// 创建context
import { createContext } from 'react';

const LevelContext = createContext(1);

// 使用context
import { useContext } from 'react';

function Index() {  
const level = useContext(LevelContext);  
// ...  
return <LevelContext.Provider value={level}>
        <Child></Child>
      </LevelContext.Provider>

}

function Child() {  
const level = useContext(LevelContext);  
// ...  
return <>
{level}
</>
}

createContext 生产Context

function createContext(defaultValue) {
  // TODO: Second argument used to be an optional `calculateChangedBits`
  // function. Warn to reserve for future use?
  var context = {
    $$typeof: REACT_CONTEXT_TYPE,
    // As a workaround to support multiple concurrent renderers, we categorize
    // some renderers as primary and others as secondary. We only expect
    // there to be two concurrent renderers at most: React Native (primary) and
    // Fabric (secondary); React DOM (primary) and React ART (secondary).
    // Secondary renderers store their context values on separate fields.
    _currentValue: defaultValue,
    _currentValue2: defaultValue,
    // Used to track how many concurrent renderers this context currently
    // supports within in a single renderer. Such as parallel server rendering.
    _threadCount: 0,
    // These are circular
    Provider: null,
    Consumer: null,
    // Add these to use same hidden class in VM as ServerContext
    _defaultValue: null,
    _globalName: null
  };
  context.Provider = {
    $$typeof: REACT_PROVIDER_TYPE,
    _context: context
  };
  var hasWarnedAboutUsingNestedContextConsumers = false;
  var hasWarnedAboutUsingConsumerProvider = false;
  var hasWarnedAboutDisplayNameOnConsumer = false;

  {
    // A separate object, but proxies back to the original context object for
    // backwards compatibility. It has a different $$typeof, so we can properly
    // warn for the incorrect usage of Context as a Consumer.
    var Consumer = {
      $$typeof: REACT_CONTEXT_TYPE,
      _context: context
    }; // $FlowFixMe[prop-missing]: Flow complains about not setting a value, which is intentional here

    Object.defineProperties(Consumer, {
      Provider: {
        get: function () {
          if (!hasWarnedAboutUsingConsumerProvider) {
            hasWarnedAboutUsingConsumerProvider = true;

            error('Rendering <Context.Consumer.Provider> is not supported and will be removed in ' + 'a future major release. Did you mean to render <Context.Provider> instead?');
          }

          return context.Provider;
        },
        set: function (_Provider) {
          context.Provider = _Provider;
        }
      },
      _currentValue: {
        get: function () {
          return context._currentValue;
        },
        set: function (_currentValue) {
          context._currentValue = _currentValue;
        }
      },
      _currentValue2: {
        get: function () {
          return context._currentValue2;
        },
        set: function (_currentValue2) {
          context._currentValue2 = _currentValue2;
        }
      },
      _threadCount: {
        get: function () {
          return context._threadCount;
        },
        set: function (_threadCount) {
          context._threadCount = _threadCount;
        }
      },
      Consumer: {
        get: function () {
          if (!hasWarnedAboutUsingNestedContextConsumers) {
            hasWarnedAboutUsingNestedContextConsumers = true;

            error('Rendering <Context.Consumer.Consumer> is not supported and will be removed in ' + 'a future major release. Did you mean to render <Context.Consumer> instead?');
          }

          return context.Consumer;
        }
      },
      displayName: {
        get: function () {
          return context.displayName;
        },
        set: function (displayName) {
          if (!hasWarnedAboutDisplayNameOnConsumer) {
            warn('Setting `displayName` on Context.Consumer has no effect. ' + "You should set it directly on the context with Context.displayName = '%s'.", displayName);

            hasWarnedAboutDisplayNameOnConsumer = true;
          }
        }
      }
    }); // $FlowFixMe[prop-missing]: Flow complains about missing properties because it doesn't understand defineProperty

    context.Consumer = Consumer;
  }

  {
    context._currentRenderer = null;
    context._currentRenderer2 = null;
  }

  return context;
}

Child 消费Context

useContext进入readContextForConsumer

function readContextForConsumer(consumer, context) {
  var value = context._currentValue ;

