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Creating an action runner

What you'll learn
  • Defining a custom action runner
  • Providing an extension property to your runner

Foscia tries to be agnostic of your data source, so sometimes you may require a custom runner to avoid code duplication.

This is a simple guide on defining a custom runner, but you may also inspire from any existing Foscia runners.

Goal​

Since Foscia is pagination agnostic, providing a first runner is not possible. Here is what we want our new first runner to do:

  • Limit the pagination to the first page and one record only
  • Fetch the first record using the one runner

In this example, we will admit a JSON:API is used with the following query parameters working:

  • page[number] describes the number of the page to fetch
  • page[size] describes the count of records to fetch (aka. limit)
tip

If you want to create a runner which does not use the action's context typing (such as defining a query parameter, etc.), you can ignore generic typing.

info

This guide is a runner version of the first enhancer described in the custom enhancers guide.

Using CLI​

npx foscia make runner first

Defining the function​

Our implementation of first will paginate the context and fetch one instance.

action/runners/first.ts
import {
  Action,
  ConsumeAdapter,
  ConsumeDeserializer,
  one,
} from '@foscia/core';
import { paginate } from '@foscia/jsonapi';

export default function first<C extends {}, RawData, Data, Deserialized>() {
  return (
    action: Action<C & ConsumeAdapter<RawData, Data> & ConsumeDeserializer<Data, Deserialized>>,
  ) => action.run(paginate({ number: 1, size: 1 }), one());
}
warning

Please note that when defining custom enhancers or runners, you should always correctly define generic types. This is very important as it will allow the context propagation through other enhancers and runners.

Using the function​

Once your runner is ready, you may use it like any other Foscia runner.

import { query } from '@foscia/core';
import action from './action';
import first from './action/runners/first';
import Post from './models/post';

const post = await action().run(query(Post), first());

Defining the extension​

Our current runner can only be used through an import and the use method of our action. To make it available for the builder pattern style calls, we must define an extension for it.

There is currently a limitation of the TypeScript language (Higher Order types are not available for now) which forces us to declare each extension manually. The goal of an extension definition is to get a type safe feature directly available on our action (and so provide autocomplete, context propagation, etc.).

Once your runner extension is ready, you will be able to use it as any other runners of Foscia.

action/runners/first.ts
import {
  Action,
  ConsumeAdapter,
  ConsumeDeserializer,
  InferConsumedInstance,
  WithParsedExtension,
  appendExtension,
  one,
} from '@foscia/core';
import { paginate } from '@foscia/jsonapi';

// Our previous enhancer code.
function first<C extends {}, RawData, Data, Deserialized>() {
  return (
    action: Action<C & ConsumeAdapter<RawData, Data> & ConsumeDeserializer<Data, Deserialized>>,
  ) => action.run(paginate({ number: 1, size: 1 }), one());
}

// New default export with typed `extension()`.
export default /* @__PURE__ */ appendExtension(
  'first',
  first,
  'run',
) as WithParsedExtension<typeof first, {
  // The extension typing.
  first<C extends {}, I extends InferConsumedInstance<C>, RawData, Data, Deserialized>(
    this: Action<C & ConsumeAdapter<RawData, Data> & ConsumeDeserializer<Data, Deserialized>>,
  ): Promise<I | null>;
}>;
warning

Here again, correctly typing our runner extension is really important to get context and action's extension propagation.