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.

從 issue 中連結的 React.Component 參考頁面的 setState 部分開始。檢視有關 callbacks 和 batching 的措辭，然後更新這些內容，使 callback 的執行時機以及 batch 後可用的 state 都明確無歧義；完成的標準是，範例不能讓人誤以為 callback 只會觀察它自己的個別更新。