Added a new lifecycle method componentWillReceiveContext()

[milesj]

I've been working with contexts a lot lately, and have run into a few issues where the current lifecycle isn't perfect. Specifically, I encounter certain situations where componentWillReceiveProps isn't triggered, even though the context has changed: #5756.

This new lifecycle may help with certain situations regarding contexts, like the following issues: #2517 #3973

[jimfb]

I don't see how this would help with #2517 or #3973 - they seem orthogonal.

More to the point, I don't think the solution is to add a new lifecycle here. The solution is to fix https://goo.gl/xWSesX to ensure componentWillReceiveProps gets called if the context changes. This appears to be the conclusion of #5756

[jimfb]

There is no guarantee that componentWillReceiveProps is called only when props change. The only guarantee is that componentWillReceiveProps will be called if props change.

[milesj]

I'm ok with either keeping componentWillReceiveContext or fixing componentWillReceiveProps. However, as it stants, the name of componentWillReceiveProps should probably just be componentWillReceive, since it's reading from 3 different sources.

Regardless, there is definitely a disconnect in how componentWillReceiveProps works, as logged here: #5756 (comment)

[jimfb]

@milesj My understanding is that the intended behavior is for componentWillReceiveProps to fire when context changes. Let's do that, since that's the spec'd behavior. We can figure out any renaming / reorganizing of lifecycles separately, but that's likely a more involved conversation.

[milesj]

Off the top of my head, we might be able to change this line here: https://github.com/facebook/react/blob/master/src/renderers/shared/reconciler/ReactCompositeComponent.js#L657

To do some kind of check that compares the previous and next contexts.

if (prevParentElement === nextParentElement && (nextContext === emptyObject || !this.diff(nextContext, prevContext)) {

Will give it a try.

[milesj]

Fixed the other issue in this pull request: #5787

[milesj]

Going to close this since #5787 was accepted.