New method to extract context value

text/0000-context-get-value.md

@@ -0,0 +1,131 @@
+- Start Date: 2018-04-27
+- RFC PR: (leave this empty)
+- React Issue: (leave this empty)
+
+# Summary
+
+Introduce a method for unwrapping context value in rendering function.
+
+# Basic example
+
+For example, two contexts are defined in an application:
+
+```javascript
+let ThemeContext = React.createContext('light');
+let LanguageContext = React.createContext('en-US');
+```
+
+Each context represents a single value and a pair of `Provider` and `Consumer`
+components to define and get access to this value. Usage details are covered in
+[RFC 0002][new-context-rfc].
+
+Current implementation may lead to the code like this:
+
+```javascript
+function ThemeAndLanguageConsumer({ children }) {
+  return (
+    <LanguageConsumer>
+      {language => (
+        <ThemeConsumer>
+          {theme => children({ language, theme })}
+        </ThemeConsumer>
+      )}
+    </LanguageConsumer>
+  );
+}
+```
+
+Suggested API extension allows to shrink the code above to something like this:
+
+```javascript
+function ThemeAndLanguageConsumer({ children }) {
+  let language = LanguageContext.getValue();
+  let theme = ThemeContext.getValue();
+  return children({ language, theme });
+}
+```
+
+# Motivation
+
+While render prop pattern is extremely useful due to the way how it separates
+concerns, it affects code complexity by adding nesting levels (basically, each
+new context used in a component means +2 levels of indent for its children).
+This introduces following concerns:
+
+- Poor readability ([even for React Core Team members][acdlite-tweet])
+- Each render method call creates new children functions (since they are inline)
+
+This RFC does not address these concerns for all possible pattern applications.
+It reduces related complexity for the components provided by React itself.
+
+# Detailed design
+
+Introduces a new method of `Context` entity:
+
+```javascript
+type Context<T> = {
+  // existing properties
+  Provider: Provider<T>,
+  Consumer: Consumer<T>,
+  // the suggested method
+  getValue: () => T,
+};
+```
+
+The method can be used in the same places where `Context.Consumer` assumed to
+be used.
+
+The upcoming React Suspense feature introduces [effects handling][effects] and
+aims at solving async data rendering problems. Despite on being synchronous by
+its nature, from the implementation perspective, context unwrapping can be
+represented as an effect that is handled in the way that wraps target component
+with necessary context consumers so the values become available during
+rendering phase. In this case, effects are thrown only once during initial
+rendering phase (for mounted component). Subsequent changes of context value
+should be covered by the fact the target component wrapped by consumers that
+combine their values in a single cache.
+
+# Drawbacks
+
+Since the new method brings yet another way for accessing context value (along
+with `Context.Consumer`) there is always a problem to choose the right option.
+It usually leads to subjective decisions based on developers experience and
+preferences.
+
+Talking about implementation in user space, there is a package that solves
+nesting problem by providing composition helpers (using render prop pattern or
+higher-order component):
+https://www.npmjs.com/package/react-compose-context-consumers
+
+# Alternatives
+
+Alternative design once has been suggested by Andrew Clark in Twitter:
+https://twitter.com/acdlite/status/971598256454098944
+
+The idea was to use higher-order components in favor of omitting render props,
+which seems to always be an alternative when it comes to comparing these two
+patterns in general. However, higher-order components bring its own impact as
+a tradeoff.
+
+# Adoption strategy
+
+Described API extension is not a breaking change. The extraction method can be
+used by developers in their new components, while being fully compatible with
+existing approach that imply using `<Consumer />`.
+
+# How we teach this
+
+Since suggested method has similar background to how React Suspense handles
+promises (going from callback style to pull-based code), the same terminology
+and flow can be used for describing the concept.
+
+# Unresolved questions
+
+- How React should propagate context value changes to consumers that are using
+`getValue()`?
+- Given suggested API being implemented, what should be the preferable way of
+getting context value?
+
+ [new-context-rfc]: https://github.com/reactjs/rfcs/blob/master/text/0002-new-version-of-context.md
+ [effects]: https://github.com/reactjs/react-basic#algebraic-effects
+ [acdlite-tweet]: https://twitter.com/acdlite/status/955955121979969537