aws-appsync-codegen

AWS Appsync GraphQL code generator

This is a tool to generate API code or type annotations based on a GraphQL schema and query documents. This project is based upon Apollo GraphQL code generator.

It currently generates Swift code, TypeScript annotations, Flow annotations, and Scala code.

See Apollo iOS for details on the mapping from GraphQL results to Swift types, as well as runtime support for executing queries and mutations.

Usage with AWS AppSync

A complete tutorial can be found in the AWS AppSync documentation which is recommended for you to review first.

Create GraphQL API

If you have never created an AWS AppSync API before please use the Quickstart Guide and then walk through the iOS client guide.

Download introspection schema

The code generaton process needs two things:

1. GraphQL introspection schema
2. GraphQL queries/mutations/subscriptions

You can get the introspection schema in a schema.json file from the AWS AppSync console. You can find this in the console by clicking on your API name in the left-hand navigation, scrolling to the bottom, selecting iOS, clicking the Export schema dropdown, and selecting schema.json.

Write GraphQL queries

Now you can write GraphQL queries and the codegen process will convert these to native Swift types. If you are unfamiliar with writing a GraphQL query please read through this guide. Once you have your queries written, save them in a file called queries.graphql. For example you might have the following in your queries.graphql file:

query AllPosts {
   allPosts {
       id
       title
       author
       content
       url
       version
   }
}

Generate Swift types

Now that you have your introspection schema and your GraphQL query, install aws-appsync-codegen and run the tool against these two files like so:

npm install -g aws-appsync-codegen

aws-appsync-codegen generate queries.graphql --schema schema.json --output API.swift

The output will be a Swift class called API.swift which you can include in your XCode project to perform a GraphQL query against AWS AppSync.

Invoke GraphQL operation from Swift

Now that you have completed the code generation, import the API.swift file into your XCode project. Then update your project's Podfile with a dependency of the AWS AppSync SDK:

target 'PostsApp' do
  use_frameworks!
  pod 'AWSAppSync' ~> '2.6.7'
end

Next, in any code you wish to run the GraphQL query against AWS AppSync, import the SDK:

import AWSAppSync

Finally, run your query:

        appSyncClient?.fetch(query: AllPostsQuery())  { (result, error) in
            if error != nil {
                print(error?.localizedDescription ?? "")
                return
            }
            self.postList = result?.data?.allPosts
        }

Note: The code generation process converted the GraphQL statement of allPosts in your queries.graphql file to AllPostsQuery() which allowed you to invoke this using appSyncClient?.fetch(). A similar process happens for mutations and subscriptions.

Automate code generation

The process defined above outlines the general flow, however you can automate this in your XCode build process.

General Usage

If you want to experiment with the tool, you can install the aws-appsync-codegen command globally:

npm install -g aws-appsync-codegen

introspect-schema

The purpose of this command is to create a JSON introspection dump file for a given graphql schema. The input schema can be fetched from a remote graphql server or from a local file. The resulting JSON introspection dump file is needed as input to the generate command.

To download a GraphQL schema by sending an introspection query to a server:

aws-appsync-codegen introspect-schema http://localhost:8080/graphql --output schema.json

You can use the header option to add additional HTTP headers to the request. For example, to include an authentication token, use --header "Authorization: Bearer <token>".

You can use the insecure option to ignore any SSL errors (for example if the server is running with self-signed certificate).

Note: The command for downloading an introspection query was named download-schema but it was renamed to introspect-schema in order to have a single command for introspecting local or remote schemas. The old name download-schema is still available is an alias for backward compatibility.

To generate a GraphQL schema introspection JSON from a local GraphQL schema:

aws-appsync-codegen introspect-schema schema.graphql --output schema.json

generate

The purpose of this command is to generate types for query and mutation operations made against the schema (it will not generate types for the schema itself).

Swift

This tool will generate Swift code by default from a set of query definitions in .graphql files:

aws-appsync-codegen generate **/*.graphql --schema schema.json --output API.swift

The --add-s3-wrapper option can be specified to add in S3 wrapper code to the generated source.

TypeScript, Flow, or Scala

You can also generate type annotations for TypeScript, Flow, or Scala using the --target option:

# TypeScript 
aws-appsync-codegen generate **/*.graphql --schema schema.json --target typescript --output operation-result-types.ts
# Flow 
aws-appsync-codegen generate **/*.graphql --schema schema.json --target flow --output operation-result-types.flow.js
# Scala 
aws-appsync-codegen generate **/*.graphql --schema schema.json --target scala --output operation-result-types.scala

gql template support

If the source file for generation is a javascript or typescript file, the codegen will try to extrapolate the queries inside the gql tag templates.

The tag name is configurable using the CLI --tag-name option.

.graphqlconfig support

Instead of using the --schema option to point out you GraphQL schema, you can specify it in a .graphqlconfig file.

In case you specify multiple schemas in your .graphqlconfig file, choose which one to pick by using the --project-name option.

Typescript and Flow

When using aws-appsync-codegen with Typescript or Flow, make sure to add the __typename introspection field to every selection set within your graphql operations.

If you're using a client like apollo-client that does this automatically for your GraphQL operations, pass in the --addTypename option to aws-appsync-codegen to make sure the generated Typescript and Flow types have the __typename field as well. This is required to ensure proper type generation support for GraphQLUnionType and GraphQLInterfaceType fields.

Why is the __typename field required?

Using the type information from the GraphQL schema, we can infer the possible types for fields. However, in the case of a GraphQLUnionType or GraphQLInterfaceType, there are multiple types that are possible for that field. This is best modeled using a disjoint union with the __typename as the discriminant.

For example, given a schema:

...
 
interface Character {
  name: String!
}
 
type Human implements Character {
  homePlanet: String
}
 
type Droid implements Character {
  primaryFunction: String
}
 
...

Whenever a field of type Character is encountered, it could be either a Human or Droid. Human and Droid objects will have a different set of fields. Within your application code, when interacting with a Character you'll want to make sure to handle both of these cases.

Given this query:

query Characters {
  characters(episode: NEW_HOPE) {
    name
 
    ... on Human {
      homePlanet
    }
 
    ... on Droid {
      primaryFunction
    }
  }
}

Apollo Codegen will generate a union type for Character.

export type CharactersQuery = {
  characters: Array<{
    __typename: 'Human',
    name: string,
    homePlanet: ?string
  } | {
    __typename: 'Droid',
    name: string,
    primaryFunction: ?string
  }>
}

This type can then be used as follows to ensure that all possible types are handled:

function CharacterFigures({ characters }: CharactersQuery) {
  return characters.map(character => {
    switch(character.__typename) {
      case "Human":
        return <HumanFigure homePlanet={character.homePlanet} name={character.name} />
      case "Droid":
        return <DroidFigure primaryFunction={character.primaryFunction} name={character.name} />
    }
  });
}

Contributing

Running tests locally:

npm install
npm test