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Eines der Hauptmerkmale des Web Components Standards ist die Möglichkeit, benutzerdefinierte Elemente (custom elements) zu erstellen, die ihre Funktionalität auf einer HTML-Seite kapseln, anstatt sich mit einem langen, verschachtelten Stapel von Elementen begnügen zu müssen, die zusammen eine benutzerdefinierte Seitenfunktion bereitstellen. Dieser Artikel stellt die Verwendung von benutzerdefinierten HTML-Elementen vor.

Hinweis: Benutzerdefinierte Elemente werden standardmäßig in Chrome und Opera unterstützt. Firefox ist sehr weit in der Entwicklung; sie sind derzeit verfügbar, wenn Sie die Einstellungen dom.webcomponents.shadowdom.enabled und dom.webcomponents.customelements.enabled auf true setzen. Die Implementierung von Firefox soll in der Version 60/61 standardmäßig aktiviert werden. Safari unterstützt bisher nur autonome benutzerdefinierte Elemente, und Edge arbeitet auch an einer Implementierung.

High-Level-Ansicht

Der Controller von benutzerdefinierten Elementen in einem Web-Dokument ist das Objekt CustomElementRegistry - mit diesem Objekt können Sie ein benutzerdefiniertes Element auf der Seite registrieren, Informationen darüber zurückgeben, welche benutzerdefinierten Elemente registriert sind etc.

Um ein benutzerdefiniertes Element auf der Seite zu registrieren, verwenden Sie die Methode CustomElementRegistry.define(). Das sind die Argumente:

  • Ein DOMString repräsentiert den Namen, den Sie dem Element geben. Beachten Sie, dass benutzerdefinierte Elementnamen einen Bindestrich erfordern; sie können keine einzelnen Wörter sein.
  • Ein Klassenobjekt, das das Verhalten des Elements definiert.
  • Optional ein Optionsobjekt, das eine extendsEigenschaft enthält, die das eingebaute Element angibt, von dem Ihr Element erbt, falls vorhanden.

So können wir z.B. ein benutzerdefiniertes Wortzählelement wie dieses definieren:

customElements.define('word-count', WordCount, { extends: 'p' });

Das Element heißt word-count, sein Klassenobjekt ist WordCount, und es erweitert das Element <p>.

Das Klassenobjekt eines benutzerdefinierten Elements wird mit der ES 2015 Standardsyntax einer Klasse geschrieben. Zum Beispiel ist WordCountso aufgebaut:

class WordCount extends HTMLParagraphElement {
  constructor() {
    // Always call super first in constructor
    super();

    // Element functionality written in here

    ...
  }
}

Dies ist nur ein einfaches Beispiel, aber es gibt noch mehr, was Sie hier tun können. Es ist möglich, spezifische Lifecycle-Callbacks innerhalb des Konstruktors zu definieren, die an bestimmten Stellen im Lebenszyklus des Elements ablaufen. Zum Beispiel wird connectedCallback aufgerufen, wenn das benutzerdefinierte Element zum ersten Mal mit dem DOM des Dokuments verbunden wird, während attributeChangedCallback aufgerufen wird, wenn eines der Attribute des benutzerdefinierten Elements hinzugefügt, entfernt oder geändert wird.

You'll learn more about these in the Using the lifecycle callbacks section below.

There are two types of custom elements:

  • Autonomous custom elements are standalone — they don't inherit from standard HTML elements. You use these on a page by literally writing them out as an HTML element. For example <popup-info>, or document.createElement("popup-info").
  • Customized built-in elements inherit from basic HTML elements. To create one of these, you have to specify which element they extend (as implied in the examples above), and they are used by writing out the basic element but specifying the name of the custom element in the is attribute (or property). For example <p is="word-count">, or document.createElement("p", { is: "word-count" }).

Working through some simple examples

At this point, let's go through some more simple examples to show you how custom elements are created in more detail.

Autonomous custom elements

Let's have a look at an example of an autonomous custom element — <popup-info-box> (see a live example). This takes an image icon and a text string, and embeds the icon into the page. When the icon is focused, it displays the text in a pop up information box to provide further in-context information.

To begin with, the JavaScript file defines a class called PopUpInfo, which extends HTMLElement. Autonomous custom elements nearly always extend HTMLElement.

class PopUpInfo extends HTMLElement {
  constructor() {
    // Always call super first in constructor
    super();

    // write element functionality in here

    ...
  }
}

The preceding code snippet contains the constructor definition for the class, which always starts by calling super() so that the correct prototype chain is established.

Inside the constructor, we define all the functionality the element will have when an instance of it is instantiated. In this case we attach a shadow root to the custom element, use some DOM manipulation to create the element's internal shadow DOM structure — which is then attached to the shadow root — and finally attach some CSS to the shadow root to style it.

