value 属性总是被赋值为新的对象：
class App extends React.Component {
render() {
return (
<MyContext.Provider value={{something: 'something'}}>
<Toolbar />
</MyContext.Provider>
);
}
}为了防止这种情况，将 value 状态提升到父节点的 state 里：
class App extends React.Component {
constructor(props) {
super(props);
this.state = {
value: {something: 'something'},
};
}
render() {
return (
<Provider value={this.state.value}>
<Toolbar />
</Provider>
);
}
}
