I've been hearing about Shadow DOM recently. While watching a video about the release of Angular 2 the presenter repeatedly made mention of Shadow DOM without clear expressions. What does Shadow DOM really mean?
The best explanation I've found is this from What the Heck is Shadow DOM? shown below:
Shadow DOM refers to the ability of the browser to include a subtree
of DOM elements into the rendering of a document, but not into the
main document DOM tree.
An important use case would be with video controls on a web page. The markup only shows a video tag, with some attributes and source tags. The addtional code for all the video operations to work is hidden in the Shadow DOM and not available to the rest of the page. The actual markup, javascript, and styles for the tag are encapsulated, which hides the implementation details of the video controls that each browser vendor has already written.
So while it's there in the DOM, its hidden from the page that renders it. So, to view the Shadow DOM, you can enable it under Dev Tools in Chrome.
The short answer is that the Shadow DOM is one of four technologies that make up Web Components.
For a definition, Web Components are: A component platform from the W3C that allows Web sites to be constructed from standardized building blocks. Web Components comprise Custom Elements, Shadow DOM and HTML Imports and Templates.
Shadow DOM is a technology of Web Components (although each can be used separately):
Custom Elements: is a capability for creating your own custom HTML tags and elements. They can have their own scripted behavior and CSS styling. They are part of Web Components but they can also be used by themselves.
HTML Templates: The HTML template element is a mechanism for holding client-side content that is not to be rendered when a page is loaded but may subsequently be instantiated during runtime using JavaScript. Think of a template as a content fragment that is being stored for subsequent use in the document.
Shadow DOM: provides encapsulation for the JavaScript, CSS, and templating in a Web Component. Shadow DOM makes it so these things remain separate from the DOM of the main document. You can also use Shadow DOM by itself, outside of a web component.
HTML Imports: is intended to be the packaging mechanism for Web Components, but you can also use HTML Imports by itself. You import an HTML file by using a tag in an HTML document.
See Introduction to the Shadow DOM.
It refers to the ability to create a "child" DOM completely sandboxed from the rest of the page. Useful for web components, reusable "widgets" which allow to not worry about their css/js affecting things they shouldn't. http://glazkov.com/2011/01/14/what-the-heck-is-shadow-dom/
Check out https://www.polymer-project.org/ if you want to see it in action.
Think of shadow DOM is as an encapsulated (private) DOM.
You can't access the shadow DOM in the manner you access regular DOM, like 'document.querySelector()'.
Let's say, you defined a reusable custom element, (which contains its DOM trees). Then you use the custom element within your app HTML.
Now, the DOM under (which is now called a "host element") has become a shadow DOM subtrees (under a shadow root), hidden from the parent structure!
I hope this help you a bit.
Related
I am using Vue & Bootstrap for an app where I generate web components according to the official Vue documentation (https://cli.vuejs.org/guide/build-targets.html#web-component). For the most part Bootstrap and my business logic is working fine within the #shadow-roots of the web components as if it were in the light DOM.
However, Bootstrap tooltips (which are based on Popper.js https://popper.js.org/) are not working within the Shadow DOM at all. I have also tried to invoke tooltips directly with Popper.js and Tippy.js (https://atomiks.github.io/tippyjs/) in the Shadow DOM encapsulated code, sidestepping Bootstrap altogether, and I still cannot get them to work.
See example here: https://jsfiddle.net/mfep6rg9/
I can guess why -- the 3rd party tooltip libraries most likely aren't finding the target DOM element because it's in a Shadow DOM.
Is there a 3rd party solution out there that accounts for Shadow DOM / web component encapsulation?
Your guess is correct. 3party solutions using document. are not querying shadowDOM.
And there probably is no 3rd party solution as a solution requires either
WebComponents to communicate Mouse positions to the outside world.
Host querying shadowDOM (and nested shadowDOMs and nested shadowDOMs)
Not much different from an (even more restricted) IFRAME
I had the same problem while building LitElement-based Web Components and found the following solution:
$(this.shadowRoot.querySelectorAll("[data-toggle='tooltip']")).tooltip();
Make sure to target the respective element's shadowRoot and run a querySelectorAll to listen to all shadowRoot child elements that listen to "data-toggle='tooltip'".
I'm trying to figure out the difference between using document.createDocumentFragment versus using an HTML <tamplate> element.
Is there a difference between them in behavior or performance?
Both <template> and document.createDocumentFragment are used for storing HTML-like data without rendering it, but the use cases are somewhat different.
The <template> tag's main purpose is to, as the name applies, store HTML for a later time, and or to be used repeatedly across the document. This tag is useful when using a template engine where the contents are usually never changed but the input may be different.
document.createDocumentFragment is used to create an entire DOM tree in JS without the browser rendering it, while still having the ability to use the DOM API with it. This useful when dynamically generating HTML by leveraging the DOM API, and to later inject the results in the actual document's DOM.
More: Template Tag and DocumentFragment
In the tutorials I see only the benefits of Shadow DOM, but there should be drawbacks as well. In which cases should we avoid using Shadow DOM?
Shadow DOM features can be seen as drawback as much as benefits:
Style isolation is a benefit if you want it but a drawback if a user wants to style a component with Shadow DOM from a global CSS stylesheet.
