graphql-lattice/es6/GQLBase.js

/** @namespace GQLBaseEnv */
// @flow

import Path from 'path'
import fs from 'fs'

import { Deferred, joinLines } from './utils'
import { typeOf } from 'ne-types'
import { SyntaxTree } from './SyntaxTree'
import { Properties } from './decorators/ModelProperties'
import { GraphQLObjectType, GraphQLEnumType } from 'graphql'
import { IDLFileHandler } from './IDLFileHandler'
import { merge } from 'lodash'
import { LatticeLogs as ll } from './utils'
import { dedent } from 'ne-tag-fns'

import AsyncFunctionExecutionError from './errors/AsyncFunctionExecutionError'
import FunctionExecutionError from './errors/FunctionExecutionError'
import AwaitingPromiseError from './errors/AwaitingPromiseError'

import EventEmitter from 'events'

/* Internal implementation to detect the existence of proxies. When present
 * additional functionality is enabled. Proxies are native in Node >= 6 */
const hasProxy = typeof global.Proxy !== 'undefined';

/* Internal Symbol referring to real accessor to GQLBase model object */
const _MODEL_KEY = Symbol.for('data-model-contents-value');

/* Internal Symbol referring to the static object containing a proxy handler */
const _PROXY_HANDLER = Symbol.for('internal-base-proxy-handler')

/**
 * Simple function to check if a supplied key matches a string of your
 * choosing and that string is not a defined property on the instance
 * passed to the check.
 *
 * @method GQLBaseEnv~notDefined
 * @memberof GQLBaseEnv
 * @since 2.5.0
 *
 * @param {string} keyToTest a String denoting the property you wish to test
 * @param {mixed} keySupplied a value, coerced `toString()`, to compare to
 * `keyToTest`
 * @param {mixed} instance an object instance to check `hasOwnProperty` on for
 * the `keyToTest` supplied.
 * @return {Boolean} true if the property matches the supplied key and that
 * property is not an ownedProperty of the instance supplied.
 */
export function notDefined(
  keyToTest: string,
  keySupplied: Object | string,
  instance: Object
) {
  return (
    new RegExp("^" + keyToTest + "$").test(keySupplied.toString())
    && !instance.hasOwnProperty(keyToTest)
  );
}

/**
 * A `Symbol` used as a key to store the backing model data. Designed as a
 * way to separate model data and GraphQL property accessors into logical bits.
 *
 * @type {Symbol}
 * @memberof GQLBaseEnv
 * @const
 */
export const MODEL_KEY = Symbol.for('data-model-contents-key');

/**
 * A `Symbol` used as a key to store the request data for an instance of the
 * GQLBase object in question.
 *
 * @type {Symbol}
 * @const
 * @inner
 * @memberof GQLBaseEnv
 */
export const REQ_DATA_KEY = Symbol.for('request-data-object-key');

/**
 * A nameless Symbol for use as a key to the internal decorator storage
 *
 * @type {Symbol}
 * @const
 * @inner
 * @memberof GQLBaseEnv
 */
export const META_KEY = Symbol();

/**
 * A Symbol used to identify calls to @Properties for properties generated
 * automatically upon instance creation.
 *
 * @type {Symbol}
 * @const
 * @inner
 * @memberOf GQLBaseEnv
 */
export const AUTO_PROPS = Symbol.for('auto-props')

/**
 * A Symbol used to identify calls to @Getters for properties generated
 * via decorator. These are stored in <class>[META_KEY][GETTERS]
 *
 * @type {Symbol}
 * @const
 * @inner
 * @memberOf GQLBaseEnv
 */
export const GETTERS = Symbol.for('getters')

/**
 * A Symbol used to identify calls to @Setters for properties generated
 * via decorator. These are stored in <class>[META_KEY][SETTERS]
 *
 * @type {Symbol}
 * @const
 * @inner
 * @memberOf GQLBaseEnv
 */
export const SETTERS = Symbol.for('setters')

/**
 * A Symbol used to identify calls to @Properties for properties generated
 * via decorator. These are stored in <class>[META_KEY][PROPS]
 *
 * @type {Symbol}
 * @const
 * @inner
 * @memberOf GQLBaseEnv
 */
export const PROPS = Symbol.for('props')

/**
 * All GraphQL Type objects used in this system are assumed to have extended
 * from this class. An instance of this class can be used to wrap an existing
 * structure if you have one.
 *
 * @class GQLBase
 */
export class GQLBase extends EventEmitter {
  fileHandler: ?IDLFileHandler;

