Hooks

The most common thing for a plugin to do is to add hooks to the builder.

The GraphQL hooks ran by newWithHooks allow you to manipulate the argument (specification) that is being passed to the GraphQL object constructors before the objects are constructed. You can think of hooks as wrappers around the original object spec, like this:

const MyType = newWithHooks(GraphQLObjectType, spec);

// is equivalent to:

const MyType = new GraphQLObjectType(hook3(hook2(hook1(spec))));

Hooks are registered via a call to builder.hook(hookName, hookFunction).

Every hookFunction must synchronously return a value - either the value that it was passed or a derivative of it (preferably immutable, but we're not strict on that).

Hook functions for one hook by default run in the order they were registered, that is why the order of plugins is sometimes relevant.

Stages of the build process

The hookName that you register the function for must match one of the supported hooks.

Here's a brief rundown of how the hooks are used by the Graphile engine. See the buildSchema and createBuild methods in the source code.

  1. A new Build object with the basic functionality is created by makeNewBuild
  2. The inflection hook allows plugins to add new inflection methods to the build.inflection object, or overwrite previously declared ones.
  3. The build hook allows plugins to add new utility methods to the build object itself, or overwrite previously declared ones.
  4. The init hook is the setup phase where the schema types get created by the plugins, using the newWithHooks (preferred) or addType (discouraged) methods of the build object. For example, the default StandardTypesPlugin registers the builtin scalars, or the PostGraphile plugins register the types found in the introspection results.
  5. The schema is constructed using newWithHooks(GraphQLSchema, …), where the query, mutation and subscription root operations are configured by the respective default plugins and other schema options can be adjusted.
  6. The finalize hook allows plugins to replace the schema that has been built with an alternative (likely derivative) schema, should that be desired. It also opens an opportunity to do something with the built schema (for example log it out) before it is returned.

During the init hook, the hooks for the respective GraphQL types are applied in the newWithHooks function, which may cause them to run in a nested fashion.

This hook system makes the library both powerful and flexible, at the expense of traceability - instead of having a clear declarative import, the origin of a called method might be in any of the used plugins, or even multiple ones. See PostGraphile's Debugging instructions for how to alleviate this.

What to do when that hook fires: hookFunction

The hookFunction that you register via builder.hook(hookName, hookFunction) will be called with 3 arguments:

  1. The input object (e.g. the spec that would be passed to the GraphQLObjectType constructor)
  2. The Build object (see below)
  3. The Context object (see below) which contains a scope property

Build object (Build)

The Build Object contains a number of helpers and sources of information relevant to the current build of the GraphQL API. If you're in watch mode then every time a new schema is generated a new build object will be used.

Plugins may extend the build object via the build hook. Once the build hook is complete the build object is frozen.

The most commonly used methods are:

  • build.extend(obj1, obj2) - returns a new object based on a non-destructive merge of obj1 and obj2 (will not overwrite keys!) - normally used at the return value for a hook
  • build.graphql - equivalent to require('graphql'), but helps ensure GraphQL version clashes do not occur
  • build.inflection - carries all the inflector functions for names

See Build Object for the rest.

Context object (Context)

The Context Object contains the information relevant to the current hook. Most importantly it contains the scope (an object based on the third argument passed to newWithHooks) but it also contains a number of other useful things. Here's some of the more commonly used ones:

  • scope - an object based on the third argument to newWithHooks or fieldWithHooks; for deeper hooks (such as GraphQLObjectType:fields:field) the scope from shallower hooks (such as GraphQLObjectType) are merged in.
  • Self - only available on deferred hooks (those that are called after the object is created, e.g.GraphQLObjectType:fields) this is the object that has been created, allowing recursive references.
  • fieldWithHooks(fieldName, spec, scope = {}) - on GraphQLObjectType:fields, used for adding a field if you need access to the field helpers (or want to define a scope)

Namespaces

Properties added to the Build object or set on the Context.scope should be namespaced so that they do not conflict; for example postgraphile uses the pg namespace: pgSql, pgIntrospection, isPgTableType, etc