Virtual Dom 虚拟DOM
| 标题 | 内容 |
|---|---|
| VirtualDOM | 为什么要有VirtualDOM? |
| VirtualDOM | 与真实DOM的区别? |
| VirtualDOM | VirtualDOM的优势 |
| VirtualDOM | VirtualDOM实例 |
| VirtualDOM | VirtualDOM应用 |
| VirtualDOM | VirtualDOM的体现, React, Vue, Angular, Sevelte |
VirtualDOM引入的原因
- 这里用过
Vue和React可能都知道VirtualDOM的作用，避免真实DOM的频繁更新从而提升渲染效率，那么真实DOM和VirtualDOM效率究竟是怎么一回事了？这是我们需要好好研究的一门学问。
真实DOM - Real DOM
- DOM(Document Object Model)文档对象模型(直译过来的含义)，它是
HTML(HyperText Markup Language)和XML(Extensible Markup Language)文档的变成接口。 - DOM提供了对文档的结构化的表述，并定义了一种方式可以使从程序中对该结构进行访问，从而改变文档的结构，样式和内容。
- DOM将文档解析为一个由节点和对象(包含属性和方法的对象)组成的结构集合。
- 总结来说，DOM会将web页面和脚本或者程序语言连接起来。
- DOM(Document Object Model)的含义有两层:
1. 基于节点和对象来表示的文档模型(the object-based representation);
2. 操作这些节点和对象的API(常见的就是JavaScript脚本语言)
- 真实DOM的实例
<!DOCTYPE html>
<html lane="en">
<head>
<meta charset="utf-8" />
<title>document object model</title>
<script type="text/javascript">
// ul list 节点信息
var list = document.getElementById("list");
console.log("list: ", list);
var firstLiItem = document.getElementsByClassName("li-item")[0];
console.log("firstLiItem: ", firstLiItem);
var secondLiItem = document.getElementsByClassName("li-item")[1];
console.log("secondLiItem: ", secondLiItem);
firstLiItem.textContent = 'Change Element li 1';
secondLiItem.textContent = 'Change Element li 2';
var newLiItem = document.createElement("li");
newLiItem.classList.add("li-item");
newLiItem.textContent = 'Element li 3';
list.appendChild(newLiItem);
</script>
</head>
<body>
<div>
<h2>Hello Real DOM<h2>
<ul id="list">
<li class="li-item">Element li 1</li>
<li class="li-item">Element li 2</li>
</ul>
</div>
</body>
</html>
- 我们可以清晰得看出来，这是一棵树的结构->DOM树也叫做Parse树(分析树)，树的每个分支的终点都是一个节点(node)，每个节点都包含对象(text, 子节点)，包含一些节点属性(class, id)。这就是基于对象来表示对象。
html
|—— head
|—— |—— meta
|—— |—— title
|—— body
|—— |—— div
|—— |—— |—— h2("Hello Real DOM")
|—— |—— |—— ul
|—— |—— |—— |—— li("Element li 1")
|—— |—— |—— |—— li("Element li 2")
- API(通常就是指JavaScript脚本)操作，如下是获取
ul节点，更新li节点中已有的内容以及增加一个新的li节点到ul节点中。
// ul list 节点信息
var list = document.getElementById("list");
console.log("list: ", list);
var firstLiItem = document.getElementsByClassName("li-item")[0];
console.log("firstLiItem: ", firstLiItem);
var secondLiItem = document.getElementsByClassName("li-item")[1];
console.log("secondLiItem: ", secondLiItem);
firstLiItem.textContent = 'Change Element li 1';
secondLiItem.textContent = 'Change Element li 2';
var newLiItem = document.createElement("li");
newLiItem.classList.add("li-item");
newLiItem.textContent = 'Element li 3';
list.appendChild(newLiItem);
- 单纯从代码出发并不能看出真实DOM的问题，我们只是对真实DOM有了一个认知和认识，我们还要继续往下去分析。
真实DOM的性能问题
- 这是一个老生常谈的问题，怎么去分析它了？
真实DOM有多少属性——一个普通的节点
var _div = document.createElement("div");
var _res = '';
for(var _key in _div) {
_res += _key + ' ';
}
console.log(_res);
// align title lang translate dir hidden accessKey draggable spellcheck autocapitalize contentEditable isContentEditable inputMode offsetParent offsetTop offsetLeft offsetWidth offsetHeight style innerText outerText onbeforexrselect onabort onblur oncancel oncanplay oncanplaythrough onchange onclick onclose oncontextmenu oncuechange ondblclick ondrag ondragend ondragenter ondragleave ondragover ondragstart ondrop ondurationchange onemptied onended onerror onfocus onformdata oninput oninvalid onkeydown onkeypress onkeyup onload onloadeddata onloadedmetadata onloadstart onmousedown onmouseenter onmouseleave onmousemove onmouseout onmouseover