  /**
   * Request data is passed to this object when constructed. Typically these
   * objects, and their children, are instantiated by its own static MUTATORS
   * and RESOLVERS. They should contain request specific state if any is to
   * be shared.
   *
   * These can be considered request specific controllers for the object in
   * question. The base class takes a single object which should contain all
   * the HTTP/S request data and the graphQLParams is provided as the object
   * { query, variables, operationName, raw }.
   *
   * When used with express-graphql, the requestData object has the format
   * { req, res, gql } where
   *   • req is an Express 4.x request object
   *   • res is an Express 4.x response object
   *   • gql is the graphQLParams object in the format of
   *     { query, variables, operationName, raw }
   *     See https://github.com/graphql/express-graphql for more info
   *
   * @memberof GQLBase
   * @method ⎆⠀constructor
   * @constructor
   *
   * @param {mixed} modelData this, typically an object, although anything
   * really is supported, represents the model data for our GraphQL object
   * instance.
   * @param {Object} requestData see description above
   */
  constructor(
    modelData: Object = {},
    requestData: ?Object = null,
    options: Object = { autoProps: true }
  ) {
    super();

    const Class = this.constructor;
    const tree = SyntaxTree.from(Class.SCHEMA);
    const outline = tree && tree.outline || null;

    if (!outline) {
      throw new FunctionExecutionError(
        new Error(dedent`
          The SDL is unparsable. Please check your SCHEMA and make sure
          it is valid GraphQL SDL/IDL. Your SCHEMA is defined as:

          ${this.SCHEMA}
        `)
      )
    }

    if (outline && !(Class.name in outline)) {
      throw new FunctionExecutionError(
        new Error(dedent`
          The class name "${Class.name}" does not match any of the types,
          enums, scalars, unions or interfaces defined in the SCHEMA for
          this class (${Object.keys(outline)}).

          \x1b[1mIn most clases this is because your class name and SCHEMA
          type do not match.\x1b[0m
        `)
      )
    }

    GQLBase.setupModel(this);
    this.setModel(modelData);
    this.requestData = requestData || {};
    this.fileHandler = new IDLFileHandler(this.constructor);

    if (options && !!options.autoProps !== false) {
      this.applyAutoProps()
    }

    // @ComputedType
    return hasProxy ? new Proxy(this, GQLBase[_PROXY_HANDLER]) : this;
  }

  /**
   * Since reading the Schema for a given GraphQL Lattice type or
   * interface is simple enough, we should be able to automatically
   * apply one to one GraphQL:Model properties.
   *
   * @instance
   * @method ⌾⠀applyAutoProps
   * @memberof GQLBase
   */
  applyAutoProps() {
    if (!this.constructor.SCHEMA || !this.constructor.SCHEMA.length) {
      ll.warn(joinLines`
        There is no SCHEMA for ${this.constructor.name}!! This will likely
        end in an error. Proceed with caution. Skipping \`applyAutoProps\`
      `)
      return
    }

    // Individual property getters do not need to be auto-created for enum
    // types. Potentially do some checks for Interfaces and Unions as well
    if (this.constructor.GQL_TYPE === GraphQLEnumType) {
      return
    }

    let Class = this.constructor
    let tree = SyntaxTree.from(Class.SCHEMA)
    let outline = tree ? tree.outline : {}
    let props = []

    // $FlowFixMe
    for (let propName of Object.keys(outline[Class.name])) {
      // $FlowFixMe
      let desc = Object.getOwnPropertyDescriptor(Class.prototype, propName)
      let hasCustomImpl = !!(
        // We have a descriptor for the property name
        desc && (
          // We have a getter function defined
          typeof desc.get !== 'undefined'
          ||
          // ...or we have a function, async or not, defined
          typeof desc.value === 'function'
        )
      )

      // Only create auto-props for non custom implementations
      if (!hasCustomImpl) {
        props.push(propName)
      }
    }

    if (props.length) {
      ll.info(`Creating auto-props for [${Class.name}]: `, props)
      try {
        Properties(...props)(Class, [AUTO_PROPS])
      }
      catch(error) {
        let parsed = /Cannot redefine property: (\w+)/.exec(error.message)
        if (parsed) {
          ll.warn(`Skipping auto-prop '${Class.name}.${parsed[1]}'`)
        }
        else {
          ll.error(`Failed to apply auto-properties\nReason: `)
          ll.error(error);
        }
      }
    }
  }

  /**
   * Getter for the internally stored model data. The contents of this
   * object are abstracted away behind a `Symbol` key to prevent collision
   * between the underlying model and any GraphQL Object Definition properties.
   *
   * @instance
   * @memberof GQLBase
   * @method ⌾⠀getModel
   * @since 2.5
   *
   * @param {Object} value any object you wish to use as a data store
   */
  getModel() {
    // @ComputedType
    return this[MODEL_KEY];
  }

  /**
   * Setter for the internally stored model data. The contents of this
   * object are abstracted away behind a `Symbol` key to prevent collision
   * between the underlying model and any GraphQL Object Definition properties.
   *
   * @instance
   * @memberof GQLBase
   * @method ⌾⠀setModel
   * @since 2.5
   *
   * @param {Object} value any object you wish to use as a data store
   */
  setModel(value: Object): GQLBase {
    // @ComputedType
    this[MODEL_KEY] = value;
    return this;
  }

