明白ReactElement和ReactClass的观点
起首让我们明白两个观点:
ReactElement
一个形貌DOM节点或component实例的字面级对象。它包含一些信息,包含组件范例type
和属性props
。就像一个形貌DOM节点的元素(假造节点)。它们能够被建立经由过程React.createElement
要领或jsx
写法
分为DOM Element
和Component Elements
两类:
DOM Elements
当节点的type属性为字符串时,它代表是一般的节点,如div,span
{
type: 'button',
props: {
className: 'button button-blue',
children: {
type: 'b',
props: {
children: 'OK!'
}
}
}
}
Component Elements
当节点的type属性为一个函数或一个类时,它代表自定义的节点
class Button extends React.Component {
render() {
const { children, color } = this.props;
return {
type: 'button',
props: {
className: 'button button-' + color,
children: {
type: 'b',
props: {
children: children
}
}
}
};
}
}
// Component Elements
{
type: Button,
props: {
color: 'blue',
children: 'OK!'
}
}
ReactClass
ReactClass是日常平凡我们写的Component组件(类或函数),比方上面的Button
类。ReactClass实例化后挪用render要领可返回DOM Element
。
react衬着历程
历程明白:
// element是 Component Elements
ReactDOM.render({
type: Form,
props: {
isSubmitted: false,
buttonText: 'OK!'
}
}, document.getElementById('root'));
挪用
React.render
要领,将我们的element
根假造节点衬着到container
元素中。element
能够是一个字符串文本元素,也能够是如上引见的ReactElement
(分为DOM Elements, Component Elements)。依据
element
的范例差别,离别实例化ReactDOMTextComponent
,ReactDOMComponent
,ReactCompositeComponent
类。这些类用来治理ReactElement
,担任将差别的ReactElement
转化成DOM(mountComponent要领),担任更新DOM(receiveComponent要领,updateComponent要领, 以下会引见)等。ReactCompositeComponent
实例挪用mountComponent
要领后内部挪用render
要领,返回了DOM Elements
。再对如图的步骤2️⃣递归。
react更新机制
每一个范例的元素都要处置惩罚好本身的更新:
自定义元素的更新,主假如更新render出的节点,做甩手掌柜交给render出的节点的对应component去治理更新。
text节点的更新很简单,直接更新案牍。
浏览器基础元素的更新,分为两块:
先是更新属性,对照出前后属性的差别,部分更新。而且处置惩罚特别属性,比方事宜绑定。
然后是子节点的更新,子节点更新主假如找出差别对象,找差别对象的时刻也会运用上面的shouldUpdateReactComponent来推断,假如是能够直接更新的就会递归挪用子节点的更新,如许也会递归查找差别对象。不可直接更新的删除之前的对象或增加新的对象。以后依据差别对象操纵dom元素(位置更改,删除,增加等)。
第一步:挪用this.setState
ReactClass.prototype.setState = function(newState) {
//this._reactInternalInstance是ReactCompositeComponent的实例
this._reactInternalInstance.receiveComponent(null, newState);
}
第二步:挪用内部receiveComponent要领
这里重要分三种状况,文本元素,基础元素,自定义元素。
自定义元素:
receiveComponent要领源码
// receiveComponent要领
ReactCompositeComponent.prototype.receiveComponent = function(nextElement, transaction, nextContext) {
var prevElement = this._currentElement;
var prevContext = this._context;
this._pendingElement = null;
this.updateComponent(
transaction,
prevElement,
nextElement,
prevContext,
nextContext
);
}
updateComponent要领源码
// updateComponent要领
ReactCompositeComponent.prototype.updateComponent = function(
transaction,
prevParentElement,
nextParentElement,
prevUnmaskedContext,
nextUnmaskedContext
) {
// 简写.....
// 不是state更新而是props更新
if (prevParentElement !== nextParentElement) {
willReceive = true;
}
if (willReceive && inst.componentWillReceiveProps) {
// 挪用生命周期componentWillReceiveProps要领
}
// 是不是更新元素
if (inst.shouldComponentUpdate) {
// 假如供应shouldComponentUpdate要领
shouldUpdate = inst.shouldComponentUpdate(nextProps, nextState, nextContext);
} else {
if (this._compositeType === CompositeTypes.PureClass) {
// 假如是PureClass,浅层对照props和state
shouldUpdate =
!shallowEqual(prevProps, nextProps) ||
!shallowEqual(inst.state, nextState);
}
}
if (shouldUpdate) {
// 更新元素
this._performComponentUpdate(
nextParentElement,
nextProps,
nextState,
nextContext,
transaction,
nextUnmaskedContext
);
} else {
// 不更新元素,但仍然设置props和state
this._currentElement = nextParentElement;
this._context = nextUnmaskedContext;
inst.props = nextProps;
inst.state = nextState;
inst.context = nextContext;
}
// .......
