Warn if calling setState in getChildContext

[blainekasten]

This code causes a maximum call stack:

getChildContext() {
  this.setState({
    foo: 'bar',
  });

  return {
    foo: this.state.foo,
  };
}

These sort of errors are hard to track down as the stack trace is deep in react. Is this a place we can give a user an invariant to let them know what they did wrong?

[jimfb]

We should warn if you call setState in getChildContext.

[raineroviir]

I'll do it!

[raineroviir]

Reproduced error (in the console)

[raineroviir]

What's a good way to start on this?

[jimfb]

@raineroviir Find the places that getChildContext is called, and the places that setState is defined, and warn if setState is called during getChildContext. I think we already do basically the same thing for calling setState within render, so you can use that as a thing to copy.

[raineroviir]

Ok, thank you!

[raineroviir]

Can you give me some more pointers.. I'm stuck figuring out whether setState is called during getChildContext.
I've tried Function.caller as in ReactComponent.prototype.setState.caller but got a type error. And trying to access the context with this hasn't revealed any clues either.

Here's the warning code shown when trying to setState within render which shows a pretty simple predicate for the warning, I'm just curious if I should try to do that as well?:

react/src/renderers/shared/reconciler/ReactUpdateQueue.js

Lines 49 to 51 in 3b96650

	ReactCurrentOwner.current == null,
	'%s(...): Cannot update during an existing state transition (such as ' +
	'within `render` or another component\'s constructor). Render methods ' +

[jimfb]

getChildContext might not be the immediate caller, so that won't work. You're going to need to set some state before and after getChildContext. We already warn when users call setState in render, take a look at that code.

[dan-weaver]

should ReactCurrentOwner.current be set during a getChildContext call? Looks like it currently is not.

as seen here

and

here

[jimfb]

@dan-weaver No, I think we only set the owner in render.

[gaearon]

FYI this is being worked on in #6121.

[SunflowerPKU]

Ok, thank you!

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