React.ComponentReact.Component 这个baseClass 而写的类Component 这一个baseClass，还有另外一个叫 PureComponentPureComponent 跟 Component 唯一区别
shouldComponentUpdate 的一个实现1 ) API 位置定位
import {Component, PureComponent} from './ReactBaseClasses';
ReactBaseClasses.js 源码
/**
* 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};
2 ）源码分析
2.1 Component
从上面看到，Component 是一个function，它是一个使用function去进行类的声明的一个方式
它接收三个参数，一个是 props，一个是 context，还有一个是 updater
关于 props 和 context 可以发现, 在这里面进行赋值
this.props = propsthis.context = context然后再声明了一个 refs，它是一个 emptyObject
关于 updater 出场率并不高，先跳过
往下看，找到 Component.prototype.setState
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');
};
partialState, 就是我们要更新的一个新的state，可以是一个对象，也可以是一个方法callback, 那么callback就是在我们state真正的更新完之后才会执行这个callbackthis.updater.enqueueSetState
接下来，进入到 Component.prototype.forceUpdate
Component.prototype.forceUpdate = function(callback) {
this.updater.enqueueForceUpdate(this, callback, 'forceUpdate');
};
this.updater.enqueueForceUpdate(this, callback, 'forceUpdate'); 这个方法接下来，在 DEV 环境的判断下，有两个即将被废弃的API
isMountedreplaceState到这里为止，可以看到 Component 的定义已经结束了, 它只有这么一点东西
这个 Component 的定义，只是用来帮助我们去承载一些信息的
没有任何的其他含义，也没有任何关于生命周期相关的一些方法, 就是这么简洁
2.2 PureComponent
其实可以认为它是继承于 Component
其实也没多少可以继承的东西，它们接收的东西也是一模一样的
/**
* 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};
pureComponentPrototype.isPureReactComponent = true;