  /**
   * Uses `_.merge()` to modify the internal backing data store for the
   * object instance. This is a shortcut for
   * `_.merge()(instance[MODEL_KEY], ...extensions)`
   *
   * @instance
   * @memberof GQLBase
   * @method ⌾⠀extendModel
   * @since 2.5
   *
   * @param {mixed} extensions n-number of valid `_.merge()` parameters
   * @return {GQLBase} this is returned
   */
  extendModel(...extensions: Array<mixed>): GQLBase {
    // $FlowFixMe
    merge(this[MODEL_KEY], ...extensions);
    return this;
  }

  /**
   * A getter that retrieves the inner request data object. When used with
   * GQLExpressMiddleware, this is an object matching {req, res, gql}.
   *
   * @instance
   * @memberof GQLBase
   * @method ⬇︎⠀requestData
   *
   * @return {Object} an object, usually matching { req, res, gql }
   */
  get requestData(): Object | null {
    // @ComputedType
    return this[REQ_DATA_KEY];
  }

  /**
   * A setter that assigns a value to the inner request data object. When
   * used with GQLExpressMiddleware, this is an object matching {req, res, gql}.
   *
   * @instance
   * @memberof GQLBase
   * @method ⬆︎⠀requestData
   *
   * @param {Object} value an object, usually matching { req, res, gql }
   */
  set requestData(value: Object): void {
    // @ComputedType
    this[REQ_DATA_KEY] = value;
  }

  /**
   * Returns the `constructor` name. If invoked as the context, or `this`,
   * object of the `toString` method of `Object`'s `prototype`, the resulting
   * value will be `[object MyClass]`, given an instance of `MyClass`
   *
   * @method ⌾⠀[Symbol.toStringTag]
   * @memberof ModuleParser
   *
   * @return {string} the name of the class this is an instance of
   * @ComputedType
   */
  get [Symbol.toStringTag]() { return this.constructor.name }

  /**
   * Properties defined for GraphQL types in Lattice can be defined as
   * a getter, a function or an async function. In the case of standard
   * functions, if they return a promise they will be handled as though
   * they were async
   *
   * Given the variety of things a GraphQL type can actually be, obtaining
   * its value can annoying. This method tends to lessen that boilerplate.
   * Errors raised will be thrown.
   *
   * @instance
   * @memberof GQLBase
   * @method ⌾⠀getProp
   *
   * @param {string|Symbol} propName the name of the property in question
   * @param {boolean} bindGetters true, by default, if the `get` or
   * `initializer` descriptor values should be bound to the current instance
   * or an object of the programmers choice before returning
   * @param {mixed} bindTo the `this` object to use for binding when
   * `bindGetters` is set to true.
   * @return {mixed} the value of the `propName` as a Function or something
   * else when the requested property name exists
   *
   * @throws {Error} errors raised in awaiting results will be thrown
   */
  getProp(propName: string, bindGetters: boolean = true, bindTo: mixed) {
    // $FlowFixMe
    let proto = Object.getPrototypeOf(this)
    let descriptor = Object.getOwnPropertyDescriptor(proto, propName)
    let result

    if (!descriptor) {
      return null;
    }

    if (descriptor) {
      if (descriptor.initializer || descriptor.get) {
        let what = descriptor.initializer || descriptor.get

        if (bindGetters) {
          result = what.bind(bindTo || this)
        }
        else {
          result = what
        }
      }
      else if (descriptor.value) {
        result = descriptor.value
      }
    }

    return result
  }

  /**
   * Properties defined for GraphQL types in Lattice can be defined as
   * a getter, a function or an async function. In the case of standard
   * functions, if they return a promise they will be handled as though
   * they were async. In addition to fetching the property, or field
   * resolver, its resulting function or getter will be invoked.
   *
   * Given the variety of things a GraphQL type can actually be, obtaining
   * its value can annoying. This method tends to lessen that boilerplate.
   * Errors raised will be thrown.
   *
   * @instance
   * @memberof GQLBase
   * @method ⌾⠀callProp
   *
   * @param {string} propName the name of the property in question
   * @param {Array<mixed>} args the arguments array that will be passed
   * to `.apply()` should the property evaluate to a `function`
   * @return {mixed} the return value of any resulting function or
   * value returned by a getter; wrapped in a promise as all async
   * functions do.
   *
   * @throws {Error} errors raised in awaiting results will be thrown
   */
  async callProp(propName: string, ...args: Array<mixed>) {
    // $FlowFixMe
    let prop = this.getProp(propName, ...args);
    let result

    if (prop && typeOf(prop) === 'AsyncFunction') {
      try {
        result = await prop.apply(this, args);
      }
      catch (error) {
        throw new AsyncFunctionExecutionError(error, prop, args, result)
      }
    }
    else if (prop && typeOf(prop) === Function.name) {
      try {
        result = prop.apply(this, args)
      }
      catch (error) {
        throw new FunctionExecutionError(error, prop, args, result)
      }

