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[Experimental] API for reading context from within any render phase function #13139

Merged
merged 5 commits into from Jul 20, 2018
Merged

[Experimental] API for reading context from within any render phase function #13139

merged 5 commits into from Jul 20, 2018

Commits on Jul 20, 2018

  1. Store list of contexts on the fiber 

    Currently, context can only be read by a special type of component,
ContextConsumer. We want to add support to all fibers, including
classes and functional components.

Each fiber may read from one or more contexts. To enable quick, mono-
morphic access of this list, we'll store them on a fiber property.
    @acdlite
    acdlite committed Jul 20, 2018
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  2. Context.unstable_read 

    unstable_read can be called anywhere within the render phase. That
includes the render method, getDerivedStateFromProps, constructors,
functional components, and context consumer render props.

If it's called outside the render phase, an error is thrown.
    @acdlite
    acdlite committed Jul 20, 2018
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  3. Remove vestigial context cursor 

    Wasn't being used.
    @acdlite
    acdlite committed Jul 20, 2018
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  4. Split fiber.expirationTime into two separate fields 

    Currently, the `expirationTime` field represents the pending work of
both the fiber itself — including new props, state, and context — and of
any updates in that fiber's subtree.

This commit adds a second field called `childExpirationTime`. Now
`expirationTime` only represents the pending work of the fiber itself.
The subtree's pending work is represented by `childExpirationTime`.

The biggest advantage is it requires fewer checks to bailout on already
finished work. For most types of work, if the `expirationTime` does not
match the render expiration time, we can bailout immediately without
any further checks. This won't work for fibers that have
`shouldComponentUpdate` semantics (class components), for which we still
need to check for props and state changes explicitly.
    @acdlite
    acdlite committed Jul 20, 2018
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  5. Performance nits 

    Optimize `readContext` for most common case
    @acdlite
    acdlite committed Jul 20, 2018
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