}
内部_performComponentUpdate要领源码
// 内部_updateRenderedComponentWithNextElement要领
ReactCompositeComponent.prototype._updateRenderedComponentWithNextElement = function() {
// 剖断两个element需不须要更新
if (shouldUpdateReactComponent(prevRenderedElement, nextRenderedElement)) {
// 假如须要更新,就继承挪用子节点的receiveComponent的要领,传入新的element更新子节点。
ReactReconciler.receiveComponent(
prevComponentInstance,
nextRenderedElement,
transaction,
this._processChildContext(context)
);
} else {
// 卸载之前的子节点,装置新的子节点
var oldHostNode = ReactReconciler.getHostNode(prevComponentInstance);
ReactReconciler.unmountComponent(
prevComponentInstance,
safely,
false /* skipLifecycle */
);
var nodeType = ReactNodeTypes.getType(nextRenderedElement);
this._renderedNodeType = nodeType;
var child = this._instantiateReactComponent(
nextRenderedElement,
nodeType !== ReactNodeTypes.EMPTY /* shouldHaveDebugID */
);
this._renderedComponent = child;
var nextMarkup = ReactReconciler.mountComponent(
child,
transaction,
this._hostParent,
this._hostContainerInfo,
this._processChildContext(context),
debugID
);
}
shouldUpdateReactComponent函数源码
function shouldUpdateReactComponent(prevElement, nextElement){
var prevEmpty = prevElement === null || prevElement === false;
var nextEmpty = nextElement === null || nextElement === false;
if (prevEmpty || nextEmpty) {
return prevEmpty === nextEmpty;
}
var prevType = typeof prevElement;
var nextType = typeof nextElement;
if (prevType === 'string' || prevType === 'number') {
// 假如先前的ReactElement对象范例是字符串或数字,新的ReactElement对象范例也是字符串或数字,则须要更新,新的ReactElement对象范例是对象,则不应当更新,直接替代。
return (nextType === 'string' || nextType === 'number');
} else {
// 假如先前的ReactElement对象范例是对象,新的ReactElement对象范例也是对象,而且标签范例和key值雷同,则须要更新
return (
nextType === 'object' &&
prevElement.type === nextElement.type &&
prevElement.key === nextElement.key
);
}
}
文本元素:
receiveComponent要领源码
ReactDOMTextComponent.prototype.receiveComponent(nextText, transaction) {
//跟之前保留的字符串比较
if (nextText !== this._currentElement) {
this._currentElement = nextText;
var nextStringText = '' + nextText;
if (nextStringText !== this._stringText) {
this._stringText = nextStringText;
var commentNodes = this.getHostNode();
// 替代文本元素
DOMChildrenOperations.replaceDelimitedText(
commentNodes[0],
commentNodes[1],
nextStringText
);
}
}
}
基础元素:
receiveComponent要领源码
ReactDOMComponent.prototype.receiveComponent = function(nextElement, transaction, context) {
var prevElement = this._currentElement;
this._currentElement = nextElement;
this.updateComponent(transaction, prevElement, nextElement, context);
}
updateComponent要领源码
ReactDOMComponent.prototype.updateComponent = function(transaction, prevElement, nextElement, context) {
// 略.....
//须要零丁的更新属性
this._updateDOMProperties(lastProps, nextProps, transaction, isCustomComponentTag);
//再更新子节点
this._updateDOMChildren(
lastProps,
nextProps,
transaction,
context
);
// ......
}
this._updateDOMChildren
要领内部挪用diff算法,请看下一节……..
react Diff算法
diff算法源码
_updateChildren: function(nextNestedChildrenElements, transaction, context) {
var prevChildren = this._renderedChildren;
var removedNodes = {};
var mountImages = [];
// 猎取新的子元素数组
var nextChildren = this._reconcilerUpdateChildren(
prevChildren,
nextNestedChildrenElements,
mountImages,
removedNodes,
transaction,
context
);
if (!nextChildren && !prevChildren) {
return;
}
var updates = null;
var name;
var nextIndex = 0;
var lastIndex = 0;
var nextMountIndex = 0;
var lastPlacedNode = null;
for (name in nextChildren) {
if (!nextChildren.hasOwnProperty(name)) {
continue;
}
var prevChild = prevChildren && prevChildren[name];
var nextChild = nextChildren[name];
if (prevChild === nextChild) {
// 同一个援用,申明是运用的同一个component,所以我们须要做挪动的操纵
// 挪动已有的子节点
// NOTICE:这里依据nextIndex, lastIndex决议是不是挪动
updates = enqueue(
updates,
this.moveChild(prevChild, lastPlacedNode, nextIndex, lastIndex)
);
// 更新lastIndex
lastIndex = Math.max(prevChild._mountIndex, lastIndex);
// 更新component的.mountIndex属性
prevChild._mountIndex = nextIndex;
} else {
if (prevChild) {
// 更新lastIndex
lastIndex = Math.max(prevChild._mountIndex, lastIndex);
}
// 增加新的子节点在指定的位置上
updates = enqueue(
updates,
this._mountChildAtIndex(
nextChild,
mountImages[nextMountIndex],
lastPlacedNode,
nextIndex,
transaction,
context
)
);
nextMountIndex++;
}
// 更新nextIndex
nextIndex++;
lastPlacedNode = ReactReconciler.getHostNode(nextChild);
}
// 移撤除不存在的旧子节点,和旧子节点和新子节点差别的旧子节点
for (name in removedNodes) {
if (removedNodes.hasOwnProperty(name)) {
updates = enqueue(
updates,
this._unmountChild(prevChildren[name], removedNodes[name])
);
}
}
}
react的长处与总结
长处
假造节点。在UI方面,不须要马上更新视图,而是天生假造DOM后一致衬着。
组件机制。各个组件自力治理,层层嵌套,互不影响,react内部完成的衬着功用。
差别算法。依据基础元素的key值,推断是不是递归更新子节点,照样删除旧节点,增加新节点。
总结
想要更好的应用react的假造DOM,diff算法的上风,我们须要准确的优化、构造react页面。比方将一个页面render的ReactElement
节点分解成多个组件。在须要优化的组件手动增加 shouldComponentUpdate
来防止不须要的 re-render
。