      if (typeOf(result) === Promise.name) {
        try {
          result = await result
        }
        catch (error) {
          throw new AwaitingPromiseError(error).setPromise(result)
        }
      }
    }

    return result
  }

  /**
   * A pass-thru method to the static function of the same name. The
   * difference being that if `requestData` is not specified, the
   * `requestData` object from this instance will be used to build the
   * resolvers in question.
   *
   * @instance
   * @method ⌾⠀getResolver
   * @memberof GQLBase
   *
   * @param {string} resolverName the name of the resolver as a string
   * @param {Object} requestData the requestData used to build the
   * resolver methods from which to choose
   * @return {Function} returns either a `function` representing the
   * resolver requested or null if there wasn't one to be found
   */
  async getResolver(resolverName: string, requestData: Object) {
    return await this.constructor.getResolver(
      resolverName,
      requestData || this.requestData
    )
  }

  /**
   * Resolvers are created in a number of different ways. OOP design
   * dictates that instances of a created class will handle field
   * resolvers, but query, mutation and subscription resolvers are
   * typically what creates these instances.
   *
   * Since a resolver can be created using `@mutator/@subscriptor/@resolver`
   * or via method on a object returned from `RESOLVERS()`, `MUTATORS()` or
   * `SUBSCRIPTIONS()`, there should be an easy to use way to fetch a
   * resolver by name; if for nothing else, code reuse.
   *
   * Pass the name of the resolver to the function and optionally pass a
   * requestData object. The `getMergedRoot()` method will build an object
   * containing all the root resolvers for the type, bound to the supplied
   * `requestData` object. It is from this object that `resolverName` will
   * be used to fetch the function in question. If one exists, it will be
   * returned, ready for use. Otherwise, null will be your answer.
   *
   *
   * @static
   * @method ⌾⠀getResolver
   * @memberof GQLBase
   *
   * @param {string} resolverName the name of the resolver as a string
   * @param {Object} requestData the requestData used to build the
   * resolver methods from which to choose
   * @return {Function} returns either a `function` representing the
   * resolver requested or null if there wasn't one to be found
   */
  static async getResolver(resolverName: string, requestData: Object) {
    const reqData = requestData || null
    const rootObj = await this.getMergedRoot(reqData)

    return rootObj[resolverName] || null
  }

  /**
   * The static version of getProp reads into the prototype to find the field
   * that is desired. If the field is either a getter or a initializer (see
   * class properties descriptors), then the option to bind that to either the
   * prototype object or one of your choosing is available.
   *
   * @memberof GQLBase
   * @method ⌾⠀getProp
   * @static
   *
   * @param {string|Symbol} propName a string or Symbol denoting the name of
   * the property or field you desire
   * @param {boolean} bindGetters true if a resulting `getter` or `initializer`
   * should be bound to the prototype or other object
   * @param {mixed} bindTo the object to which to bind the `getter` or
   * `initializer` functions to if other than the class prototype.
   * @return {mixed} a `Function` or other mixed value making up the property
   * name requested
   */
  static getProp(
    propName: string,
    bindGetters: boolean = false,
    bindTo: mixed
  ) {
    let descriptor = Object.getOwnPropertyDescriptor(this.prototype, propName)

    if (descriptor) {
      if (descriptor.get || descriptor.initializer) {
        let what = descriptor.initializer || descriptor.get

        if (bindGetters) {
          bindTo = bindTo || this.prototype

          return what.bind(bindTo)
        }
        else {
          return what
        }
      }
      else {
        return descriptor.value
      }
    }
    else {
      return null
    }
  }

  /**
   * Until such time as the reference implementation of Facebook's GraphQL
   * SDL AST parser supports comments, or until we take advantage of Apollo's
   * AST parser, this is how comments will be applied to a built schema.
   *
   * Several constants are defined on the GQLBase object itself, and thereby
   * all its subclasses. They pertain to how to define description fields
   * for various parts of your GQL implementation.
   *
   * ```
   * // To define a description on the top level class
   * [this.DOC_CLASS]: string
   *
   * // To define a description on a field (getter, function or async function)
   * [this.DOC_FIELDS]: {
   *   fieldName: string
   * }
   *
   * // To define a description on a query, mutation or subscription field
   * [this.DOC_QUERIES || this.DOC_MUTATORS || this.DOC_SUBSCRIPTIONS]: {
   *   fieldName: string
   * }
   * ```
   *
   * To make writing code easier, the `joinLines()` template function is
   * available so your source code can look nice and neat and your descriptions
   * won't get annoying line breaks and spaces as part of that process.
   *
   * @static
   * @memberof GQLBase
   * @method apiDocs
   *
   * @return {Object} an object with various keys and values denoting
   * description fields that should be applied to the final schema object
   */
  static apiDocs(): Object {
    return {
      [this.DOC_CLASS]: joinLines`
        GQLBase class implementation. GQLBase is the root class used in
        graphql-lattice to describe a GraphQLObjectType. If you are reading
        this, the person using lattice failed to provide documentation for
        their type. :)
      `,

