Updated Chrome Apps codelab from I/O 2013

chrome/common/extensions/docs/templates/articles/app_codelab_filesystem.html

+<h1 id="export-to-filesystem">
+  <span class="h1-step">Step 6:</span>
+  Export Todos to the Filesystem
+</h1>
+
+<p class="note">
+  <strong>Want to start fresh from here?</strong>
+  Find the previous step's code in the <a href="https://github.com/mangini/io13-codelab/archive/master.zip">reference code zip</a> under <strong><em>cheat_code > solution_for_step5</strong></em>.
+</p>
+
+<p>In this step, you will learn:</p>
+
+<ul>
+  <li>How to get a reference to a file in the external filesystem.</li>
+  <li>How to write to the filesystem.</li>
+</ul>
+
+<p>
+  <em>Estimated time to complete this step: 20 minutes.</em>
+  <br>
+  To preview what you will complete in this step, <a href="#launch">jump down to the bottom of this page &#8595;</a>.
+</p>
+
+<h2 id="export-todos">Export todos</h2>
+
+<p>In this step, we will add an export button to the app. When clicked, the current 
+todo items will be saved to a text file selected by the user. If the file 
+exists, it will be replaced. Otherwise, a new file will be created.</p>
+
+<h3 id="update-permissions">Update permissions</h3>
+
+<p>File system permissions can be requested as a string for read-only access, or an Object with additional properties. For example:</p>
+
+<pre>
+// Read only
+"permissions": [<b>"fileSystem"</b>]
+
+// Read and write
+"permissions": [<b>{"fileSystem": ["write"]}</b>]
+
+// Read, write, autocomplate previous input, and select folder directories instead of files
+"permissions": [<b>{"fileSystem": ["write", "retainEntries", "directory"]}</b>] 
+</pre>
+
+<p>For our purposes, we need read and write access. In <strong><em>manifest.json</em></strong>, request the <code>{fileSystem: [ "write" ] }</code>  permission:</p>
+
+<pre data-filename="manifest.json">
+"permissions": ["storage", "alarms", "notifications", "webview",
+                 "&lt;all_urls&gt;"<b>, { "fileSystem": ["write"] }</b> ],
+</pre>
+
+
+<h3 id="update-html-view">Update HTML view</h3>
+
+<p>In <strong><em>index.html</em></strong>, add an <b>Export to disk</b> button and 
+a <code>div</code> where we will show a status message:</p>
+
+<pre data-filename="index.html">
+&lt;footer id="info"&gt;
+  &lt;button id="toggleAlarm"&gt;Activate alarm&lt;/button&gt;
+  <b>&lt;button id="exportToDisk"&gt;Export to disk&lt;/button&gt;</b>
+  <b>&lt;div id="status"&gt;&lt;/div&gt;</b>
+  ...
+&lt;/footer&gt;
+</pre>
+
+<p>Also in <em>index.html</em>, load the <em>export.js</em> script we will create next:</p>
+
+<pre data-filename="index.html">
+...
