move misc to js/ui

2-ui/99-ui-misc/01-mutation-observer/article.md

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+
+# Mutation observer
+
+`MutationObserver` is a built-in object that observes a DOM element and fires a callback in case of changes.
+
+We'll first see syntax, and then explore a real-world use case.
+
+## Syntax
+
+`MutationObserver` is easy to use.
+
+First, we create an observer with a callback-function:
+
+```js
+let observer = new MutationObserver(callback);
+```
+
+And then attach it to a DOM node:
+
+```js
+observer.observe(node, config);
+```
+
+`config` is an object with boolean options "what kind of changes to react on":
+- `childList` -- changes in the direct children of `node`,
+- `subtree` -- in all descendants of `node`,
+- `attributes` -- attributes of `node`,
+- `attributeOldValue` -- record the old value of attribute (infers `attributes`),
+- `characterData` -- whether to observe `node.data` (text content),
+- `characterDataOldValue` -- record the old value of `node.data` (infers `characterData`),
+- `attributeFilter` -- an array of attribute names, to observe only selected ones.
+
+Then after any changes, the `callback` is executed, with a list of [MutationRecord](https://dom.spec.whatwg.org/#mutationrecord) objects as the first argument, and the observer itself as the second argument.
+
+[MutationRecord](https://dom.spec.whatwg.org/#mutationrecord) objects have properties:
+
+- `type` -- mutation type, one of
+    - `"attributes"`: attribute modified
+    - `"characterData"`: data modified, used for text nodes,
+    - `"childList"`: child elements added/removed,
+- `target` -- where the change occurred: an element for "attributes", or text node for "characterData", or an element for a "childList" mutation,
+- `addedNodes/removedNodes`  -- nodes that were added/removed,
+- `previousSibling/nextSibling` -- the previous and next sibling to added/removed nodes,
+- `attributeName/attributeNamespace` -- the name/namespace (for XML) of the changed attribute,
+- `oldValue` -- the previous value, only for attribute or text changes.
+
+
+For example, here's a `<div>` with `contentEditable` attribute. That attribute allows us to focus on it and edit.
+
+```html run
+<div contentEditable id="elem">Click and <b>edit</b>, please</div>
+
+<script>
+let observer = new MutationObserver(mutationRecords => {
+  console.log(mutationRecords); // console.log(the changes)
+});
+observer.observe(elem, {
+  // observe everything except attributes
+  childList: true,
+  subtree: true,
+  characterDataOldValue: true
+});
+</script>
+```
+
+If we change the text inside `<b>me</b>`, we'll get a single mutation:
+
+```js
+mutationRecords = [{
+  type: "characterData",
+  oldValue: "me",
+  target: <text node>,
+  // other properties empty
+}];
+```
+
+If we select and remove the `<b>me</b>` altogether, we'll get multiple mutations:
+
+```js
+mutationRecords = [{
+  type: "childList",
+  target: <div#elem>,
+  removedNodes: [<b>],
+  nextSibling: <text node>,
+  previousSibling: <text node>
+  // other properties empty
+}, {
+  type: "characterData"
+  target: <text node>
+  // ...details depend on how the browser handles the change
+  // it may coalesce two adjacent text nodes "Edit " and ", please" into one node
+  // or it can just delete the extra space after "Edit".
+  // may be one mutation or a few
+}];
+```
+
+## Observer use case
+
+When `MutationObserver` is needed? Is there a scenario when such thing can be useful?
+
+We can track something like `contentEditable` and implement "undo/redo" functionality (record mutations and rollback/redo them on demand). There are also cases when `MutationObserver` is good from architectural standpoint.
+
+Let's say we're making a website about programming. Naturally, articles and other materials may contain source code snippets.
+
+An HTML markup of a code snippet looks like this:
+```html
+...
+<pre class="language-javascript"><code>
+  // here's the code
+  let hello = "world";
+</code></pre>
+...
+```
+
+Also we'll use a JavaScript highlighting library on our site, e.g. [Prism.js](https://prismjs.com/). A call to `Prism.highlightElem(pre)` examines the contents of such `pre` elements and adds into them special tags and styles for colored syntax highlighting, similar to what you see in examples here, at this page.
+
+When to run that method? We can do it on `DOMContentLoaded` event, or at the bottom of the page. At that moment we have DOM ready, can search for elements `pre[class*="language"]` and call `Prism.highlightElem` on them:
+
+```js
+// highlight all code snippets on the page
+document.querySelectorAll('pre[class*="language"]').forEach(Prism.highlightElem);
+```
+