      [this.DOC_QUERY]: joinLines`
        ## Welcome to GraphQL Lattice
        **Query**

        You will want to define a \`DOC_QUERY\` apiDoc comment with something
        more meaningful to your particular Schema here.
      `,

      [this.DOC_MUTATION]: joinLines`
        ## Welcome to GraphQL Lattice
        **Mutation**

        You will want to define a \`DOC_MUTATION\` apiDoc comment with
        something more meaningful to your particular Schema here.
      `,

      [this.DOC_SUBSCRIPTION]: joinLines`
        ## Welcome to GraphQL Lattice
        **Subscription**

        You will want to define a \`DOC_SUBSCRIPTION\` apiDoc comment with
        something more meaningful to your particular Schema here.
      `,

      [this.DOC_FIELDS]: {
        // fieldName: `fieldDescription`,
      },

      [this.DOC_QUERIES]: {
        // queryName: `queryDescription`,
      },

      [this.DOC_MUTATORS]: {
        // mutatorName: `mutatorDescription`
      },

      [this.DOC_SUBSCRIPTIONS]: {
        // subscriptionName: `subscriptionDescription`
      }
    }
  }

  /**
   * Defined in a base class, this getter should return either a String
   * detailing the full IDL schema of a GraphQL handler or one of two
   * types of Symbols.
   *
   * The first Symbol type is the constant `ADJACENT_FILE`. If this Symbol is
   * returned, the system assumes that next to the source file in question is
   * a file of the same name with a .graphql extension. This file should be
   * made of the GraphQL IDL schema definitions for the object types being
   * created.
   *
   * Example:
   * ```js
   *   static get SCHEMA(): string | Symbol {
   *     return GQLBase.ADJACENT_FILE
   *   }
   * ```
   *
   * The primary advantage of this approach is allowing an outside editor that
   * provides syntax highlighting rather than returning a string from the
   * SCHEMA getter.
   *
   * Alternatively, the static method IDLFilePath can be used to point to an
   * alternate location where the GraphQL IDL file resides. The extension can
   * also be changed from .graphql to something else if need be using this
   * method.
   *
   * Example:
   * ```js
   *   static get SCHEMA(): string | Symbol {
   *     return GQLBase.IDLFilePath('/path/to/file', '.idl')
   *   }
   * ```
   *
   * @instance
   * @memberof GQLBase
   * @method ⬇︎⠀SCHEMA
   * @readonly
   * @static
   *
   * @return {string|Symbol} a valid IDL string or one of the Symbols
   * described above.
   *
   * @see {@link GQLBase#ADJACENT_FILE}
   * @see {@link GQLBase#IDLFilePath}
   */
  static get SCHEMA(): string | Symbol {
    return ''
  }

  /**
   * This method should return a promise that resolves to an object of
   * functions matching the names of the mutation operations. These are to be
   * injected into the root object when used by `GQLExpressMiddleware`.
   *
   * @instance
   * @memberof GQLBase
   * @method ⌾⠀MUTATORS
   * @readonly
   * @static
   *
   * @param {Object} requestData typically an object containing three
   * properties; {req, res, gql}
   * @return {Promise} a promise that resolves to an object; see above for more
   * information.
   */
  static async MUTATORS(requestData: Object): Promise<Object> {
    // define in base class
    return {};
  }

  /**
   * This method should return a promise that resolves to an object of
   * functions matching the names of the query operations. These are to be
   * injected into the root object when used by `GQLExpressMiddleware`.
   *
   * @instance
   * @memberof GQLBase
   * @method ⌾⠀RESOLVERS
   * @readonly
   * @static
   *
   * @param {Object} requestData typically an object containing three
   * properties; {req, res, gql}
   * @return {Promise} a promise that resolves to an object; see above for more
   * information.
   */
  static async RESOLVERS(requestData: Object): Promise<Object> {
    // define in base class
    return {};
  }

  /**
   * @see {@link GQLBase#SCHEMA}
   *
   * @memberof GQLBase
   * @method ⬇︎⠀ADJACENT_FILE
   * @static
   * @const
   *
   * @return {Symbol} the Symbol, when returned from SCHEMA, causes
   * the logic to load an IDL Schema from an associated file with a .graphql
   * extension and bearing the same name.
   */
  static get ADJACENT_FILE(): Symbol {
    return Symbol.for('.graphql file located adjacent to source')
  }

  /**
   * Determines the default type targeted by this GQLBase class. Any
   * type will technically be valid but only will trigger special behavior
   *
   * @memberof GQLBase
   * @method ⬇︎⠀GQL_TYPE
   * @static
   * @const
   *
   * @return {Function} a type, such as `GraphQLObjectType` or
   * `GraphQLInterfaceType`
   */
  static get GQL_TYPE(): Function {
    return GraphQLObjectType;
  }

