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Maven plugin for generating graphql clients
# Apollo GraphQL code generator
[](https://raw.githubusercontent.com/apollographql/apollo-ios/master/LICENSE) [](https://www.npmjs.com/package/apollo-codegen) [](http://www.apollostack.com/#slack)
This is a tool to generate API code or type annotations based on a GraphQL schema and query documents.
It currently generates Swift code, TypeScript annotations and Flow annotations, we hope to add support for other targets in the future.
See [Apollo iOS](https://github.com/apollographql/apollo-ios) for details on the mapping from GraphQL results to Swift types, as well as runtime support for executing queries and mutations.
## Usage
If you want to experiment with the tool, you can install the `apollo-codegen` command globally:
```sh
npm install -g apollo-codegen
```
### `introspect-schema`
To download a GraphQL schema by sending an introspection query to a server:
```sh
apollo-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 "`.
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:
```sh
apollo-codegen introspect-schema schema.graphql --output schema.json
```
### `generate`
This tool will generate Swift code by default from a set of query definitions in `.graphql` files:
```sh
apollo-codegen generate **/*.graphql --schema schema.json --output API.swift
```
You can also generate type annotations for TypeScript or Flow using the `--target` option:
```sh
# TypeScript
apollo-codegen generate **/*.graphql --schema schema.json --target typescript --output schema.ts
# Flow
apollo-codegen generate **/*.graphql --schema schema.json --target flow --output schema.flow.js
```
#### `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](https://github.com/apollographql/graphql-tag) templates.
The tag name is configurable using the CLI `--tag-name` option.
## Typescript and Flow
When using `apollo-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 `apollo-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:
```graphql
...
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:
```graphql
query Characters {
characters(episode: NEW_HOPE) {
name
... on Human {
homePlanet
}
... on Droid {
primaryFunction
}
}
}
```
Apollo Codegen will generate a union type for Character.
```javascript
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:
```javascript
function CharacterFigures({ characters }: CharactersQuery) {
return characters.map(character => {
switch(character.__typename) {
case "Human":
return