WIP: failing test proving that inputs can't be empty

src/rxreact.tsx

@@ -30,7 +30,7 @@ function withViewModelFactory<S, A, P extends S & ActionMap<A>>(
       constructor(props: Difference<P, S & ActionMap<A>>) {
         super(props);
         let viewModel = viewModelFactory(this.propsObservable);
-        this.observableState = combineObservables(viewModel.inputs);
+        this.observableState = Object.keys(viewModel.inputs).length ? combineObservables(viewModel.inputs) : Observable.of({} as S);
         this.actions = subjectMapToActionMap(viewModel.outputs);
       }
 

test/rxreact.test.tsx

@@ -106,6 +106,50 @@ describe("withViewModel", () => {
           expect(rendered.find("#other").text()).toContain("moldy cheese");
         });
       });
+
+      describe("when `inputs` are empty", () => {
+        it("still renders", () => {
+          interface OutputOnlyComponentProps {
+            inputNumber: (_: number) => void;
+          }
+          let Component: React.SFC<OutputOnlyComponentProps> = () =>
+            <div>Just checking</div>;
+          let viewModel = {
+            inputs: {},
+            outputs: {inputNumber: numberSubject}
+          };
+          let ComponentWithViewModel = withViewModel(viewModel, Component);
+          const subject = mount(<ComponentWithViewModel/>);
+
+          expect(subject.text()).toEqual("Just checking");
+        });
+      });
+
+      describe("when `outputs` are empty", () => {
+        it("still renders", () => {
+          interface InputOnlyComponentProps {
+            derived1: string;
+            derived2: string;
+          }
+
+          let Component: React.SFC<InputOnlyComponentProps> = ({
+                                                                 derived1,
+                                                                 derived2,
+                                                               }) =>
+            <div>We have {derived1} {derived2}</div>;
+          let viewModel = {
+            inputs: {
+              derived1: derived1Signal,
+              derived2: derived2Signal,
+            },
+            outputs: {}
+          };
+          let ComponentWithViewModel = withViewModel(viewModel, Component);
+          const subject = mount(<ComponentWithViewModel/>);
+
+          expect(subject.text()).toEqual("We have 2 bananas applesauce");
+        });
+      });
     });
 
     describe("with a view model that has no initial value for state", () => {