onClick = () => {
    this.setState({ quantity: this.state.quantity + 1 });
    this.setState({ quantity: this.state.quantity + 1 });
}
// react中，這個(gè)方法最終會(huì)變成
Object.assign(
    previousState,
    {quantity: state.quantity + 1},
    {quantity: state.quantity + 1},
    ...
)

ReactComponent.prototype.setState = function(partialState, callback) {
  this.updater.enqueueSetState(this, partialState);
  if (callback) {
    this.updater.enqueueCallback(this, callback, 'setState');
  }
}

enqueueSetState: function(publicInstance, partialState) {
    var internalInstance = getInternalInstanceReadyForUpdate(
      publicInstance,
      'setState',
    );
    if (!internalInstance) {
      return;
    }
    var queue =
      internalInstance._pendingStateQueue ||
      (internalInstance._pendingStateQueue = []);
    queue.push(partialState);
    enqueueUpdate(internalInstance);
}
enqueueCallback: function(publicInstance, callback, callerName) {
    ReactUpdateQueue.validateCallback(callback, callerName);
    var internalInstance = getInternalInstanceReadyForUpdate(publicInstance);
    if (!internalInstance) {
      return null;
    }
    if (internalInstance._pendingCallbacks) {
      internalInstance._pendingCallbacks.push(callback);
    } else {
      internalInstance._pendingCallbacks = [callback];
    }
    enqueueUpdate(internalInstance);
  }

function enqueueUpdate(component) {
  ensureInjected();
  if (!batchingStrategy.isBatchingUpdates) {
    batchingStrategy.batchedUpdates(enqueueUpdate, component);
    return;
  }
  dirtyComponents.push(component);
  if (component._updateBatchNumber == null) {
    component._updateBatchNumber = updateBatchNumber + 1;
  }
}

var transaction = new ReactDefaultBatchingStrategyTransaction();
var ReactDefaultBatchingStrategy = {
  isBatchingUpdates: false,
  batchedUpdates: function(callback, a, b, c, d, e) {
    var alreadyBatchingUpdates = ReactDefaultBatchingStrategy.isBatchingUpdates;
    ReactDefaultBatchingStrategy.isBatchingUpdates = true;
    if (alreadyBatchingUpdates) {
      return callback(a, b, c, d, e);
    } else {
      return transaction.perform(callback, null, a, b, c, d, e);
    }
  },
};

var RESET_BATCHED_UPDATES = {
  initialize: emptyFunction,
  close: function() {
    ReactDefaultBatchingStrategy.isBatchingUpdates = false;
  },
};
var FLUSH_BATCHED_UPDATES = {
  initialize: emptyFunction,
  close: ReactUpdates.flushBatchedUpdates.bind(ReactUpdates),
};

var flushBatchedUpdates = function() {
  while (dirtyComponents.length || asapEnqueued) {
    if (dirtyComponents.length) {
      var transaction = ReactUpdatesFlushTransaction.getPooled();
      transaction.perform(runBatchedUpdates, null, transaction);
      ReactUpdatesFlushTransaction.release(transaction);
    }
    if (asapEnqueued) {
      asapEnqueued = false;
      var queue = asapCallbackQueue;
      asapCallbackQueue = CallbackQueue.getPooled();
      queue.notifyAll();
      CallbackQueue.release(queue);
    }
  }
};
function runBatchedUpdates(transaction) {
  var len = transaction.dirtyComponentsLength;
  dirtyComponents.sort(mountOrderComparator);
  updateBatchNumber++;
  for (var i = 0; i < len; i++) {
    // dirtyComponents中取出一個(gè)component
    var component = dirtyComponents[i];
    // 取出dirtyComponent中的未執(zhí)行的callback
    var callbacks = component._pendingCallbacks;
    component._pendingCallbacks = null;
    var markerName;
    if (ReactFeatureFlags.logTopLevelRenders) {
      var namedComponent = component;
      if (component._currentElement.type.isReactTopLevelWrapper) {
        namedComponent = component._renderedComponent;
      }
      markerName = 'React update: ' + namedComponent.getName();
      console.time(markerName);
    }
    // 執(zhí)行updateComponent
    ReactReconciler.performUpdateIfNecessary(
      component,
      transaction.reconcileTransaction,
      updateBatchNumber,
    );
    // 執(zhí)行dirtyComponent中之前未執(zhí)行的callback
    if (callbacks) {
      for (var j = 0; j < callbacks.length; j++) {
        transaction.callbackQueue.enqueue(
          callbacks[j],
          component.getPublicInstance(),
        );
      }
    }
  }
}

performUpdateIfNecessary: function(
    internalInstance,
    transaction,
    updateBatchNumber,
  ) {
    internalInstance.performUpdateIfNecessary(transaction);
  },
};

  performUpdateIfNecessary: function(transaction) {
    if (this._pendingElement != null) {
      ReactReconciler.receiveComponent(
        this,
        this._pendingElement,
        transaction,
        this._context,
      );
    } else if (this._pendingStateQueue !== null || this._pendingForceUpdate) {
      this.updateComponent(
        transaction,
        this._currentElement,
        this._currentElement,
        this._context,
        this._context,
      );
    } else {
      this._updateBatchNumber = null;
    }
  },

var Transaction = require('Transaction');
var emptyFunction = require('emptyFunction');
var RESET_BATCHED_UPDATES = {
  initialize: emptyFunction,
  close: function() {
    ReactDefaultBatchingStrategy.isBatchingUpdates = false;
  },
};
var FLUSH_BATCHED_UPDATES = {
  initialize: emptyFunction,
  close: ReactUpdates.flushBatchedUpdates.bind(ReactUpdates),
};
var TRANSACTION_WRAPPERS = [FLUSH_BATCHED_UPDATES, RESET_BATCHED_UPDATES];
function ReactDefaultBatchingStrategyTransaction() {
  this.reinitializeTransaction();
}
Object.assign(ReactDefaultBatchingStrategyTransaction.prototype, Transaction, {
  getTransactionWrappers: function() {
    return TRANSACTION_WRAPPERS;
  },
});

dispatchEvent: function(topLevelType, nativeEvent) {
    if (!ReactEventListener._enabled) {
      return;
    }
    var bookKeeping = TopLevelCallbackBookKeeping.getPooled(
      topLevelType,
      nativeEvent,
    );
    try {
      // Event queue being processed in the same cycle allows
      // `preventDefault`.
      ReactUpdates.batchedUpdates(handleTopLevelImpl, bookKeeping);
    } finally {
      TopLevelCallbackBookKeeping.release(bookKeeping);
    }
}