  if (lastFullyObservedContext === context) ; else {
    var contextItem = {
      context: context,
      memoizedValue: value,
      next: null
    };

    if (lastContextDependency === null) {
      if (consumer === null) {
        throw new Error('Context can only be read while React is rendering. ' + 'In classes, you can read it in the render method or getDerivedStateFromProps. ' + 'In function components, you can read it directly in the function body, but not ' + 'inside Hooks like useReducer() or useMemo().');
      } // This is the first dependency for this component. Create a new list.


      lastContextDependency = contextItem;
      consumer.dependencies = {
        lanes: NoLanes,
        firstContext: contextItem
      };
    } else {
      // Append a new context item.
      lastContextDependency = lastContextDependency.next = contextItem;
    }
  }

  return value;
}

Child fiber的dependencies = { lanes: NoLanes, firstContext: contextItem };

关联上了Context

注意dependencies，后面conText更新的时候就是根据这个字段来找到Child的

Index 找到消费者

beginWork LevelContext.Provider 进入updateContextProvider

function updateContextProvider (current, workInProgress, renderLanes) {
// ...
pushProvider(workInProgress, context, newValue);
{
    if (oldProps !== null) {
      var oldValue = oldProps.value;

      if (objectIs(oldValue, newValue)) {
        // No change. Bailout early if children are the same.
        if (oldProps.children === newProps.children && !hasContextChanged()) {
          return bailoutOnAlreadyFinishedWork(current, workInProgress, renderLanes);
        }
      } else {
        // The context value changed. Search for matching consumers and schedule
        // them to update.
        propagateContextChange(workInProgress, context, renderLanes);
      }
    }
  }

  var newChildren = newProps.children;
  reconcileChildren(current, workInProgress, newChildren, renderLanes);
  return workInProgress.child;
}

pushProvider 会将上下文推入，并且_currentValue取新值

function pushProvider(providerFiber, context, nextValue) {
  {
    push(valueCursor, context._currentValue, providerFiber);
    context._currentValue = nextValue;

    {
      push(rendererCursorDEV, context._currentRenderer, providerFiber);

      if (context._currentRenderer !== undefined && context._currentRenderer !== null && context._currentRenderer !== rendererSigil) {
        error('Detected multiple renderers concurrently rendering the ' + 'same context provider. This is currently unsupported.');
      }

      context._currentRenderer = rendererSigil;
    }
  }
}

如果value没有发生变化，则和普通的fiber一样

发生变化了进入propagateContextChange

function propagateContextChange(workInProgress, context, renderLanes) {
  {
    propagateContextChange_eager(workInProgress, context, renderLanes);
  }
}

function propagateContextChange_eager(workInProgress, context, renderLanes) {

  var fiber = workInProgress.child;

  if (fiber !== null) {
    // Set the return pointer of the child to the work-in-progress fiber.
    fiber.return = workInProgress;
  }

  while (fiber !== null) {
    var nextFiber = void 0; // Visit this fiber.

    var list = fiber.dependencies;

    if (list !== null) {
      nextFiber = fiber.child;
      var dependency = list.firstContext;

      while (dependency !== null) {
        // Check if the context matches.
        if (dependency.context === context) {
          // Match! Schedule an update on this fiber.
          if (fiber.tag === ClassComponent) {
            // Schedule a force update on the work-in-progress.
            var lane = pickArbitraryLane(renderLanes);
            var update = createUpdate(lane);
            update.tag = ForceUpdate; // TODO: Because we don't have a work-in-progress, this will add the
            // update to the current fiber, too, which means it will persist even if
            // this render is thrown away. Since it's a race condition, not sure it's
            // worth fixing.
            // Inlined `enqueueUpdate` to remove interleaved update check

            var updateQueue = fiber.updateQueue;

            if (updateQueue === null) ; else {
              var sharedQueue = updateQueue.shared;
              var pending = sharedQueue.pending;

              if (pending === null) {
                // This is the first update. Create a circular list.
                update.next = update;
              } else {
                update.next = pending.next;
                pending.next = update;
              }

              sharedQueue.pending = update;
            }
          }

          fiber.lanes = mergeLanes(fiber.lanes, renderLanes);
          var alternate = fiber.alternate;

          if (alternate !== null) {
            alternate.lanes = mergeLanes(alternate.lanes, renderLanes);
          }

          scheduleContextWorkOnParentPath(fiber.return, renderLanes, workInProgress); // Mark the updated lanes on the list, too.

