Add support for @OneOf directives in printSchema (graphql#3969)

fixes graphql#3968

Author: hayes

src/utilities/__tests__/printSchema-test.ts

@@ -509,6 +509,23 @@ describe('Type System Printer', () => {
     `);
   });
 
+  it('Print Input Type with @oneOf directive', () => {
+    const InputType = new GraphQLInputObjectType({
+      name: 'InputType',
+      isOneOf: true,
+      fields: {
+        int: { type: GraphQLInt },
+      },
+    });
+
+    const schema = new GraphQLSchema({ types: [InputType] });
+    expectPrintedSchema(schema).to.equal(dedent`
+      input InputType @oneOf {
+        int: Int
+      }
+    `);
+  });
+
   it('Custom Scalar', () => {
     const OddType = new GraphQLScalarType({ name: 'Odd' });
 

src/utilities/printSchema.ts

@@ -204,7 +204,12 @@ function printInputObject(type: GraphQLInputObjectType): string {
   const fields = Object.values(type.getFields()).map(
     (f, i) => printDescription(f, '  ', !i) + '  ' + printInputValue(f),
   );
-  return printDescription(type) + `input ${type.name}` + printBlock(fields);
+  return (
+    printDescription(type) +
+    `input ${type.name}` +
+    (type.isOneOf ? ' @oneOf' : '') +
+    printBlock(fields)
+  );
 }
 
 function printFields(type: GraphQLObjectType | GraphQLInterfaceType): string {