Crate cynic

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Cynic

Cynic is a GraphQL query builder & data mapper for Rust.

This documentation is primarily intended to be a reference for specific functions, for a guide to using Cynic see the website: cynic-rs.dev.

Overview

To get started with Cynic you’ll need a GraphQL schema for the API you wish to query. The examples will be using the star wars API.

Generating a Query DSL

Once you’ve got your schema installed locally, you’ll need to use the use_schema macro to generate a schema module:

mod schema {
    cynic::use_schema!("../schemas/starwars.schema.graphql");
}

This macro generates a few things:

  1. Some structs to represent the Input types the underlying schema. You may need to use these to build mutations or as parameters to queries.
  2. Definitions of all the Enums in the provided schema. You’ll need these if you want to use any enum types.
  3. Type safe selection set functions. These can be used to build up a query manually, though it’s usually easier to use the QueryFragment derive functionality explained below. Hopefully you’ll not need to use these directly too often.

Though using macros to generate these is convenient, it does leave a lot of code to the imagination. You can get a glimpse of the things this defines by running cargo doc --document-private-items and having a look in the schema module. It’s not ideal, but at least provides some visibility into the various enum types.

Creating QueryFragments

Now that you have a schema defined, you can start building some queries. Cynic lets you do this by deriving QueryFragment for a struct. For example, if we wanted to know what director title & director a Star Wars film had, we could define this QueryFragment:


#[derive(cynic::QueryFragment)]
#[cynic(schema_path = "../schemas/starwars.schema.graphql")]
struct Film {
    title: Option<String>,
    director: Option<String>
}

// This `Film` struct can now be used as the type of a field on any other
// `QueryFragment` struct and cynic will know how to turn that into a GraphQL
// query, and populate the `Film` struct from the response.

// For example, if we wanted to know the Director for a particular film:

#[derive(cynic::QueryFragment)]
#[cynic(
    schema_path = "../schemas/starwars.schema.graphql",
    graphql_type = "Root"
)]
struct FilmDirectorQuery {
    // Here we use the `#[arguments()]` attribute on the `film` field to provide a
    // hard coded film ID to look up.  Though useful for demonstration, hard coded
    // arguments like this aren't much use in reality.  For more details on providing
    // runtime arguments please see below.
    #[arguments(id = cynic::Id::new("ZmlsbXM6MQ=="))]
    film: Option<Film>,
}

// You can then build a `cynic::Operation` from this fragment
use cynic::{QueryBuilder};
let operation = FilmDirectorQuery::build(());

operation above implements serde::Serialize so can be used with any HTTP client. A selection of HTTP client integrations are provided in cynic::http - see the docs there for examples of using a cynic::Operation

ⓘ
let response = reqwest::blocking::Client::new()
                    .post("a_url")
                    .json(&operation)
                    .send()?;
let result = response.json::<GraphQlResponse<FilmDirectorQuery>>()?;

After this code has run, result will be an instance of GraphQlResponse<FilmDirectorQuery> with the film populated appropriately.

Dynamic Query Arguments

The query above was useful for demonstration, but you’ll usually want to be able to provide parameters to your query. To do this, you should define a struct that contains all of the parameters you want to provide:


// Deriving `QueryVariables` allows this struct to be used as variables in a
// `QueryFragment`, whether it represents part of a query or a whole query.
#[derive(cynic::QueryVariables)]
struct FilmArguments {
    id: Option<cynic::Id>
}

// You can now define a query to use these arguments on.  For example, to make
// `FilmDirectorQuery` a bit more dynamic:
#[derive(cynic::QueryFragment)]
#[cynic(
    schema_path = "../schemas/starwars.schema.graphql",
    graphql_type = "Root",
    // By adding the `variables` parameter to our `QueryFragment` we've made a variable
    // named `args` available for use in the `arguments` attribute.
    variables = "FilmArguments"
)]
struct FilmDirectorQueryWithArgs {
    // Here we use `args`, which we've declared above to be an instance of `FilmArguments`
    #[arguments(id: $id)]
    film: Option<Film>,
}

// Then we can build a query using this new struct;
use cynic::QueryBuilder;
let operation = FilmDirectorQueryWithArgs::build(
    FilmArguments{ id: Some("ZmlsbXM6MQ==".into()) }
);

Feature Flags

Cynic has a few features that are controlled by feature flags.

  • surf adds integration with the surf http client.
  • reqwest adds async integration with the reqwest http client.
  • reqwest-blocking adds blocking integration with the reqwest http client.

It’s worth noting that each of these features pulls in extra dependencies, which may impact your build size. Particularly if you’re targeting WASM. In particular the url crate has known issues when targeting web assembly.

Re-exports

Modules

  • coercions
    GraphQL has a number of coercion rules that make it easier to use and allow certain changes to be made in a backwards compatible way.
  • http
    HTTP client support for cynic.
  • queries
    Tools for building GraphQL queries in cynic
  • schema
    Types for describing a GraphQL schema in Rust.
  • variables
    Types & traits for working with GraphQL Variables

Macros

  • assert_impl
    Asserts that the type implements all of the given traits.
  • assert_type_eq_all
    Asserts that all types in a list are equal to each other.
  • impl_coercions
    Implements the default GraphQL list & option coercion rules for a type.
  • impl_scalar
    Implements [cynic::Scalar] for a given type & type lock.
  • use_schema
    Imports a schema for use by cynic.

Structs

  • GraphQlError
    A model describing an error which has taken place during execution.
  • GraphQlErrorLocation
    A line and column offset describing the location of an error within a GraphQL document.
  • GraphQlResponse
    The response to a GraphQl operation
  • Id
    A GraphQL ID
  • Operation
    An Operation that can be sent to a remote GraphQL server.
  • StreamingOperation
    A StreamingOperation is an Operation that expects a stream of results.

Enums

Traits

  • Enum
    A GraphQL Enum.
  • InlineFragments
    A QueryFragment that contains a set of inline fragments
  • InputObject
    A GraphQL input object.
  • MutationBuilder
    Provides a build function on QueryFragments that represent a mutation
  • QueryBuilder
    Provides a build function on QueryFragments that represent a query
  • QueryFragment
    Indicates that a type may be used as part of a graphql query.
  • SubscriptionBuilder
    Provides a build function on QueryFragments that represent a subscription

Attribute Macros

  • schema_for_derives
    An attribute macro to automatically add schema attributes to cynic derives.

Derive Macros