Add ReferenceIO and use it for SoluteIO, see phetsims/molarity#54

js/types/ReferenceIO.js

@@ -0,0 +1,78 @@
+// Copyright 2019, University of Colorado Boulder
+
+/**
+ * ReferenceIO uses reference identity for toStateObject/fromStateObject
+ *
+ * @author Sam Reid (PhET Interactive Simulations)
+ */
+define( require => {
+  'use strict';
+
+  // modules
+  const ObjectIO = require( 'TANDEM/types/ObjectIO' );
+  const phetioInherit = require( 'TANDEM/phetioInherit' );
+  const tandemNamespace = require( 'TANDEM/tandemNamespace' );
+  const validate = require( 'AXON/validate' );
+
+  // ifphetio
+  const phetioEngine = require( 'ifphetio!PHET_IO/phetioEngine' );
+
+  /**
+   * Main constructor for ReferenceIO base IO type.
+   * @param {Object} instance
+   * @param {string} phetioID
+   * @constructor
+   * @abstract
+   */
+  function ReferenceIO( instance, phetioID ) {
+    ObjectIO.call( this, instance, phetioID );
+  }
+
+  // ReferenceIO inherits from window.Object because it starts with its prototype in phetioInherit.inheritBase
+  // However, when serialized, the ReferenceIO supertype is reported as null (not sent in the JSON).
+  phetioInherit( ObjectIO, 'ReferenceIO', ReferenceIO, {}, {
+
+    /**
+     * A validator object to be used to validate the core types that IOTypes wrap.
+     * @type {ValidatorDef}
+     * @public
+     * @override
+     */
+    validator: ObjectIO.validator,
+
+    /**
+     * Documentation that appears in PhET-iO Studio, supports HTML markup.
+     * @public
+     */
+    documentation: 'Uses reference identity for toStateObject/fromStateObject',
+
+    /**
+     * Return the json that ReferenceIO is wrapping.  This can be overridden by subclasses, or types can use ReferenceIO type
+     * directly to use this implementation.
+     * @param {Object} o
+     * @returns {Object}
+     * @public
+     */
+    toStateObject: function( o ) {
+      validate( o, this.validator );
+      return o.tandem.phetioID;
+    },
+
+    /**
+     * Decodes the object from a state, used in PhetioStateEngine.setState.  This can be overridden by subclasses, or types can
+     * use ReferenceIO type directly to use this implementation.
+     * @param {Object} o
+     * @returns {Object}
+     * @public
+     */
+    fromStateObject: function( o ) {
+      const phetioObject = phetioEngine.getPhetioObject( o );
+      validate( phetioObject, this.validator );
+      return phetioObject;
+    }
+  } );
+
+  tandemNamespace.register( 'ReferenceIO', ReferenceIO );
+
+  return ReferenceIO;
+} );