DOM Shadowing is a benefit in some cases, but a drawback if an external script/library or extension needs to parse or select the content.
There are many 3rd party libraries (or extensions) that won't work with Shadow DOM content because they were not designed to deal with it, or need some additional configuration to work with Shadow DOM.
Examples:
Disqus comments integration
CodeMirror editor integration
Wiris math editor integration
Also, extensions that parse HTML will be blocked at the Shadow DOM boundary: a benefit if you don't want to spied, a drawback if you consider them as a useful service.
Event propagation is different inside and outside the Shadow DOM. So you may have some difficulties dealing with UI events.
Example :
TinyMCE integration
Conclusion
Use Shadow DOM only if you want CSS style or DOM isolation.
Don't use Shadow DOM if you need to interact with some not compliant third party components or library.
Uh, ah, this technology's specification has not stabilized and is not supported by many browsers. I would call that a drawback.
See: https://developer.mozilla.org/en-US/docs/Web/API/Element/attachShadow
For now I would say; Avoid in most cases, except when you want to experiment with new stuff and it is not for the production environment.
UPDATE: By now all modern browsers support this feature.
In Google Chrome’s Developer Tools, I see a #shadow-root right under <html lang="en"> tag. what does it do and what is it used for? I don’t see it in Firefox nor in IE; only in Chrome, is this a special feature?
If I open it, it shows <head> and <body> and a link beside named reveal, by clicking, it points to the <head> and <body>, nothing else.
This is a special indicator that a Shadow DOM exists. These have existed for years, but developers have never been given APIs into it until recently. Chrome has had this functionality for a while, other browsers are still catching up. It can be toggled in the DevTools Settings under the "Elements" section. Uncheck the "Show User Agent Shadow DOM". This will at least hide away any Shadow DOMs created internally (like select elements.) I am unsure right away if it affects user-created ones, such as custom elements.
These come up in things like iframes as well, where you have a separate DOM tree nested inside of another.
The Shadow DOM is simply saying that some part of the page, has its own DOM within it. Styles and scripting can be scoped within that element so what runs in it only executes in that boundary.
This is one of the primary pieces needed for Web Components to work. Which is a new technology allowing developers to build their own encapsulated components that developers can use just like any other HTML element.
As an example of Shadow DOM, when you have a <video> tag on a web page, its shown as just one tag in the main DOM, but if you enable Shadow DOM, you will be able to see the video player's HTML(player DOM).
This is explained aptly in this article, http://webcomponents.org/articles/introduction-to-shadow-dom/
In the case of web components, there is a fundamental problem that makes widgets built out of HTML and JavaScript hard to use.
Problem: The DOM tree inside a widget isn’t encapsulated from the rest of the page. This lack of encapsulation means your document stylesheet might accidentally apply to parts inside the widget; your JavaScript might accidentally modify parts inside the widget; your IDs might overlap with IDs inside the widget and so on.
Shadow DOM addresses the DOM tree encapsulation problem.
For example, if you had markup like this:
<button>Hello, world!</button>
<script>
var host = document.querySelector('button');
var root = host.createShadowRoot();
root.textContent = 'こんにちは、影の世界!';
</script>
then instead of
Hello, world!
your page looks like
こんにちは、影の世界!
Not only that, if JavaScript on the page asks what the button’s textContent is, it isn’t going to get “こんにちは、影の世界!”, but “Hello, world!” because the DOM subtree under the shadow root is encapsulated.
NOTE: I have picked up above content from https://www.html5rocks.com/en/tutorials/webcomponents/shadowdom/ as it helped me understand shadow DOM a little better than answers already here. I have added relevant content here so that it helps others but do take a look at the link for detailed discussion on same.
I have studied Shadow DOM recently, and I was wondering what are the aims of using it instead of the main one.
What does it gives ? Why dont we use standard DOM instead of it (except for styling scoping) ?
It allows you to encapsulate functionality, effectively putting it in a black box. It means you can create [reusable] components whose inner workings aren't exposed; this is impossible using standard DOM.
As an example, take HTML input elements. So, say, the file type of input. To use it on an HTML page, you simply add <input type="file" />, and it works. You don't need to add any extra code or HTML or CSS to handle how it works, it just does, and you can't access the internal bits of it. If you were to write a piece of UI, using HTML/CSS/JS, that did the same thing, it would be fairly complex. But the file input is just a single tag that you can use anywhere, it always does the same thing. The web component family of specs allow you to create your own elements that work in this way, and the Shadow DOM is a critical part of this. You can create a new element, like <my-fantastic-file-input />, with its functionality encapsulated. It has its own internal DOM subtree, but that subtree isn't directly accessible; ditto with scoped styles. The new component does not expose its implementation details to the document.
You can do most of this stuff using the DOM, but the implementation will be wholly tied into the document/application structure. With components, you extract that implementation, and you can reuse it, pass it around, publish it and let other people drop it into their applications/documents, and be sure it will work in exactly the same way, anywhere. You cannot really do that as things currently stand by using the standard DOM.
This is from 2011, and slightly out-of-date, but it's a list of some possible use cases for the component model: http://www.w3.org/2008/webapps/wiki/Component_Model_Use_Cases