Documentation should state that React preserves setState callback this context

I just recently found out that setState's callback calls with the correct this context per this line in the React source. Nowhere in the documentation is it stated that React internally calls callbacks with context. This should be made clear as many assume it doesn't, leading to unnecessary measures such as arrow functions, explicit binding, etc.

The lack of explicit documentation also creates confusion with other things that do require binding, such as traditional class methods used as event handlers.

先从所引用的 ReactFiberUpdateQueue.js 行开始，确认 setState 回调如何接收上下文，然后在 React 文档中找到 setState 回调的文档。更新指南，说明回调的 this 上下文，并将其与用作事件处理器的类方法区分开来。