setState() callback function: potential confusion of call order

This paragraph from the React.Component reference page:

setState() does not always immediately update the component. It may batch or defer the update until later. This makes reading this.state right after calling setState() a potential pitfall. Instead, use componentDidUpdate or a setState callback (setState(updater, callback)), either of which are guaranteed to fire after the update has been applied.

can create the impression that instead of doing:

this.setState({ value: 5 });
console.log(this.state.value); // => not necessarily 5

you can use the callback function to safely read the state back:

this.setState({ value: 5 }, () => console.log(this.state.value)); // => "guaranteed" 5

(especially if you draw a parallel between the callback function and the updater function)

and it may actually work in some cases where there's no setState batching, but as more calls get batched in the future, these constructs may generate unintended side-effects.

Instead, we could clarify that all callback functions for a batch of setState() calls get called after the entire batch has been processed.

从 issue 中链接的 React.Component 参考页面的 setState 部分开始。检查有关 callbacks 和 batching 的措辞，然后更新这些内容，使 callback 的执行时机以及 batch 后可用的 state 都明确无歧义；完成的标准是，示例不能让人误以为 callback 只会观察它自己的单个更新。