+Now the `<pre>` snippet looks like this (without line numbers by default):
+
+```js
+// here's the code
+let hello = "world";
+```
+
+Everything's simple so far, right? There are `<pre>` code snippets in HTML, we highlight them.
+
+Now let's go on. Let's say we're going to dynamically fetch materials from a server. We'll study methods for that [later in the tutorial](info:fetch-basics). For now it only matters that we fetch an HTML article from a webserver and display it on demand:
+
+```js
+let article = /* fetch new content from server */
+articleElem.innerHTML = article;
+```
+
+The new `article` HTML may contain code snippets. We need to call `Prism.highlightElem` on them, otherwise they won't get highlighted.
+
+**Where and when to call `Prism.highlightElem` for a dynamically loaded article?**
+
+We could append that call to the code that loads an article, like this:
+
+```js
+let article = /* fetch new content from server */
+articleElem.innerHTML = article;
+
+*!*
+let snippets = articleElem.querySelectorAll('pre[class*="language-"]');
+snippets.forEach(Prism.highlightElem);
+*/!*
+```
+
+...But imagine, we have many places in the code where we load contents: articles, quizzes, forum posts. Do we need to put the highlighting call everywhere? That's not very convenient, and also easy to forget.
+
+And what if the content is loaded by a third-party module? E.g. we have a forum written by someone else, that loads contents dynamically, and we'd like to add syntax highlighting to it. No one likes to patch third-party scripts.
+
+Luckily, there's another option.
+
+We can use `MutationObserver` to automatically detect code snippets inserted in the page and highlight them.
+
+So we'll handle the highlighting functionality in one place, relieving us from the need to integrate it.
+
+## Dynamic highlight demo
+
+Here's the working example.
+
+If you run this code, it starts observing the element below and highlighting any code snippets that appear there:
+
+```js run
+let observer = new MutationObserver(mutations => {
+
+  for(let mutation of mutations) {
+    // examine new nodes
+
+    for(let node of mutation.addedNodes) {
+      // we track only elements, skip other nodes (e.g. text nodes)
+      if (!(node instanceof HTMLElement)) continue;
+
+      // check the inserted element for being a code snippet
+      if (node.matches('pre[class*="language-"]')) {
+        Prism.highlightElement(node);
+      }
+
+      // maybe there's a code snippet somewhere in its subtree?
+      for(let elem of node.querySelectorAll('pre[class*="language-"]')) {
+        Prism.highlightElement(elem);
+      }
+    }
+  }
+
+});
+
+let demoElem = document.getElementById('highlight-demo');
+
+observer.observe(demoElem, {childList: true, subtree: true});
+```
+
+<p id="highlight-demo" style="border: 1px solid #ddd">Demo element with <code>id="highlight-demo"</code>, obverved by the example above.</p>
+
+The code below populates `innerHTML`. Please run the code above first, it will watch and highlight the new content:
+
+```js run
+let demoElem = document.getElementById('highlight-demo');
+
+// dynamically insert content with code snippets
+demoElem.innerHTML = `A code snippet is below:
+  <pre class="language-javascript"><code> let hello = "world!"; </code></pre>
+  <div>Another one:</div>
+  <div>
+    <pre class="language-css"><code>.class { margin: 5px; } </code></pre>
+  </div>
+`;
+```
+
+Now we have `MutationObserver` that can track all highlighting in observed elements or the whole `document`. We can add/remove code snippets in HTML without thinking about it.
+
+## Additional methods
+
+There's a method to stop observing the node:
+
+- `observer.disconnect()` -- stops the observation.
+
+Additionally:
+
+- `mutationRecords = observer.takeRecords()` -- gets a list of unprocessed mutation records, those that happened, but the callback did not handle them.
+
+```js
+// we're going to disconnect the observer
+// it might have not yet handled some mutations
+let mutationRecords = observer.takeRecords();
+// process mutationRecords
+
+// now all handled, disconnect
+observer.disconnect();
+```
+
+## Garbage collection
+
+Observers use weak references to nodes internally. That is: if a node is removed from DOM, and becomes unreachable, then it becomes garbage collected, an observer doesn't prevent that.
+
+## Summary  
+
+`MutationObserver` can react on changes in DOM: attributes, added/removed elements, text content.
+
+We can use it to track changes introduced by other parts of our own or 3rd-party code.
+
+For example, to post-process dynamically inserted content, as demo `innerHTML`, like highlighting in the example above.

2-ui/99-ui-misc/index.md

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+
+# Miscellaneous