Apollo GraphQL

https://www.apollographql.com/docs/react/

June 25, 2020

On this page

Queries
- useQuery hook
- Example component
Mutations
Fragments
Resolvers

Queries

jsx

import gql from 'graphql-tag'
const GET_DOGS = gql`
  {
    dogs {
      id
      breed
    }
  }
`

useQuery hook

useQuery Docs

import { useQuery } from '@apollo/react-hooks'
const { data, loading, error } = useQuery(GET_DOGS)
// useQuery with a `variable`
const { data, loading, error } = useQuery(GET_DOG_PHOTO, {
  variables: { breed },
})

Example component

jsx

const Dogs = ({ onDogSelected }) => {
  const { data, loading, error } = useQuery(GET_DOGS)
  if (loading) {
    return <Loader />
  }
  if (error) {
    return <p>Error {JSON.stringify(error, null, 2)}</p>
  }
  return (
    <select name="dog" onChange={onDogSelected}>
      {data.dogs.map((dog) => (
        <option key={dog.id} value={dog.breed}>
          {dog.breed}
        </option>
      ))}
    </select>
  )
}

Mutations

Mutations docs

Queries enable clients to fetch data, but not to modify data. To enable clients to modify data, our schema needs to define some mutations.

useMutation hook

useMutation Docs

import { useMutation } from '@apollo/react-hooks'
const [addTodo, { data, loading, error }] = useMutation(ADD_TODO)
// usage
addTodo({ variables: { type: 'some value' } })

Example component

jsx

import gql from 'graphql-tag'
import { useMutation } from '@apollo/react-hooks'
const ADD_TODO = gql`
  mutation AddTodo($type: String!) {
    addTodo(type: $type) {
      id
      type
    }
  }
`
const AddTodo = () => {
  let input
  const [addTodo, { data }] = useMutation(ADD_TODO)
  return (
    <div>
      <form
        onSubmit={(e) => {
          e.preventDefault()
          addTodo({ variables: { type: input.value } })
          input.value = ''
        }}
      >
        <input
          ref={(node) => {
            input = node
          }}
        />
        <button type="submit">Add Todo</button>
      </form>
    </div>
  )
}

Updating the cache

If a mutation modifies multiple entities, or if it creates or deletes entities, the Apollo Client cache is not automatically updated to reflect the result of the mutation. To resolve this, your call to useMutation can include an update function.

The purpose of an update function is to modify your cached data to match the modifications that a mutation makes to your back-end data. In the example in Executing a mutation, the update function for the ADD_TODO mutation should add the same item to our cached version of the to-do list.

const GET_TODOS = gql`
  query GetTodos {
    todos
  }
`
const AddTodo = () => {
  let input
  const [addTodo] = useMutation(ADD_TODO, {
    update(cache, { data: { addTodo } }) {
      const { todos } = cache.readQuery({ query: GET_TODOS })
      cache.writeQuery({
        query: GET_TODOS,
        data: { todos: todos.concat([addTodo]) },
      })
    },
  })
  return (
    <div>
      <form
        onSubmit={(e) => {
          e.preventDefault()
          addTodo({ variables: { type: input.value } })
          input.value = ''
        }}
      >
        <input
          ref={(node) => {
            input = node
          }}
        />
        <button type="submit">Add Todo</button>
      </form>
    </div>
  )
}

Fragments

Fragments docs

gql

fragment NameParts on Person {
  firstName
  lastName
}
query GetPerson {
  people(id: "7") {
    ...NameParts
    avatar(size: LARGE)
  }
}

Resolvers

Resolvers provide the instructions for turning a GraphQL operation (a query, mutation, or subscription) into data. They either return the same type of data we specify in our schema or a promise for that data.

Before we can start writing resolvers, we need to learn more about what a resolver function looks like. Resolver functions accept four arguments:

fieldName: (parent, args, context, info) => data

parent: An object that contains the result returned from the resolver on the parent type
args: An object that contains the arguments passed to the field
context: An object shared by all resolvers in a GraphQL operation. We use the context to contain per-request state such as authentication information and access our data sources.
info: Information about the execution state of the operation which should only be used in advanced cases

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