add method for adding synthetic fields to messages

lib/descriptors/MessageDescriptor.js

@@ -5,6 +5,7 @@
 
 var util = require('util')
 var Descriptor = require('./Descriptor')
+var FieldDescriptor = require('./FieldDescriptor')
 var OneofDescriptor = require('./OneofDescriptor')
 var helper = require('../helper')
 
@@ -97,6 +98,18 @@ MessageDescriptor.prototype.addField = function (field) {
   return this
 }
 
+/**
+ * Adds a synthetic field to this message
+ * e.g. for use by a post-processor to add another field to a message after parsing before compiling
+ *
+ * @param {string|FieldType} type The synthetic field type, e.g. string or int32
+ * @param {string} name The field name
+ * @param {number} tag The tag number for this field
+ * @return {MessageDescriptor}
+ */
+MessageDescriptor.prototype.addSyntheticField = function (type, name, tag) {
+  return this.addField(new FieldDescriptor(type, name, tag))
+}
 
 /**
  * @param {string} name

tests/project_test.js

@@ -131,6 +131,18 @@ builder.add(function testRemoveField(test) {
   test.done()
 })
 
+builder.add(function testAddSyntheticField(test) {
+  var project = new Project(baseDir)
+      .addProto('protos/common.proto')
+
+  var color = project.getProtos('protos/common.proto')[0].getMessage('Color')
+  test.equal(3, color.toTemplateObject().fields.length)
+
+  color.addSyntheticField('int32', 'alpha', 4)
+  test.equal(4, color.toTemplateObject().fields.length)
+  test.done()
+})
+
 builder.add(function testTypeResolution(test) {
   var project = new Project(baseDir)
       .addProto('protos/person.proto')