+&lt;script src="js/alarms.js"&gt;&lt;/script&gt;
+<b>&lt;script src="js/export.js"&gt;&lt;/script&gt;</b>
+</pre>
+
+<h3 id="create-js">Create export script</h3>
+
+<p>Create a new JavaScript file named <strong><em>export.js</em></strong> using the code below. Save it in the <strong><em>js</em></strong>
+   folder.</p>
+
+<pre data-filename="export.js">
+(function() {
+
+  var dbName = 'todos-vanillajs';
+
+  var savedFileEntry, fileDisplayPath;
+
+  function getTodosAsText(callback) {
+  }
+
+  function exportToFileEntry(fileEntry) {
+  }
+
+  function doExportToDisk() {
+  }
+
+  document.getElementById('exportToDisk').addEventListener('click', doExportToDisk);
+
+})();
+</pre>
+
+<p>Right now, <em>export.js</em> only contains a click listener on the <strong>Export to disk</strong> button and stubs for <code>getTodosAsText()</code>, <code>exportToFileEntry</code>, and <code>doExportToDisk()</code>.</p>
+
+<h3 id="get-todos-as-text">Get todo items as text</h3>
+
+<p>Update <code>getTodosAsText()</code> so that it reads todos from <code>chrome.storage.local</code> and generates a textual representation of them:</p>
+
+<pre data-filename="export.js">
+function getTodosAsText(callback) {
+<b>  chrome.storage.local.get(dbName, function(storedData) {
+    var text = '';
+
+    if ( storedData[dbName].todos ) {
+      storedData[dbName].todos.forEach(function(todo) {
+          text += '- ';
+          if ( todo.completed ) {
+            text += '[DONE] ';
+          }
+          text += todo.title;
+          text += '\n';
+        }, '');
+    }
+
+    callback(text);
+
+  }.bind(this));</b>
+}
+</pre>
+
+<h3 id="choose-file">Choose a file</h3>
+
+<p>Update <code>doExportToDisk()</code> with <code><a href="/apps/fileSystem#method-chooseEntry">chrome.fileSystem.chooseEntry()</a></code> to allow the user to choose a file:</p>
+
+<pre data-filename="export.js">
+function doExportToDisk() {
+
+<b>  if (savedFileEntry) {
+
+    exportToFileEntry(savedFileEntry);
+
+  } else {
+
+    chrome.fileSystem.chooseEntry( {
+      type: 'saveFile',
+      suggestedName: 'todos.txt',
+      accepts: [ { description: 'Text files (*.txt)',
+                   extensions: ['txt']} ],
+      acceptsAllTypes: true
+    }, exportToFileEntry);
+
+  }</b>
+}
+</pre>
+
+<p>The first parameter of <code>chrome.fileSystem.chooseEntry()</code> is an object of options. The second parameter is a callback method.</p>
+
+<p>If we already have a saved <code>FileEntry</code>, we'll use that instead when we call <code>exportToFileEntry()</code>. 
+File references will exist for the lifetime of the object representing the <code>FileEntry</code>. 
+In our example, we tied <code>FileEntry</code> to the app window so the JavaScript code can write to the 
+selected file without any user interaction as long as the app window remains open.</p>
+
+<h3 id="use-fileentry">Use FileEntry to write todos items to disk</h3>
+
+<p>Update <code>exportToFileEntry()</code> to save the todos as text via the <code>FileEntry</code> Web API:</p>
+
+<pre data-filename="export.js">
+function exportToFileEntry(fileEntry) {
+<b>  savedFileEntry = fileEntry;
+
+  var status = document.getElementById('status');
+
+  // Use this to get a file path appropriate for displaying
+  chrome.fileSystem.getDisplayPath(fileEntry, function(path) {
+    fileDisplayPath = path;
+    status.innerText = 'Exporting to '+path;
+  });
+
+  getTodosAsText( function(contents) {
+
+    fileEntry.createWriter(function(fileWriter) {
+
+      var truncated = false;
+      var blob = new Blob([contents]);
+
+      fileWriter.onwriteend = function(e) {
+        if (!truncated) {
+          truncated = true;
+          // You need to explicitly set the file size to truncate
+          // any content that might have been there before
+          this.truncate(blob.size);
+          return;
+        }
+        status.innerText = 'Export to '+fileDisplayPath+' completed';
+      };
+
+      fileWriter.onerror = function(e) {
+        status.innerText = 'Export failed: '+e.toString();
+      };
+
+      fileWriter.write(blob);
+
+    });
+  });</b>
+}
+</pre>
+
+<p><code><a href="/apps/fileSystem#method-getDisplayPath">chrome.fileSystem.getDisplayPath()</a></code> is used to get a displayable file path that we'll output to the status <code>div</code>.</p>
