Context 提供一种方法，让一个componet tree里面传递数据无需通过 props 从上到下传递。
React.createContext('light');打印一下
$$typeof: Symbol(react.context)
Consumer: {$$typeof: Symbol(react.context), _context: {…}, _calculateChangedBits: null, …}
Provider: {$$typeof: Symbol(react.provider), _context: {…}}
_calculateChangedBits: null
_currentRenderer: null
_currentRenderer2: null
_currentValue: "light"
_currentValue2: "light"
_threadCount: 0我们先借个小例子说明一下
//传统做法 通过props 一级一级往下传
class App extends React.Component {
render() {
return <Toolbar theme="dark" />;
}
}
function Toolbar(props) {
// The Toolbar component must take an extra "theme" prop
// and pass it to the ThemedButton. This can become painful
// if every single button in the app needs to know the theme
// because it would have to be passed through all components.
return (
<div>
<ThemedButton theme={props.theme} />
</div>
);
}
class ThemedButton extends React.Component {
render() {
return <Button theme={this.props.theme} />;
}
}如果使用 context ，避免一级一级传递数据
// Context lets us pass a value deep into the component tree
// without explicitly threading it through every component.
// Create a context for the current theme (with "light" as the default).
const ThemeContext = React.createContext('light');
class App extends React.Component {
render() {
// Use a Provider to pass the current theme to the tree below.
// Any component can read it, no matter how deep it is.
// In this example, we're passing "dark" as the current value.
return (
<ThemeContext.Provider value="dark">
<Toolbar />
</ThemeContext.Provider>
);
}
}
// A component in the middle doesn't have to
// pass the theme down explicitly anymore.
function Toolbar(props) {
return (
<div>
<ThemedButton />
</div>
);
}
class ThemedButton extends React.Component {
// Assign a contextType to read the current theme context.
// React will find the closest theme Provider above and use its value.
// In this example, the current theme is "dark".
static contextType = ThemeContext;
render() {
return <Button theme={this.context} />;
}
const MyContext = React.createContext(defaultValue);创建一个 context , 当一个 component 订阅这个context，组件将匹配包裹它的最近的 Provider 的 value 值。 如果没有 Provider 将读取默认值。
<MyContext.Provider value={/* some value */}>Provider 可以让后代组件可以订阅 value 值的变化,
class MyClass extends React.Component {
componentDidMount() {
let value = this.context;
/* perform a side-effect at mount using the value of MyContext */
}
componentDidUpdate() {
let value = this.context;
/* ... */
}
componentWillUnmount() {
let value = this.context;
/* ... */
}
render() {
let value = this.context;
/* render something based on the value of MyContext */
}
}
MyClass.contextType = MyContext;contextType 可以绑定一个 context object 到组件上，这样就可以读取最近的 Provider 的 Value 值。
<MyContext.Consumer>
{value => /* render something based on the context value */}
</MyContext.Consumer>这里 {value => {}} 其实就是一个 function component 组件, 可以随时订阅 value 的变化
晓得了基本用法，让我们来看看源码：
// context
function createContext(defaultValue, calculateChangedBits) {
if (calculateChangedBits === undefined) {
calculateChangedBits = null;
} else {
{
!(calculateChangedBits === null || typeof calculateChangedBits === 'function') ? warningWithoutStack$1(false, 'createContext: Expected the optional second argument to be a ' + 'function. Instead received: %s', calculateChangedBits) : void 0;
}
}
var context = {
// Symbol.for('react.context')
$$typeof: REACT_CONTEXT_TYPE,
_calculateChangedBits: calculateChangedBits,
// As a workaround to support multiple concurrent renderers, we categorize
// some renderers as primary and others as secondary. We only expect
// there to be two concurrent renderers at most: React Native (primary) and
// Fabric (secondary); React DOM (primary) and React ART (secondary).
// Secondary renderers store their context values on separate fields.
_currentValue: defaultValue,
_currentValue2: defaultValue,
// Used to track how many concurrent renderers this context currently
// supports within in a single renderer. Such as parallel server rendering.
_threadCount: 0,
// These are circular
Provider: null,
Consumer: null
};
context.Provider = {
// Symbol.for('react.provider')
$$typeof: REACT_PROVIDER_TYPE,
_context: context
};
var hasWarnedAboutUsingNestedContextConsumers = false;
var hasWarnedAboutUsingConsumerProvider = false;
{
// A separate object, but proxies back to the original context object for
// backwards compatibility. It has a different $$typeof, so we can properly
// warn for the incorrect usage of Context as a Consumer.
var Consumer = {
$$typeof: REACT_CONTEXT_TYPE,
_context: context,
_calculateChangedBits: context._calculateChangedBits
};
// $FlowFixMe: Flow complains about not setting a value, which is intentional here
Object.defineProperties(Consumer, {
Provider: {
get: function () {
if (!hasWarnedAboutUsingConsumerProvider) {
hasWarnedAboutUsingConsumerProvider = true;
warning$1(false, 'Rendering <Context.Consumer.Provider> is not supported and will be removed in ' + 'a future major release. Did you mean to render <Context.Provider> instead?');
}
return context.Provider;
},
set: function (_Provider) {
context.Provider = _Provider;
}
},
_currentValue: {
get: function () {
return context._currentValue;
},
set: function (_currentValue) {
context._currentValue = _currentValue;
}
},
_currentValue2: {
get: function () {
return context._currentValue2;
},
set: function (_currentValue2) {
context._currentValue2 = _currentValue2;
}
},
_threadCount: {
get: function () {
return context._threadCount;
},
set: function (_threadCount) {
context._threadCount = _threadCount;
}
},
Consumer: {
get: function () {
if (!hasWarnedAboutUsingNestedContextConsumers) {
hasWarnedAboutUsingNestedContextConsumers = true;
warning$1(false, 'Rendering <Context.Consumer.Consumer> is not supported and will be removed in ' + 'a future major release. Did you mean to render <Context.Consumer> instead?');
}
return context.Consumer;
}
}
});
// $FlowFixMe: Flow complains about missing properties because it doesn't understand defineProperty
context.Consumer = Consumer;
}
{
context._currentRenderer = null;
context._currentRenderer2 = null;
}
return context;
}源码还是相对简单理解的，具体关联稍后再说。