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 refkey 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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