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A hook from the Recoil Sync library to define an external store for atoms to sync with using the syncEffect() atom effect.

function useRecoilSync(options: {
storeKey?: string,

read?: ReadItem,
write?: WriteItems,
listen?: ListenToItems,
}): void

The storeKey is used to match up which atoms should sync with this external store.

Read Interface

The read() callback defines how to read an item from the external store. This is used when attempting to initialize the atom value based on the external store. It may also be called from other complex mappings.

type ReadItem = ItemKey =>
| DefaultValue
| Promise<DefaultValue | mixed>
| Loadable<DefaultValue | mixed>
| mixed;

You may return the actual value of the item in the store. If the item is not set or available in the store you may return DefaultValue. It is also possible to return an async Promise to the value if it requires an async operation to read from the store. You can also give a Loadable representation, which is useful for providing an error state if necessary.

Write Interface

The write() callback is called when atom states are mutated so you can define how to write these changes to the external store.

type ItemDiff = Map<ItemKey, DefaultValue | any>;
type ItemSnapshot = Map<ItemKey, DefaultValue | mixed>;

type WriteItems = ({
diff: ItemDiff,
allItems: ItemSnapshot,
}) => void;

The write() callback is provided some named parameters:

  • diff - A map of item keys and their new values that have changed for this atomic transaction.
  • allItems - A map of the keys and values for all items that have been used in this store.

The value for an item may be a DefaultValue object, which means that the item should be reset or deleted. If the syncEffect() specifies the syncDefault option, then the actual default values will be provided instead of the DefaultValue placeholder object.

Listen Interface

The listen() callback allows you to subscribe to async updates from the external store and mutate the atom state to keep them in sync.

type UpdateItem = <T>(ItemKey, DefaultValue | T) => void;
type UpdateAllKnownItems = ItemSnapshot => void;

type ListenToItems = ({
updateItem: UpdateItem,
updateAllKnownItems: UpdateAllKnownItems,
}) => void | (() => void);

The listen() callback is provided some callbacks in its parameter that allow you to update items to a new value. Any atoms which have read from those items are "subscribed" and will have their state updated by reading from the updated items.

  • updateItem() - This will update the value of a single item by providing a key and value. If the value is DefaultValue then it will reset the item to the default. This only updates a single item, other items will not be affected.
  • updateAllKnownItems() - This will update multiple items by providing a Map of item keys and values. Again, if the value of any is DefaultValue then it will reset those items. This function will update all of the known items that have been read by atoms syncing with this store. That means if an item key is not included in the provided Map then that item will be reset to default.

You can return a callback handler function from your listen() implementation that will be called when the store effect is cleaned up. This can be used to cleanup your subscription to the external store.


See the "Implementing a Store" guide for examples.