introduce createDecorator

src/vs/base/common/decorators.ts

@@ -5,6 +5,27 @@
 
 'use strict';
 
+export function createDecorator(mapFn: (fn: Function) => Function): Function {
+	return (target: any, key: string, descriptor: any) => {
+		let fnKey: string = null;
+		let fn: Function = null;
+
+		if (typeof descriptor.value === 'function') {
+			fnKey = 'value';
+			fn = descriptor.value;
+		} else if (typeof descriptor.get === 'function') {
+			fnKey = 'get';
+			fn = descriptor.get;
+		}
+
+		if (!fn) {
+			throw new Error('not supported');
+		}
+
+		descriptor[fnKey] = mapFn(fn);
+	};
+}
+
 export function memoize(target: any, key: string, descriptor: any) {
 	let fnKey: string = null;
 	let fn: Function = null;