class="ot">=</span> <span class="st">"a"</span>))</span>
<span id="cb36-10"><a href=""></a> <span class="fu">expect_snapshot_file</span>(<span class="fu">r_to_json</span>(x), <span class="st">"demo.json"</span>)</span>
<span id="cb36-11"><a href=""></a>})</span></code><button title="Copy to Clipboard" class="code-copy-button"><i class="bi"></i></button></pre></div>
<div class="cell-output cell-output-stdout">
<pre><code>Test passed 😀</code></pre>
</div>
</div>
</section><section id="what-test-failure-looks-like-2" class="slide level2 smaller"><h2>What test failure looks like</h2>
<p>If the new output doesn’t match the expected one, the test will fail:</p>
<div class="sourceCode" id="cb38" data-code-line-numbers="11"><pre class="sourceCode numberSource r number-lines code-with-copy"><code class="sourceCode r"><span id="cb38-1"><a href=""></a><span class="co"># File: tests/testthat/test-write-json.R</span></span>
<span id="cb38-2"><a href=""></a><span class="fu">test_that</span>(<span class="st">"json writer works"</span>, {</span>
<span id="cb38-3"><a href=""></a> r_to_json <span class="ot"><-</span> <span class="cf">function</span>(x) {</span>
<span id="cb38-4"><a href=""></a> path <span class="ot"><-</span> <span class="fu">tempfile</span>(<span class="at">fileext =</span> <span class="st">".json"</span>)</span>
<span id="cb38-5"><a href=""></a> jsonlite<span class="sc">::</span><span class="fu">write_json</span>(x, path)</span>
<span id="cb38-6"><a href=""></a> path</span>
<span id="cb38-7"><a href=""></a> }</span>
<span id="cb38-8"><a href=""></a></span>
<span id="cb38-9"><a href=""></a> x <span class="ot"><-</span> <span class="fu">list</span>(<span class="dv">1</span>, <span class="fu">list</span>(<span class="st">"x"</span> <span class="ot">=</span> <span class="st">"b"</span>))</span>
<span id="cb38-10"><a href=""></a> <span class="fu">expect_snapshot_file</span>(<span class="fu">r_to_json</span>(x), <span class="st">"demo.json"</span>)</span>
<span id="cb38-11"><a href=""></a>})</span></code><button title="Copy to Clipboard" class="code-copy-button"><i class="bi"></i></button></pre></div>
<div class="cell">
<div class="cell-output cell-output-stdout">
<pre><code>── Failure: json writer works ──────────────────────────────────────────────────
Snapshot of `r_to_json(x)` to 'slides.qmd/demo.json' has changed
Run `testthat::snapshot_review('slides.qmd/')` to review changes</code></pre>
</div>
<div class="cell-output cell-output-error">
<pre><code>Error:
! Test failed</code></pre>
</div>
</div>
</section><section id="fixing-tests-2" class="slide level2 smaller"><h2>Fixing tests</h2>
<p>Running <code>snapshot_review()</code> launches a Shiny app which can be used to either accept or reject the new output(s).</p>
<img data-src="media/json_snapshot.png" width="622" class="r-stretch"></section><section id="further-reading-2" class="slide level2 smaller"><h2>Further reading</h2>
<p>Documentation for <a href="https://testthat.r-lib.org/reference/expect_snapshot_file.html" target="_blank"><code>expect_snapshot_file()</code></a></p>
</section></section><section><section id="testing-shiny-applications" class="title-slide slide level1 center"><h1>Testing Shiny applications</h1>
<p>To write formal tests for Shiny applications, you will use <a href="https://testthat.r-lib.org">testthat</a> extension package: <a href="https://rstudio.github.io/shinytest2/" target="_blank"><code>{shinytest2}</code></a>.</p>
</section><section id="how-does-shinytest2-work" class="slide level2 smaller"><h2>How does <code>{shinytest2}</code> work?</h2>
<p><a href="https://rstudio.github.io/shinytest2/">shinytest2</a> uses a Shiny app (how meta! 😅) to record user interactions with the app and generate snapshots of the application’s state. Future behaviour of the app will be compared against these snapshots to check for any changes.</p>
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<p>Exactly how tests for Shiny apps in R package are written depends on how the app is stored. There are two possibilities, and you will discuss them both separately.</p>
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<p><strong>Stored in <code>/inst</code> folder</strong></p>
<pre><code>├── DESCRIPTION
├── R
├── inst
│ └── sample_app
│ └── app.R</code></pre>
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<p><strong>Returned by a function</strong></p>
<pre><code>├── DESCRIPTION
├── R
│ └── app-function.R</code></pre>
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</section></section><section><section id="shiny-app-in-subdirectory" class="title-slide slide level1 center"><h1>Shiny app in subdirectory</h1>
<p><br></p>
<pre><code>├── DESCRIPTION
├── R
├── inst
│ └── sample_app
│ └── app.R</code></pre>
</section><section id="example-app" class="slide level2 smaller"><h2>Example app</h2>
<p>Let’s say this app resides in the <code>inst/unitConverter/app.R</code> file.</p>
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