Document custom directives

Author: rmosolgo

guides/css/main.scss

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guides/type_definitions/directives.md

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+---
+layout: guide
+doc_stub: false
+search: true
+section: Type Definitions
+title: Directives
+desc: Special instructions for the GraphQL runtime
+index: 10
+class_based_api: true
+experimental: true
+---
+
+
+Directives are system-defined keywords that modify execution. All GraphQL systems have at least _two_ directives, `@skip` and `@include`. For example:
+
+```ruby
+query ProfileView($renderingDetailedProfile: Boolean!){
+  viewer {
+    handle
+    # These fields will be included only if the check passes:
+    ... @include(if: $renderingDetailedProfile) {
+      location
+      homepageUrl
+    }
+  }
+}
+```
+
+Here's how the two built-in directives work:
+
+- `@skip(if: ...)` skips the selection if the `if: ...` value is truthy ({{ "GraphQL::Schema::Directive::Skip" | api_doc }})
+- `@include(if: ...)` includes the selection if the `if: ...` value is truthy ({{ "GraphQL::Schema::Directive::Include" | api_doc }})
+
+GraphQL-Ruby also supports custom directives for use with the {% internal_link "interpreter runtime", "/queries/interpreter" %}.
+
+## Custom Directives
+
+Custom directives extend {{ "GraphQL::Schema::Directive" | api_doc }}:
+
+```ruby
+# app/graphql/directives/my_directive.rb
+class Directives::MyDirective < GraphQL::Schema::Directive
+  description "A nice runtime customization"
+end
+```
+
+Then, they're hooked up to the schema using `directive(...)`:
+
+```ruby
+class MySchema < GraphQL::Schema
+  # Custom directives are only supported by the Interpreter runtime
+  use GraphQL::Execution::Interpreter
+  # Attach the custom directive to the schema
+  directive(Directives::MyDirective)
+end
+```
+
+{{ "GraphQL::Schema::Directive::Feature" | api_doc }} and {{ "GraphQL::Schema::Directive::Transform" | api_doc }} are included in the library as examples.
+
+### Arguments
+
+Like fields, directives may have {% internal_link "arguments", "/fields/arguments" %} :
+
+```ruby
+argument :if, Boolean, required: true,
+  description: "Skips the selection if this condition is true"
+```
+
+### Runtime hooks
+
+Directive classes may implement the following class methods to interact with the runtime:
+
+- `def self.include?(obj, args, ctx)`: If this hook returns `false`, the nodes flagged by this directive will be skipped at runtime.
+- `def self.resolve(obj, args, ctx)`: Wraps the resolution of flagged nodes. Resolution is passed as a __block__, so `yield` will continue resolution. The return value of this method will be treated as the return value of the flagged field.
+
+Looking for a runtime hook that isn't listed here? {% open_an_issue "New directive hook: @something", "<!-- Describe how the directive would be used and then how you might implement it --> " %} to start the conversation!
+
+### Directive object lifecycle
+
+Currently, Directive classes are never initialized. A later version of GraphQL may initialize these objects for runtime application.