onmouseup onmousewheel onpause onplay onplaying onprogress onratechange onreset onresize onscroll onsecuritypolicyviolation onseeked onseeking onselect onslotchange onstalled onsubmit onsuspend ontimeupdate ontoggle onvolumechange onwaiting onwebkitanimationend onwebkitanimationiteration onwebkitanimationstart onwebkittransitionend onwheel onauxclick ongotpointercapture onlostpointercapture onpointerdown onpointermove onpointerup onpointercancel onpointerover onpointerout onpointerenter onpointerleave onselectstart onselectionchange onanimationend onanimationiteration onanimationstart ontransitionrun ontransitionstart ontransitionend ontransitioncancel oncopy oncut onpaste dataset nonce autofocus tabIndex attachInternals blur click focus enterKeyHint virtualKeyboardPolicy onpointerrawupdate namespaceURI prefix localName tagName id className classList slot attributes shadowRoot part assignedSlot innerHTML outerHTML scrollTop scrollLeft scrollWidth scrollHeight clientTop clientLeft clientWidth clientHeight attributeStyleMap onbeforecopy onbeforecut onbeforepaste onsearch elementTiming onfullscreenchange onfullscreenerror onwebkitfullscreenchange onwebkitfullscreenerror children firstElementChild lastElementChild childElementCount previousElementSibling nextElementSibling after animate append attachShadow before closest computedStyleMap getAttribute getAttributeNS getAttributeNames getAttributeNode getAttributeNodeNS getBoundingClientRect getClientRects getElementsByClassName getElementsByTagName getElementsByTagNameNS getInnerHTML hasAttribute hasAttributeNS hasAttributes hasPointerCapture insertAdjacentElement insertAdjacentHTML insertAdjacentText matches prepend querySelector querySelectorAll releasePointerCapture remove removeAttribute removeAttributeNS removeAttributeNode replaceChildren replaceWith requestFullscreen requestPointerLock scroll scrollBy scrollIntoView scrollIntoViewIfNeeded scrollTo setAttribute setAttributeNS setAttributeNode setAttributeNodeNS setPointerCapture toggleAttribute webkitMatchesSelector webkitRequestFullScreen webkitRequestFullscreen ariaAtomic ariaAutoComplete ariaBusy ariaChecked ariaColCount ariaColIndex ariaColSpan ariaCurrent ariaDescription ariaDisabled ariaExpanded ariaHasPopup ariaHidden ariaKeyShortcuts ariaLabel ariaLevel ariaLive ariaModal ariaMultiLine ariaMultiSelectable ariaOrientation ariaPlaceholder ariaPosInSet ariaPressed ariaReadOnly ariaRelevant ariaRequired ariaRoleDescription ariaRowCount ariaRowIndex ariaRowSpan ariaSelected ariaSetSize ariaSort ariaValueMax ariaValueMin ariaValueNow ariaValueText getAnimations nodeType nodeName baseURI isConnected ownerDocument parentNode parentElement childNodes firstChild lastChild previousSibling nextSibling nodeValue textContent ELEMENT_NODE ATTRIBUTE_NODE TEXT_NODE CDATA_SECTION_NODE ENTITY_REFERENCE_NODE ENTITY_NODE PROCESSING_INSTRUCTION_NODE COMMENT_NODE DOCUMENT_NODE DOCUMENT_TYPE_NODE DOCUMENT_FRAGMENT_NODE NOTATION_NODE DOCUMENT_POSITION_DISCONNECTED DOCUMENT_POSITION_PRECEDING DOCUMENT_POSITION_FOLLOWING DOCUMENT_POSITION_CONTAINS DOCUMENT_POSITION_CONTAINED_BY DOCUMENT_POSITION_IMPLEMENTATION_SPECIFIC appendChild cloneNode compareDocumentPosition contains getRootNode hasChildNodes insertBefore isDefaultNamespace isEqualNode isSameNode lookupNamespaceURI lookupPrefix normalize removeChild replaceChild addEventListener dispatchEvent removeEventListener
// 总共是302个属性
Thinking in JackDan