  /**
   * Creates an appropriate Symbol crafted with the right data for use by
   * the IDLFileHandler class below.
   *
   * @static
   * @memberof GQLBase
   * @method ⌾⠀IDLFilePath
   *
   * @param {string} path a path to the IDL containing file
   * @param {string} [extension='.graphql'] an extension, including the
   * prefixed period, that will be added to the supplied path should it not
   * already exist.
   * @return Symbol
   *
   * @see {@link GQLBase#SCHEMA}
   */
  static IDLFilePath(path: string, extension: string = '.graphql'): Symbol {
    return Symbol.for(`Path ${path} Extension ${extension}`);
  }

  /**
   * A file handler for fetching the IDL schema string from the file system
   * for those `GQLBase` extended classes that have indicated to do so by
   * returning a `Symbol` for their `SCHEMA` property.
   *
   * @static
   * @memberof GQLBase
   * @method ⬇︎⠀handler
   *
   * @return {IDLFileHandler} instance of IDLFileHandler, created if one does
   * not already exist, for fetching the contents from disk.
   */
  static get handler(): IDLFileHandler {
    const key = Symbol.for(`${IDLFileHandler.name}.${this.name}`);

    // @ComputedType
    if (!this[key]) {
      // @ComputedType
      this[key] = new IDLFileHandler(this);
    }

    // @ComputedType
    return this[key];
  }

  /**
   * Returns the module object where your class is created. This needs to be
   * defined on your class, as a static getter, in the FILE where you are
   * defining your Class definition.
   *
   * @static
   * @memberof GQLBase
   * @method ⬇︎⠀module
   * @const
   *
   * @return {Object} the reference to the module object defined and injected
   * by node.js' module loading system.
   *
   * @see https://nodejs.org/api/modules.html
   */
  static get module(): Object {
    return module;
  }

  /**
   * The internal data model has some custom `EventEmitter` code wrapped
   * it here. When the data model is set via `setModel` or by accessing it
   * via `instance[MODEL_KEY]`, an event `EVENT_MODEL_SET` is emitted. Any
   * listener listening for this event receives an object with two keys
   * ```
   * {
   *   model: The actual model being set; changes are persisted
   *   instance: The GQLBase instance the model is associated with
   * }
   * ```
   *
   * Subsequently, the events `EVENT_MODEL_PROP_CHANGE` and
   * `EVENT_MODEL_PROP_DELETE` can be listened to if your version of node
   * supports Proxy objects. They allow you to be notified whenever your
   * model has a property changed or deleted, respectively.
   *
   * The callback for `change` receives an object with four properties
   * ```
   * {
   *   model: The model object the value is being changed on
   *   old: The old value being replaced; undefined if it is the first time
   *   key: The property key for the value being changed
   *   value: The new value being set
   * }
   * ```
   *
   * The callback for `delete` receives an object with four properties
   * ```
   * {
   *   model: The model object the value is deleted from
   *   key: The property key for the deleted value
   *   deleted: The deleted value
   * }
   * ```
   *
   * @static
   * @memberof GQLBase
   * @method ⌾⠀setupModel
   *
   * @param {GQLBase} instance typically `this` as passed in from a call in
   * the constructor
   */
  static setupModel(instance: GQLBase) {
    const changeHandler: Object = {
      /**
       * Proxy set() handler. This is where the change events are fired from
       *
       * @method GQLBase~set
       * @param {Object} target the `GQLBase` model object
       * @param {string} key the property name
       * @param {mixed} value the new property value
       */
      set(target, key, value) {
        const old = target[key];

        target[key] = value;
        instance.emit(GQLBase.EVENT_MODEL_PROP_CHANGE, {
          model: target,
          old,
          key,
          value
        })
      },

      /**
       * Proxy deleteProperty() handler. This is where the delete property
       * events are fired from
       *
       * @method GQLBase~deleteProperty
       * @param {Object} target the `GQLBase` model object
       * @param {string} key the property name
       */
      deleteProperty(target, key) {
        const deleted = target[key];

        delete target[key];
        instance.emit(GQLBase.EVENT_MODEL_PROP_DELETE, {
          model: target,
          key,
          deleted
        })
      }
    }

    /**
     * 'Publicly' the Symbol for accessing the `GQLBase` model is `MODEL_KEY`.
     * In truth it is stored under a Symbol defined in `setupModel` and
     * referred to as `_MODEL_KEY` in this code. This is done so a getter and
     * setter can be wrapped around the usage of the instance's data model.
     *
     * When being read, if `Proxy` exists in the node environment and if there
     * are any registered `EVENT_MODEL_PROP_CHANGE` or `EVENT_MODEL_PROP_DELETE`
     * events, then the returned model is a Proxy around the real model that
     * allows us to capture the changes and deletion of keys
     *
     * When being assigned, the event `EVENT_MODEL_WILL_BE_SET` and the event
     * `EVENT_MODEL_HAS_BEEN_SET` are emitted to allow listeners to modify and
     * see the final data around the setting of a model object. Both events
     * receive an object with two keys
     *
     * ```
     * {
     *   model: The object being or having been set
     *   instance: The GQLBase instance receiving the model
     * }
     * ```
     */
    Object.defineProperty(instance, MODEL_KEY, {
      get: function() {
        let model = this[_MODEL_KEY]
        let hasListeners =
          this.listenerCount(GQLBase.EVENT_MODEL_PROP_CHANGE) +
          this.listenerCount(GQLBase.EVENT_MODEL_PROP_DELETE)