// Create a shadow root
var shadow = this.attachShadow({mode: 'open'});

// Create spans
var wrapper = document.createElement('span');
wrapper.setAttribute('class','wrapper');
var icon = document.createElement('span');
icon.setAttribute('class','icon');
icon.setAttribute('tabindex', 0);
var info = document.createElement('span');
info.setAttribute('class','info');

// Take attribute content and put it inside the info span
var text = this.getAttribute('text');
info.textContent = text;

// Insert icon
var imgUrl;
if(this.hasAttribute('img')) {
  imgUrl = this.getAttribute('img');
} else {
  imgUrl = 'img/default.png';
}
var img = document.createElement('img');
img.src = imgUrl;
icon.appendChild(img);

// Create some CSS to apply to the shadow dom
var style = document.createElement('style');

style.textContent = '.wrapper {' +
// CSS truncated for brevity

// attach the created elements to the shadow dom

shadow.appendChild(style);
shadow.appendChild(wrapper);
wrapper.appendChild(icon);
wrapper.appendChild(info);

Finally, we register our custom element on the CustomElementRegistry using the define() method we mentioned earlier — in the parameters we specify the element name, and then the class name that defines its functionality:

customElements.define('popup-info', PopUpInfo);

It is now available to use on our page. Over in our HTML, we use it like so:

<popup-info img="img/alt.png" text="Your card validation code (CVC)
  is an extra security feature — it is the last 3 or 4 numbers on the
  back of your card.">

Note: You can see the full JavaScript source code here.

Customized built-in elements

Now let's have a look at another customized built in element example — expanding-list (see it live also). This turns any unordered list into an expanding/collapsing menu.

First of all, we define our element's class, in the same manner as before:

class ExpandingList extends HTMLUListElement {
  constructor() {
    // Always call super first in constructor
    super();

    // write element functionality in here

    ...
  }
}

We will not explain the element functionality in any detail here, but you can discover how it works by checking out the source code. The only real difference here is that our element is extending the HTMLUListElement interface, and not HTMLElement. So it has all the characteristics of a <ul> element with the functionality we define built on top, rather than being a standalone element. This is what makes it a customized built-in, rather than an autonomous element.

Next, we register the element using the define() method as before, except that this time it also includes an options object that details what element our custom element inherits from:

customElements.define('expanding-list', ExpandingList, { extends: "ul" });

Using the built-in element in a web document also looks somewhat different:

<ul is="expanding-list">

  ...

</ul>

You use a <ul> element as normal, but specify the name of the custom element inside the is attribute.

Note: Again, you can see the full JavaScript source code here.

Using the lifecycle callbacks

You can define several different callbacks inside a custom element's constructor, which fire at different points in the element's lifecycle:

  • connectedCallback: Invoked when the custom element is first connected to the document's DOM.
  • disconnectedCallback: Invoked when the custom element is disconnected from the document's DOM.
  • adoptedCallback: Invoked when the custom element is moved to a new document.
  • attributeChangedCallback: Invoked when one of the custom element's attributes is added, removed, or changed.

Let's look at an example of these in use. The code below is taken from the life-cycle-callbacks example (see it running live). This is a trivial example that simply generates a colored square of a fixed size on the page. The custom element looks like this:

<custom-square l="100" c="red"></custom-square>

The class constructor is really simple — here we attach a shadow DOM to the element, then attach empty <div> and <style> elements to the shadow root:

var shadow = this.attachShadow({mode: 'open'});

var div = document.createElement('div');
var style = document.createElement('style');
shadow.appendChild(style);
shadow.appendChild(div);

The key function in this example is updateStyle() — this takes an element, gets its shadow root, finds its <style> element, and adds width, height, and background-color to the style.

function updateStyle(elem) {
  var shadow = elem.shadowRoot;
  var childNodes = shadow.childNodes;
  for(var i = 0; i < childNodes.length; i++) {
    if(childNodes[i].nodeName === 'STYLE') {
      childNodes[i].textContent = 'div {' +
                          ' width: ' + elem.getAttribute('l') + 'px;' +
                          ' height: ' + elem.getAttribute('l') + 'px;' +
                          ' background-color: ' + elem.getAttribute('c');
    }
  }
}

The actual updates are all handled by the life cycle callbacks, which are placed inside the constructor. The connectedCallback() runs when the element is added to the DOM — here we run the updateStyle() function to make sure the square is styled as defined in its attributes:

connectedCallback() {
  console.log('Custom square element added to page.');
  updateStyle(this);
}

the disconnectedCallback() and adoptedCallback() callbacks log simple messages to the console to inform us when the element is either removed from the DOM, or moved to a different page:

disconnectedCallback() {
  console.log('Custom square element removed from page.');
}

adoptedCallback() {
  console.log('Custom square element moved to new page.');
}

The attributeChangedCallback() callback is run whenever one of the element's attributes is changed in some way. As you can see from its properties, it is possible to act on attributes individually, looking at their name, and old and new attribute values. In this case however, we are just running the updateStyle() function again to make sure that the square's style is updated as per the new values:

attributeChangedCallback(name, oldValue, newValue) {
  console.log('Custom square element attributes changed.');
  updateStyle(this);
}

Note that to get the attributeChangedCallback() callback to fire when an attribute changes, you have to observe the attributes. This is done by calling the observedAttributes() getter inside the constructor, including inside it a return statement that returns an array containing the names of the attributes you want to observe:

static get observedAttributes() {return ['w', 'l']; }

This is placed right at the top of the constructor, in our example.

Note: Find the full JavaScript source here.

Schlagwörter des Dokuments und Mitwirkende

Mitwirkende an dieser Seite: td8
Zuletzt aktualisiert von: td8,