useResource$()

This method allows you to generate computed values asynchronously. The asynchronous function passed as its first argument will be called when the component is mounted and when the tracked values change.

Just like all the use- methods, it must be called within the context of component$(), and all the hook rules apply.

const store = useStore({
  bar: 'foo'
})
const resource = useResource$(async (ctx) => {
  ctx.track(store, 'bar'); // the resource will rerun when store.bar changes.
  ctx.track(props, 'url'); // track() can be called multiple times, to track multiple values
  ctx.cleanup(() => {
    // In case the resource need to be cleaned up, this function will be called.
    // Allowing to clean resources like timers, subscriptions, fetch requests, etc.
  });

  // cleanup() can also be called multiple times.
  ctx.cleanup(() => console.log('cleanup'));

  // Resources can contain async computations
  const value = await expensiveCompute(store.bar, props.url);
  return value;
});

As we see in the example above, useResouce$() returns a ResourceReturn<T> object that works like a glorified, fully reactive promise, containing the data and the resource state.

The state resource.state can be one of the following strings:

  • 'pending' - the data is not yet available.
  • 'resolved' - the data is available.
  • 'rejected' - the data is not available due to an error or timeout.

The callback passed to useResource$() runs right after the useMount$() and useWatch$() callbacks complete. Please refer to the Lifecycle section for more details.

<Resource>

<Resource> is a component that renders its children when the resource is resolved, and renders a fallback when the resource is pending or rejected.

  <Resource
    value={weatherResource}
    onPending={() => <div>Loading...</div>}
    onRejected={() => <div>Failed to load weather</div>}
    onResolved={(weather) => {
      return <div>Temperature: {weather.temp}</div>;
    }}
  />

Notice that <Resource> is not necessary to use useResource$(). It is just a convenient way to render the resource state.

Example using useResource$() with <Resource>

Example showing how useResource is used to perform a fetch call to request the weather whenever the input city name changes.

export const Cmp = component$(() => {
  const store = useStore({
    city: '',
  });

  const weatherResource = useResource$<any>(async ({ track, cleanup }) => {
    const cityName = track(store, 'city');
    const abortController = new AbortController();
    cleanup(() => abortController.abort('cleanup'));
    const res = await fetch(`http://weatherdata.com?city=${cityName}`, {
      signal: abortController.signal,
    });
    const data = res.json();
    return data;
  });

  return (
    <div>
      <input name="city" onInput$={(ev: any) => (store.city = ev.target.value)} />
      <Resource
        value={weatherResource}
        onPending={() => <div>Loading...</div>}
        onResolved={(weather) => {
          return <div>Temperature: {weather.temp}</div>;
        }}
      />
    </div>
  );
});

Example without <Resource>

export const Cmp = component$(() => {
  const store = useStore({
    city: '',
  });

  const weatherResource = useResource$<any>(async ({ track, cleanup }) => {
    const cityName = track(store, 'city');
    const abortController = new AbortController();
    cleanup(() => abortController.abort('cleanup'));
    const res = await fetch(`http://weatherdata.com?city=${cityName}`, {
      signal: abortController.signal,
    });
    const data = res.json();
    return data;
  });

  return (
    <div>
      <input name="city" onInput$={(ev: any) => (store.city = ev.target.value)} />
      {weatherResource.state === 'pending' && (
        <div>Loading...</div>
      )}
      {weatherResource.state === 'resolved' && (
        <div>Temperature: {weatherResource.resolved.temp}</div>
      )}
    </div>
  );
});

NOTE We strongly recommend to use <Resource/> whenever possible, since this component will more efficiently avoid rerenders and it is also optimized for SSR.

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