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React 源码浅析之 - ReactElement
ReactElement
这个模块定义了 React 元素的行为和方法,首先看 ReactElement 函数:
ReactElement
var ReactElement = function (type, key, ref, self, source, owner, props) {
var element = {
// This tag allow us to uniquely identify this as a React Element
$$typeof: REACT_ELEMENT_TYPE,
// Built-in properties that belong on the element
type: type,
key: key,
ref: ref,
props: props,
// Record the component responsible for creating this element.
_owner: owner,
};
if (__DEV__) {
// The validation flag is currently mutative. We put it on
// an external backing store so that we can freeze the whole object.
// This can be replaced with a WeakMap once they are implemented in
// commonly used development environments.
element._store = {};
// To make comparing ReactElements easier for testing purposes, we make
// the validation flag non-enumerable (where possible, which should
// include every environment we run tests in), so the test framework
// ignores it.
Object.defineProperty(element._store, "validated", {
configurable: false,
enumerable: false,
writable: true,
value: false,
});
// self and source are DEV only properties.
Object.defineProperty(element, "_self", {
configurable: false,
enumerable: false,
writable: false,
value: self,
});
// Two elements created in two different places should be considered
// equal for testing purposes and therefore we hide it from enumeration.
Object.defineProperty(element, "_source", {
configurable: false,
enumerable: false,
writable: false,
value: source,
});
if (Object.freeze) {
Object.freeze(element.props);
Object.freeze(element);
}
}
return element;
};
抛去其中 DEV 里面的内容,其他部分很简单,定义了一个 React 元素应有属性。包括: type, key, ref, self, source, owner, props.
还有一个 $$typeof
属性,是一个常量,用来判断这个对象是不是一个 React 元素。
var REACT_ELEMENT_TYPE =
(typeof Symbol === "function" && Symbol.for && Symbol.for("react.element")) ||
0xeac7;
ReactElement.createElement
ReactElement.createElement = function (type, config, children) {
var propName;
// Reserved names are extracted
var props = {};
var key = null;
var ref = null;
var self = null;
var source = null;
if (config != null) {
// ref 和 key 都属于保留 props key 值,所以这里需要做判断
if (hasValidRef(config)) {
ref = config.ref;
}
if (hasValidKey(config)) {
key = "" + config.key;
}
// __self 和 __source 这两个属性目前没有看到他们的作用,先放着
self = config.__self === undefined ? null : config.__self;
source = config.__source === undefined ? null : config.__source;
// Remaining properties are added to a new props object
// 其他的属性添加到新的 props 对象上,同时需要排除掉保留字段 RESERVED_PROPS
// var RESERVED_PROPS = {key: true, ref: true, __self: true, __source: true,};
for (propName in config) {
if (
hasOwnProperty.call(config, propName) &&
!RESERVED_PROPS.hasOwnProperty(propName)
) {
props[propName] = config[propName];
}
}
}
// Children 可以传递一个以上的参数,这些 children 参数都会作为新分配的 props 的属性
var childrenLength = arguments.length - 2;
if (childrenLength === 1) {
props.children = children;
} else if (childrenLength > 1) {
var childArray = Array(childrenLength);
for (var i = 0; i < childrenLength; i++) {
childArray[i] = arguments[i + 2];
}
if (__DEV__) {
if (Object.freeze) {
Object.freeze(childArray);
}
}
props.children = childArray;
}
// 设置 defaultProps 属性
if (type && type.defaultProps) {
var defaultProps = type.defaultProps;
for (propName in defaultProps) {
if (props[propName] === undefined) {
props[propName] = defaultProps[propName];
}
}
}
// 开发环境下,如果使用了保留字段 key 和 ref ,那么进行控制台报错提醒
if (__DEV__) {
if (key || ref) {
if (
typeof props.$$typeof === "undefined" ||
props.$$typeof !== REACT_ELEMENT_TYPE
) {
var displayName =
typeof type === "function"
? type.displayName || type.name || "Unknown"
: type;
if (key) {
defineKeyPropWarningGetter(props, displayName);
}
if (ref) {
defineRefPropWarningGetter(props, displayName);
}
}
}
}
// 返回一个 React 元素, ReactCurrentOwner.current 是指当前正处于构建过程中的组件,这里默认是 null
return ReactElement(
type,
key,
ref,
self,
source,
ReactCurrentOwner.current,
props,
);
};
用给定的参数来创建一个 React 元素,需要注意的是 ref
和 key
属于保留字段,不能作为 props 的属性传递。
ReactElement.createFactory
一个简单的工厂函数,用来方便的创建同类型的组件。
ReactElement.createFactory = function (type) {
var factory = ReactElement.createElement.bind(null, type);
// Expose the type on the factory and the prototype so that it can be
// easily accessed on elements. E.g. `<Foo />.type === Foo`.
// This should not be named `constructor` since this may not be the function
// that created the element, and it may not even be a constructor.
// Legacy hook TODO: Warn if this is accessed
factory.type = type;
return factory;
};
ReactElement.cloneAndReplaceKey
这个 API 没用过,可以用来替换一个 React 元素的保留属性: key 值。
ReactElement.cloneAndReplaceKey = function (oldElement, newKey) {
var newElement = ReactElement(
oldElement.type,
newKey,
oldElement.ref,
oldElement._self,
oldElement._source,
oldElement._owner,
oldElement.props,
);
return newElement;
};
ReactElement.cloneElement
cloneElement 方法和 createElement 基本一样,只是前者是通过现有的 React 元素来复制一个新的元素出来。
ReactElement.isValidElement
用来判断一个对象是不是一个合法的 React 元素。
ReactElement.isValidElement = function (object) {
return (
typeof object === "object" &&
object !== null &&
object.$$typeof === REACT_ELEMENT_TYPE
);
};
用到了上面一开始定义的 $$typeof 属性。
export
module.exports = ReactElement;
ReactElement 模块就是这么些东西了,主要定义了一个 React 元素应有的属性,以及操作元素的一些方法。
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本文发布于 2017年9月21日,最后更新于 2017年9月21日,距今已有 2937 天,文章内容可能已经过时。
React 源码浅析之 - ReactElement
https://blog.kisnows.com/zh-CN/2017/09/21/react-source-code-analyze-4/