+
+<p>Use <code>fileEntry.createWriter()</code> to create a <code>FileWriter</code> object. <code>fileWriter.write()</code> can then write a <a href="https://developer.mozilla.org/en-US/docs/Web/API/Blob">Blob</a> to the filesystem. Use <code>fileWriter.onwriteend()</code> and <code>fileWriter.onerror()</code> to update the status <code>div</code>.</p>
+
+<!-- <code>fileWriter.truncate()</code> -->
+
+<p>For more information about <code>FileEntry</code>, read <a href="http://www.html5rocks.com/en/tutorials/file/filesystem/"><em>Exploring the FileSystem APIs</em></a> on HTML5Rocks, or refer to the <code><a href="https://developer.mozilla.org/en-US/docs/Web/API/FileEntry">FileEntry docs</a></code> on MDN.</p>
+
+<h3 id="persistance">Persist FileEntry objects</h3>
+
+<p><strong>Advanced</strong>: <code>FileEntry</code> objects cannot be persisted indefinitely. This means 
+that your app will need to ask the user to choose a file every time the app is 
+launched. However, <a href="/apps/fileSystem#method-restoreEntry">restoreEntry()</a> is an 
+option to 
+restore a <code>FileEntry</code> if your app was forced to restart due to a runtime crash or update.</p>
+
+<p>If you wish, experiment by saving the ID returned by 
+<a href="/apps/fileSystem.html#method-retainEntry">retainEntry()</a>
+and restoring it on app restart. (Hint: Add a listener to the <code>onRestarted</code> event 
+in the background page.)</p>
+
+<h2 id="launch">Launch your finished Todo app</h2>
+
+<p>You are done Step 6! Reload your app and add some todos. Click <b>Export to disk</b> to export your todos to a .txt file.</p>
+
+<figure>
+  <img src="{{static}}/images/app_codelab/step6-completed.png" alt="The Todo app with exported todos"/>
+</figure>
+
+<h2 id="recap">Recap APIs referenced in this step</h2>
+
+<p>For more detailed information about some of the APIs introduced in this step, refer to:</p>
+
+<ul>
+  <li>
+    <a href="/apps/app_storage#filesystem" title="Read 'Using the Chrome Filesystem API' in the Chrome developer docs">Using the Chrome Filesystem API</a>
+    <a href="#export-todos" class="anchor-link-icon" title="This feature mentioned in 'Export todos'">&#8593;</a>
+  </li>
+  <li>
+    <a href="/apps/declare_permissions" title="Read 'Declare Permissions' in the Chrome developer docs">Declare Permissions</a>
+    <a href="#update-permissions" class="anchor-link-icon" title="This feature mentioned in 'Update permissions'">&#8593;</a>
+  </li>
+  <li>
+    <a href="/apps/storage#method-StorageArea-get" title="Read 'chrome.storage.local.get()' in the Chrome developer docs">chrome.storage.local.get()</a>
+    <a href="#get-todos-as-text" class="anchor-link-icon" title="This feature mentioned in 'Get todo items as text'">&#8593;</a>
+  </li>
+  <li>
+    <a href="/apps/fileSystem#method-chooseEntry" title="Read 'chrome.fileSystem.chooseEntry()' in the Chrome developer docs">chrome.fileSystem.chooseEntry()</a>
+    <a href="#choose-file" class="anchor-link-icon" title="This feature mentioned in 'Choose a file'">&#8593;</a>
+  </li>
+  <li>
+    <a href="/apps/fileSystem#method-getDisplayPath" title="Read 'chrome.fileSystem.getDisplayPath()' in the Chrome developer docs">chrome.fileSystem.getDisplayPath()</a>
+    <a href="#use-fileentry" class="anchor-link-icon" title="This feature mentioned in 'Use FileEntry to write todos items to disk'">&#8593;</a>
+  </li>
+  <li>
+    <a href="/apps/fileSystem#method-restoreEntry" title="Read 'chrome.fileSystem.restoreEntry()' in the Chrome developer docs">chrome.fileSystem.restoreEntry()</a>
+    <a href="#persistance" class="anchor-link-icon" title="This feature mentioned in 'Persist FileEntry objects'">&#8593;</a>
+  </li>
+  <li>
+    <a href="/apps/fileSystem#method-retainEntry" title="Read 'chrome.fileSystem.retainEntry()' in the Chrome developer docs">chrome.fileSystem.retainEntry()</a>
+    <a href="#persistance" class="anchor-link-icon" title="This feature mentioned in 'Persist FileEntry objects'">&#8593;</a>
+  </li>
+</ul>
+
+<p>Ready to continue onto the next step? Go to <a href="app_codelab_publish.html">Step 7 - Publish your app &raquo;</a></p>