          list.lanes = mergeLanes(list.lanes, renderLanes); // Since we already found a match, we can stop traversing the
          // dependency list.

          break;
        }

        dependency = dependency.next;
      }
    } else if (fiber.tag === ContextProvider) {
      // Don't scan deeper if this is a matching provider
      nextFiber = fiber.type === workInProgress.type ? null : fiber.child;
    } else if (fiber.tag === DehydratedFragment) {
      // If a dehydrated suspense boundary is in this subtree, we don't know
      // if it will have any context consumers in it. The best we can do is
      // mark it as having updates.
      var parentSuspense = fiber.return;

      if (parentSuspense === null) {
        throw new Error('We just came from a parent so we must have had a parent. This is a bug in React.');
      }

      parentSuspense.lanes = mergeLanes(parentSuspense.lanes, renderLanes);
      var _alternate = parentSuspense.alternate;

      if (_alternate !== null) {
        _alternate.lanes = mergeLanes(_alternate.lanes, renderLanes);
      } // This is intentionally passing this fiber as the parent
      // because we want to schedule this fiber as having work
      // on its children. We'll use the childLanes on
      // this fiber to indicate that a context has changed.


      scheduleContextWorkOnParentPath(parentSuspense, renderLanes, workInProgress);
      nextFiber = fiber.sibling;
    } else {
      // Traverse down.
      nextFiber = fiber.child;
    }

    if (nextFiber !== null) {
      // Set the return pointer of the child to the work-in-progress fiber.
      nextFiber.return = fiber;
    } else {
      // No child. Traverse to next sibling.
      nextFiber = fiber;

      while (nextFiber !== null) {
        if (nextFiber === workInProgress) {
          // We're back to the root of this subtree. Exit.
          nextFiber = null;
          break;
        }

        var sibling = nextFiber.sibling;

        if (sibling !== null) {
          // Set the return pointer of the sibling to the work-in-progress fiber.
          sibling.return = nextFiber.return;
          nextFiber = sibling;
          break;
        } // No more siblings. Traverse up.


        nextFiber = nextFiber.return;
      }
    }

    fiber = nextFiber;
  }
}

根据dependencies找到 Child fiber 然后进入scheduleContextWorkOnParentPath

function scheduleContextWorkOnParentPath(parent, renderLanes, propagationRoot) {
  // Update the child lanes of all the ancestors, including the alternates.
  var node = parent;

  while (node !== null) {
    var alternate = node.alternate;

    if (!isSubsetOfLanes(node.childLanes, renderLanes)) {
      node.childLanes = mergeLanes(node.childLanes, renderLanes);

      if (alternate !== null) {
        alternate.childLanes = mergeLanes(alternate.childLanes, renderLanes);
      }
    } else if (alternate !== null && !isSubsetOfLanes(alternate.childLanes, renderLanes)) {
      alternate.childLanes = mergeLanes(alternate.childLanes, renderLanes);
    } else ;

    if (node === propagationRoot) {
      break;
    }

    node = node.return;
  }

  {
    if (node !== propagationRoot) {
      error('Expected to find the propagation root when scheduling context work. ' + 'This error is likely caused by a bug in React. Please file an issue.');
    }
  }
}

Child 向上一直到Index，中间的childLanes都标记为2。这样beginWith才能进到Child。

之后进入reconcileChildren 和 正常的Index fiber一样，生成新的子fiber

Child消费新的Context

useContext拿到context._currentValue，然后重新渲染