React 深度學習：2. Component、PureComponent

來看看 React 官網的第一個示例代碼：

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" />
複製代碼

在 React 中建立組件有如下方式：

• 函數式
• class 方式
• React.createElement

在上述的代碼中就屬於使用 class 的方式建立組件。咱們先來看看 React.Component 的源碼。

源碼

源碼位置：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}; 複製代碼

這段源碼包含了 Component 和 PureComponent 的源碼。

Component

當 ShoppingList 組件進行實例化的時候它天然也會帶有上圖中的全部實例屬性和原型方法。

使用組件：

<ShoppingList name="Mark" />複製代碼

看到這段代碼，產生幾個疑問：

• 繼承自 Component 的組件是如何進行實例話的呢？
  
• ES6 中繼承自其餘類的類在實例化的須要顯式的調用的 super()，爲何在 React 中不須要呢？

JSX

以上定義 ShoppingList 組件的語法便是 JSX 語法。在 深刻 JSX 章節中講到，JSX 僅僅只是 React.createElement(component, props, ...children) 函數的語法糖。它最後會被編譯爲  React.createElement 函數。

<MyButton color="blue" shadowSize={2}> Click Me </MyButton>複製代碼

會被編譯爲：

React.createElement(
  MyButton,
  {color: 'blue', shadowSize: 2},
  'Click Me'
)複製代碼

那 JSX 是如何被編譯的呢？

在項目中 JSX 被相似  @babel/plugin-transform-react-jsx  的 Babel 插件編譯，關於此插件，之後的章節中再進行研究。

最後的一切都指向了 React.createElement 函數，在下一個章節中，咱們來探究  React.createElement 

PurComponent

基本跟 Component 相同