Switch the Context to use the Parent tree instead of the Owner tree

[sebmarkbage]

I'm pretty convinced at this point that Contexts are more useful in the Parent tree than the Owner tree. It also opens up new use cases for parent->child communication that wasn't possible before.

[1st1]

I'd be a big +1 for this. This would really make it much easier to design complex components in react.

[syranide]

@sebmarkbage I'm not entirely sure exactly what problem contexts are meant to solve. But in the hypothetical context of reinvented styles (without selectors) I can imagine a benefit in being able to propagate hoveringOverTheBigButton (active/etc) to all descendants... and same for implicitly passing props from far up the hierarchy to far down (as is often requested). This should obviously be in the Parent tree then.

It seems that using a GUID style approach would be preferable for avoiding any and all conflicts, components would instead opt-in. Something like:

var Child = React.createClass({
  contextSources: {
    ancestorIsHovered: Parent...hovered
  },
  render: function() {
    return <div>{this.context.ancestorIsHovered ? 'Ancestor hovered' : 'Ancestor not hovered'}</div>;
  }
});

var Parent = React.createClass({
  childContextTypes: {
    hovered: React.PropTypes.boolean
  },
  getChildContext: function() {
    return {hovered: true};
  },
  render: function() {
    return <Child />;
  }
});

But again, I'm not exactly sure what contexts are meant to solve so I could be way way of the mark here.

[mjackson]

If someone who is more familiar with the React internals would care to put together a short summary of what needs to be done here, I'd be glad to give it a shot and make a PR. react-router depends heavily on context, and it's starting to break for some people's use cases.

Things I don't fully understand:

• What exactly is the difference between the parent and the owner trees? Is one DOM-based and the other a React thing?
• How might this change affect users currently using context?

[appsforartists]

I'd love to know more about when/if this might happen and what it will take to get there. I share @1st1's belief that this will make component/library composition better. In my particular case, I'm working on a library that wraps ReactRouter, and I'd like to be able to pass data down from my library, through the routes, and into the app's components. I can't presently do this without either a) changes by ReactRouter to support this directly, or b) the ability to pass a context into an app's already-created routes.

[sophiebits]

@sebmarkbage I assume we'll want to support reparenting someday… is there anything we need to keep in mind while doing this? Guessing not but wanted to check.

[mjackson]

Things I learned yesterday while talking to @sebmarkbage about this:

• The parent tree is made up of parent components, not necessarily DOM elements. The owner tree is made up of components that were on the bottom of the render stack at the time a given component was rendered.
• Changing context to use the parent tree gets us one step closer to being able to remove _owner entirely, which is a good thing.
• The vast majority of people currently using context should be unaffected by this change since the owner tree is by definition a subset of the parent tree.

[sebmarkbage]

cc @JSFB