React 深度学习：2. Component、PureComponent

来看看 React 官网的第一个示例代码：

class ShoppingList extends React.Component {
  render() {
    return (
      <div className="shopping-list"> <h1>Shopping List for {this.props.name}</h1> <ul> <li>Instagram</li> <li>WhatsApp</li> <li>Oculus</li> </ul> </div>
    );
  }
}

// 用法示例: <ShoppingList name="Mark" />
复制代码

在 React 中建立组件有如下方式：

• 函数式
• class 方式
• React.createElement

在上述的代码中就属于使用 class 的方式建立组件。咱们先来看看 React.Component 的源码。

源码

源码位置：packages/react/src/ReactBaseClasses.js

/**
 * Copyright (c) Facebook, Inc. and its affiliates.
 *
 * This source code is licensed under the MIT license found in the
 * LICENSE file in the root directory of this source tree.
 */

import invariant from 'shared/invariant';
import lowPriorityWarning from 'shared/lowPriorityWarning';

import ReactNoopUpdateQueue from './ReactNoopUpdateQueue';

const emptyObject = {};
if (__DEV__) {
  Object.freeze(emptyObject);
}

/**
 * Base class helpers for the updating state of a component.
 */
function Component(props, context, updater) {
  this.props = props;
  this.context = context;
  // If a component has string refs, we will assign a different object later.
  this.refs = emptyObject;
  // We initialize the default updater but the real one gets injected by the
  // renderer.
  this.updater = updater || ReactNoopUpdateQueue;
}

Component.prototype.isReactComponent = {};

/**
 * Sets a subset of the state. Always use this to mutate
 * state. You should treat `this.state` as immutable.
 *
 * There is no guarantee that `this.state` will be immediately updated, so
 * accessing `this.state` after calling this method may return the old value.
 *
 * There is no guarantee that calls to `setState` will run synchronously,
 * as they may eventually be batched together.  You can provide an optional
 * callback that will be executed when the call to setState is actually
 * completed.
 *
 * When a function is provided to setState, it will be called at some point in
 * the future (not synchronously). It will be called with the up to date
 * component arguments (state, props, context). These values can be different
 * from this.* because your function may be called after receiveProps but before
 * shouldComponentUpdate, and this new state, props, and context will not yet be
 * assigned to this.
 *
 * @param {object|function} partialState Next partial state or function to
 *        produce next partial state to be merged with current state.
 * @param {?function} callback Called after state is updated.
 * @final
 * @protected
 */
Component.prototype.setState = function(partialState, callback) {
  invariant(
    typeof partialState === 'object' ||
      typeof partialState === 'function' ||
      partialState == null,
    'setState(...): takes an object of state variables to update or a ' +
      'function which returns an object of state variables.',
  );
  this.updater.enqueueSetState(this, partialState, callback, 'setState');
};

/**
 * Forces an update. This should only be invoked when it is known with
 * certainty that we are **not** in a DOM transaction.
 *
 * You may want to call this when you know that some deeper aspect of the
 * component's state has changed but `setState` was not called. * * This will not invoke `shouldComponentUpdate`, but it will invoke * `componentWillUpdate` and `componentDidUpdate`. * * @param {?function} callback Called after update is complete. * @final * @protected */ Component.prototype.forceUpdate = function(callback) { this.updater.enqueueForceUpdate(this, callback, 'forceUpdate'); }; /** * Deprecated APIs. These APIs used to exist on classic React classes but since * we would like to deprecate them, we're not going to move them over to this
 * modern base class. Instead, we define a getter that warns if it's accessed. */ if (__DEV__) { const deprecatedAPIs = { isMounted: [ 'isMounted', 'Instead, make sure to clean up subscriptions and pending requests in ' + 'componentWillUnmount to prevent memory leaks.', ], replaceState: [ 'replaceState', 'Refactor your code to use setState instead (see ' + 'https://github.com/facebook/react/issues/3236).', ], }; const defineDeprecationWarning = function(methodName, info) { Object.defineProperty(Component.prototype, methodName, { get: function() { lowPriorityWarning( false, '%s(...) is deprecated in plain JavaScript React classes. %s', info[0], info[1], ); return undefined; }, }); }; for (const fnName in deprecatedAPIs) { if (deprecatedAPIs.hasOwnProperty(fnName)) { defineDeprecationWarning(fnName, deprecatedAPIs[fnName]); } } } function ComponentDummy() {} ComponentDummy.prototype = Component.prototype; /** * Convenience component with default shallow equality check for sCU. */ function PureComponent(props, context, updater) { this.props = props; this.context = context; // If a component has string refs, we will assign a different object later. this.refs = emptyObject; this.updater = updater || ReactNoopUpdateQueue; } const pureComponentPrototype = (PureComponent.prototype = new ComponentDummy()); pureComponentPrototype.constructor = PureComponent; // Avoid an extra prototype jump for these methods. Object.assign(pureComponentPrototype, Component.prototype); pureComponentPrototype.isPureReactComponent = true; export {Component, PureComponent}; 复制代码

这段源码包含了 Component 和 PureComponent 的源码。

Component

当 ShoppingList 组件进行实例化的时候它天然也会带有上图中的全部实例属性和原型方法。

使用组件：

<ShoppingList name="Mark" />复制代码

看到这段代码，产生几个疑问：

• 继承自 Component 的组件是如何进行实例话的呢？
  
• ES6 中继承自其余类的类在实例化的须要显式的调用的 super()，为何在 React 中不须要呢？

JSX

以上定义 ShoppingList 组件的语法便是 JSX 语法。在 深刻 JSX 章节中讲到，JSX 仅仅只是 React.createElement(component, props, ...children) 函数的语法糖。它最后会被编译为  React.createElement 函数。

<MyButton color="blue" shadowSize={2}> Click Me </MyButton>复制代码

会被编译为：

React.createElement(
  MyButton,
  {color: 'blue', shadowSize: 2},
  'Click Me'
)复制代码

那 JSX 是如何被编译的呢？

在项目中 JSX 被相似  @babel/plugin-transform-react-jsx  的 Babel 插件编译，关于此插件，之后的章节中再进行研究。

最后的一切都指向了 React.createElement 函数，在下一个章节中，咱们来探究  React.createElement 

PurComponent

基本跟 Component 相同