add singleton as source helper

lib/index.js

@@ -12,4 +12,6 @@ exports.fs =
 exports.dir = require('./sources/dir');
 exports.node = deprecate.function(exports.dir, 'Container#node: Use Container#dir instead');
 
+exports.singleton = require('./sources/singleton');
+
 exports.node_modules = require('./sources/node_modules');

lib/sources/singleton.js

@@ -0,0 +1,25 @@
+/**
+ * Sources objects from a js singleton.
+ *
+ * This source allows to inject manually singletons as dependencies
+ *
+ * This typically allows to override specific modules for testing purposes
+ *
+ * Examples:
+ *
+ *    IoC.use(IoC.singleton('settings', {foo : 'bar'}));
+ *
+ * @param {string} id - the name of the component
+ * @param {any} sglt - the singleton that is injected
+ * @return {function}
+ * @public
+ */
+module.exports = function(id, sglt) {
+	return function app(idLocal){
+		if (idLocal === id) {
+			const singleton = Promise.resolve(sglt);
+			singleton['@singleton'] = true;
+			return singleton;
+		}	
+	}
+};

test/sources/singleton.test.js

@@ -0,0 +1,31 @@
+var singleton = require('../../lib/sources/singleton');
+
+describe('source/singleton', function() {
+	var sglt = {foo: 'bar'};
+  var source = singleton('settings', sglt)
+
+	var obj = source('settings')
+
+  it('should return @singleton component', function() {
+		expect(obj['@singleton']).to.be.equal(true);
+  });
+
+  it('should return undefined if module not found', function() {
+    var obj = source('fubar');
+    expect(obj).to.be.undefined;
+  });
+
+	let resolved;
+
+	before(function(done) {
+		obj.then(function(o) {
+			resolved = o;
+			done();
+		}, done);
+	});
+
+  it('should be resolved with settings', function() {
+    expect(resolved).to.be.equal(sglt);
+  });
+
+});