createResource

This creates a signal that returns the result of an async request.

type ResourceReturn<T> = [
  {
    (): T | undefined
    state: "unresolved" | "pending" | "ready" | "refreshing" | "errored"
    loading: boolean
    error: any
    latest: T | undefined
  },
  {
    mutate: (v: T | undefined) => T | undefined
    refetch: (info: unknown) => Promise<T> | T
  }
]

export type ResourceOptions<T, S = unknown> = {
  initialValue?: T
  name?: string
  deferStream?: boolean
  ssrLoadFrom?: "initial" | "server"
  storage?: (
    init: T | undefined
  ) => [Accessor<T | undefined>, Setter<T | undefined>]
  onHydrated?: (k: S | undefined, info: { value: T | undefined }) => void
}

function createResource<T, U = true>(
  fetcher: (
    k: U,
    info: { value: T | undefined; refetching: boolean | unknown }
  ) => T | Promise<T>,
  options?: ResourceOptions<T, U>
): ResourceReturn<T>

function createResource<T, U>(
  source: U | false | null | (() => U | false | null),
  fetcher: (
    k: U,
    info: { value: T | undefined; refetching: boolean | unknown }
  ) => T | Promise<T>,
  options?: ResourceOptions<T, U>
): ResourceReturn<T>

createResource takes an asynchronous fetcher function and returns a signal that is updated with the resulting data when the fetcher completes.

There are two ways to use createResource: you can pass the fetcher function as the sole argument, or you can additionally pass a source signal as the first argument. The source signal will retrigger the fetcher whenever it changes, and its value will be passed to the fetcher.

const [data, { mutate, refetch }] = createResource(fetchData)

const [data, { mutate, refetch }] = createResource(source, fetchData)

In these snippets, the fetcher is the function fetchData, and data() is undefined until fetchData finishes resolving. In the first case, fetchData will be called immediately. In the second, fetchData will be called as soon as source has any value other than false, null, or undefined. It will be called again whenever the value of source changes, and that value will always be passed to fetchData as its first argument.

You can call mutate to directly update the data signal (it works like any other signal setter). You can also call refetch to rerun the fetcher directly, and pass an optional argument to provide additional info to the fetcher e.g refetch(info).

data works like a normal signal getter: use data() to read the last returned value of fetchData. But it also has extra reactive properties: data.loading tells you if the fetcher has been called but not returned, and data.error tells you if the request has errored out; if so, it contains the error thrown by the fetcher. (Note: if you anticipate errors, you may want to wrap createResource in an ErrorBoundary.)

As of v1.4.0, data.latest will return the last returned value and won't trigger Suspense and transitions; if no value has been returned yet, data.latest acts the same as data(). This can be useful if you want to show the out-of-date data while the new data is loading.

loading, error, and latest are reactive getters and can be tracked.

The fetcher

The fetcher is the async function that you provide to createResource to actually fetch the data. It is passed two arguments: the value of the source signal (if provided), and an info object with two properties: value and refetching. The value property tells you the previously fetched value. The refetching property is true if the fetcher was triggered using the refetch function and false otherwise. If the refetch function was called with an argument (refetch(info)), refetching is set to that argument.

async function fetchData(source, { value, refetching }) {
  // Fetch the data and return a value.
  //`source` tells you the current value of the source signal;
  //`value` tells you the last returned value of the fetcher;
  //`refetching` is true when the fetcher is triggered by calling `refetch()`,
  // or equal to the optional data passed: `refetch(info)`
}

const [data, { mutate, refetch }] = createResource(getQuery, fetchData)

// read value
data()

// check if loading
data.loading

// check if errored
data.error

// directly set value without creating promise
mutate(optimisticValue)

// refetch the last request explicitly
refetch()

Version 1.4.0 and Later

v1.4.0

If you're using renderToStream, you can tell Solid to wait for a resource before flushing the stream using the deferStream option:

// fetches a user and streams content as soon as possible
const [user] = createResource(() => params.id, fetchUser)

// fetches a user but only streams content after this resource has loaded
const [user] = createResource(() => params.id, fetchUser, {
  deferStream: true,
})

v1.5.0

1. We've added a new state field which covers a more detailed view of the Resource state beyond loading and error. You can now check whether a Resource is unresolved, pending, ready, refreshing, or error.

2. When server rendering resources especially when fetching when embedding Solid in other system that fetch before render, you might want to initiate the resource with this prefetched value instead of fetching again and having the resource serialize it in it's own state. You can use the new ssrLoadFrom option for this. Instead of using the default server value, you can pass initial and the resource will use initialValue as if it were the result of the first fetch for both SSR and hydration.

const [data, { mutate, refetch }] = createResource(() => params.id, fetchUser, {
  initialValue: preloadedData,
  ssrLoadFrom: "initial",
})

3. Resources can be set with custom defined storage with the same signature as a Signal by using the storage option. For example using a custom reconciling store could be done this way:

function createDeepSignal<T>(value: T): Signal<T> {
  const [store, setStore] = createStore({
    value,
  })
  return [
    () => store.value,
    (v: T) => {
      const unwrapped = unwrap(store.value)
      typeof v === "function" && (v = v(unwrapped))
      setStore("value", reconcile(v))
      return store.value
    },
  ] as Signal<T>
}

const [resource] = createResource(fetcher, {
  storage: createDeepSignal,
})

This option is still experimental and may change in the future.

Options

The createResource function takes an optional third argument, an options object. The options are: