Function readContext

lib/assets/react-source/development/react.js:15779–15826

(context, observedBits)

	  }
	}
	function readContext(context, observedBits) {
	  {
	    // This warning would fire if you read context inside a Hook like useMemo.
	    // Unlike the class check below, it's not enforced in production for perf.
	    if (isDisallowedContextReadInDEV) {
	      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().');
	    }
	  }

	  if (lastContextWithAllBitsObserved === context) ; else if (observedBits === false || observedBits === 0) ; else {
	    var resolvedObservedBits; // Avoid deopting on observable arguments or heterogeneous types.

	    if (typeof observedBits !== 'number' || observedBits === MAX_SIGNED_31_BIT_INT) {
	      // Observe all updates.
	      lastContextWithAllBitsObserved = context;
	      resolvedObservedBits = MAX_SIGNED_31_BIT_INT;
	    } else {
	      resolvedObservedBits = observedBits;
	    }

	    var contextItem = {
	      context: context,
	      observedBits: resolvedObservedBits,
	      next: null
	    };

	    if (lastContextDependency === null) {
	      if (!(currentlyRenderingFiber !== null)) {
	        {
	          throw 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;
	      currentlyRenderingFiber.dependencies = {
	        expirationTime: NoWork,
	        firstContext: contextItem,
	        responders: null
	      };
	    } else {
	      // Append a new context item.
	      lastContextDependency = lastContextDependency.next = contextItem;
	    }
	  }

	  return  context._currentValue ;
	}

	var UpdateState = 0;
	var ReplaceState = 1;