        if (hasProxy && hasListeners) {
          model = new Proxy(model, changeHandler);
        }

        return model
      },

      set: function(model) {
        const instance = this;

        this.emit(GQLBase.EVENT_MODEL_WILL_BE_SET, { model, instance });
        instance[_MODEL_KEY] = model;
        this.emit(GQLBase.EVENT_MODEL_HAS_BEEN_SET, { model, instance })
      }
    });
  }

  /**
   * If ES6 Proxies are supported in your execution environment, all GQLBase
   * extended classes are also proxies. By default the internal proxy handler
   * provides backwards compatibility with the removal of the default getters
   * and setters for the 'model' property as long as you do not define a
   * top level 'model' property of your own.
   *
   * @method ⬇︎⠀[_PROXY_HANDLER]
   * @memberof GQLBase
   * @static
   * @const
   * @since 2.5.0
   *
   * @type {Object}
   * @ComputedType
   */
  static get [_PROXY_HANDLER]() {
    return {
      get(target, key, lastResult) {
        const model = target[_MODEL_KEY];

        // Allow backwards compatibility for 'model' property if one is not
        // explicitly defined on your instance.
        if (notDefined('model', key, target)) {
          // Be sure to use the public MODEL_KEY to ensure events fire
          return target[MODEL_KEY];
        }

        return target[key]
      }
    }
  }

  /**
   * Applies the same logic as {@link #[Symbol.toStringTag]} but on a static
   * scale. So, if you perform `Object.prototype.toString.call(MyClass)`
   * the result would be `[object MyClass]`.
   *
   * @method ⌾⠀[Symbol.toStringTag]
   * @memberof ModuleParser
   * @static
   *
   * @return {string} the name of this class
   * @ComputedType
   */
  static get [Symbol.toStringTag]() { return this.name }

  /**
   * A constant used to register an event listener for when the internal
   * model object is assigned a new value. This event fires before the model
   * is set. Changes to the model value at this point will affect the contents
   * before the value assignment takes place.
   *
   * @static
   * @memberof GQLBase
   * @method ⬇︎⠀EVENT_MODEL_WILL_BE_SET
   * @const
   *
   * @type {string}
   */
  static get EVENT_MODEL_WILL_BE_SET() { return 'E: Int. model will be set' }

  /**
   * A constant used to register an event listener for when the internal
   * model object is assigned a new value. This event fires after the model
   * is set.
   *
   * @static
   * @memberof GQLBase
   * @method ⬇︎⠀EVENT_MODEL_HAS_BEEN_SET
   * @const
   *
   * @type {string}
   */
  static get EVENT_MODEL_HAS_BEEN_SET() { return 'E: Int. model has been set' }

  /**
   * A constant used to register an event listener for when a property of the
   * internal model object is set to a new or intial value.
   *
   * @static
   * @memberof GQLBase
   * @method ⬇︎⠀EVENT_MODEL_PROP_CHANGE
   * @const
   *
   * @type {string}
   */
  static get EVENT_MODEL_PROP_CHANGE() { return 'E: Int. model prop changed' }

  /**
   * A constant used to register an event listener for when a property of the
   * internal model object has been deleted. This event fires after the value
   * has been deleted.
   *
   * @static
   * @memberof GQLBase
   * @method ⬇︎⠀EVENT_MODEL_PROP_DELETE
   * @const
   *
   * @type {string}
   */
  static get EVENT_MODEL_PROP_DELETE() { return 'E: Int. model prop deleted' }

  /**
   * A constant key used to identify a comment for a class description
   *
   * @static
   * @memberof GQLBase
   * @method ⬇︎⠀DOC_CLASS
   * @const
   *
   * @type {string}
   */
  static get DOC_CLASS() { return 'class' }

  /**
   * A constant key used to identify a comment for a type field description
   *
   * @static
   * @memberof GQLBase
   * @method ⬇︎⠀DOC_FIELDS
   * @const
   *
   * @type {string}
   */
  static get DOC_FIELDS() { return 'fields' }

  /**
   * A constant key used to identify a comment for the top level query
   * description
   *
   * @static
   * @memberof GQLBase
   * @method ⬇︎⠀DOC_QUERY
   * @const
   *
   * @type {string}
   */
  static get DOC_QUERY() { return 'query' }

