Field resolution hooks

In many cases, it might be desired to perform some aditional action before or after some field is resolved. Most common use-case could be authorization of user.

Hooks are special kind of functions added to field with @Before or @After decorator.

@Before and @After hooks

Let's say we want to send informations about useage of some field of our graphql api to analitycs server. We could achieve that with:

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import { ObjectType, Field, After } from 'decapi'

​

@ObjectType()

class Person {

  @Field()

  id: number

​

  @Field()

  @After(() => {

    loggingService.sendLog(`Some user may have been removed`)

  })

  remove(): boolean {

    const isDeleted = userService.removeById(this.id)

    return isDeleted

  }

}

Using resolver informations inside hook function

Arguments passed to resolver are exactly the same, as ones passed to native graphql resolver funtion. Signature of a @Before hook function is

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({source, arguments, context, info}) => void | Promise<any>

for @After, it is:

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(resolvedValue, {source, arguments, context, info}) => void | Promise<any>

In case, we'd like to add id of removed user to logs, we'd change our code to:

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@ObjectType()

class Person {

  @Field()

  id: number

​

  @Field()

  @After((isDeleted, { source }) => {

    if (isDeleted) {

      loggingService.sendLog(`User with id ${this.id} was removed`)

      //loggingService.sendLog(`User with id ${source.id} was removed`) could be used alternatively

    }

  })

  remove(): boolean {

    const isDeleted = userService.removeById(this.id)

    return isDeleted

  }

}

Notes

• If one field has many hooks of the same type - they're executed in parallel.
• Field resolver is not called until all @Before hooks are resolved