  /**
   * A constant key used to identify a comment for a query description
   *
   * @static
   * @memberof GQLBase
   * @method ⬇︎⠀DOC_QUERIES
   * @const
   *
   * @type {string}
   */
  static get DOC_QUERIES() { return 'queries' }

  /**
   * A constant key used to identify a comment for the top level mutation
   * description
   *
   * @static
   * @memberof GQLBase
   * @method ⬇︎⠀DOC_MUTATION
   * @const
   *
   * @type {string}
   */
  static get DOC_MUTATION() { return 'mutation' }

  /**
   * A constant key used to identify a comment for a mutator description
   *
   * @static
   * @memberof GQLBase
   * @method ⬇︎⠀DOC_MUTATORS
   * @const
   * @deprecated Use `DOC_MUTATIONS` instead
   *
   * @type {string}
   */
  static get DOC_MUTATORS() { return 'mutators' }

  /**
   * A constant key used to identify a comment for a mutator description
   *
   * @static
   * @memberof GQLBase
   * @method ⬇︎⠀DOC_MUTATORS
   * @const
   *
   * @type {string}
   */
  static get DOC_MUTATIONS() { return 'mutators' }

  /**
   * A constant key used to identify a comment for the top level subscription
   * description
   *
   * @static
   * @memberof GQLBase
   * @method ⬇︎⠀DOC_SUBSCRIPTION
   * @const
   *
   * @type {string}
   */
  static get DOC_SUBSCRIPTION() { return 'subscription' }

  /**
   * A constant key used to identify a comment for a subscription description
   *
   * @static
   * @memberof GQLBase
   * @method ⬇︎⠀DOC_SUBSCRIPTIONS
   * @const
   *
   * @type {string}
   */
  static get DOC_SUBSCRIPTIONS() { return 'subscriptions' }

  /**
   * A shortcut to the utils/joinLines function to make it easier to get
   * the tools to write docs for your types in a friendly fashion.
   *
   * @memberof GQLBase
   * @method ⬇︎⠀joinLines
   * @static
   * @const
   *
   * @type {Function}
   */
  static get joinLines(): Function { return joinLines }

  /**
   * An simple pass-thru method for fetching a types merged root object.
   *
   * @method ⌾⠀getMergedRoot
   * @memberof GQLBase
   * @static
   *
   * @param {Object} requestData an object containing the request data such as
   * request, response or graphql context info that should be passed along to
   * each of the resolver creators
   * @return {Object} the merged root object with all the query, mutation and
   * subscription resolvers defined and created within.
   */
  static async getMergedRoot(
    requestData: Object,
    separateByType: boolean = false
  ): Object {
    const root = {};
    const Class = this;

    let _ = {
      // $FlowFixMe
      resolvers: Class[META_KEY].resolvers || [],
      // $FlowFixMe
      mutators: Class[META_KEY].mutators || [],
      // $FlowFixMe
      subscriptors: Class[META_KEY].subscriptors || []
    }

    let convert = f => {
      let isFactoryClass = (c) => {
        return !!Class[META_KEY][Symbol.for('Factory Class')]
      }

      if (isFactoryClass(Class)) {
        return {
          [f.name]: function(...args) {
            return f.apply(Class, [Class, requestData, ...args])
          }
        }
      }
      else {
        return {
          [f.name]: function(...args) {
            return f.apply(Class, [requestData, ...args])
          }
        }
      }
    }
    let reduce = (p, c) => merge(p, c)

    _.resolvers = _.resolvers.map(convert).reduce(reduce, {})
    _.mutators = _.mutators.map(convert).reduce(reduce, {})
    _.subscriptors = _.subscriptors.map(convert).reduce(reduce, {})

    if (separateByType) {
      // Apollo wants all the resolvers to grouped by top level type.
      // The field resolvers aren't an issue in Lattice defined types
      // but the root types do need to be sorted; so let's do that here
      merge(
        root,
        { Query: await Class.RESOLVERS(requestData) },
        { Mutation: await Class.MUTATORS(requestData) },
        { Query: _.resolvers },
        { Mutation: _.mutators },
        { Subscription: _.subscriptors }
      );

      // When using lattice with apollo server, it is quite particular about
      // empty Query, Mutation or Subscription resolver maps.
      if (!Object.keys(root.Query).length) delete root.Query
      if (!Object.keys(root.Mutation).length) delete root.Mutation
      if (!Object.keys(root.Subscription).length) delete root.Subscription
    }
    else {
      merge(
        root,
        await Class.RESOLVERS(requestData),
        await Class.MUTATORS(requestData),
        _.resolvers,
        _.mutators,
        _.subscriptors
      );
    }

    return root;
  }

  /**
   * An object used to store data used by decorators and other internal
   * proccesses.
   * @ComputedType
   */
  static get [META_KEY]() {
    let storage = this[Symbol.for(this.name)]

    if (!storage) {
      storage = (this[Symbol.for(this.name)] = {})
    }

    return storage;
  }
}

export default GQLBase;