fix(createSelector): memoize projector function (#228)

Closes #226

modules/store/spec/selector.spec.ts

@@ -41,6 +41,23 @@ describe('Selectors', () => {
       expect(projectFn).toHaveBeenCalledWith(countOne, countTwo);
     });
 
+    it('should call the projector function only when the value of a dependent selector change', () => {
+      const firstState = { first: 'state', unchanged: 'state' };
+      const secondState = { second: 'state', unchanged: 'state' };
+      const neverChangingSelector = jasmine
+        .createSpy('unchangedSelector')
+        .and.callFake((state: any) => {
+          return state.unchanged;
+        });
+      const projectFn = jasmine.createSpy('projectionFn');
+      const selector = createSelector(neverChangingSelector, projectFn);
+
+      selector(firstState);
+      selector(secondState);
+
+      expect(projectFn).toHaveBeenCalledTimes(1);
+    });
+
     it('should memoize the function', () => {
       const firstState = { first: 'state' };
       const secondState = { second: 'state' };

modules/store/src/selector.ts

@@ -115,19 +115,24 @@ export function createSelector(...args: any[]): Selector<any, any> {
       selector.release && typeof selector.release === 'function'
   );
 
-  const { memoized, reset } = memoize(function(state: any) {
+  const memoizedProjector = memoize(function(...selectors: any[]) {
+    return projector.apply(null, selectors);
+  });
+
+  const memoizedState = memoize(function(state: any) {
     const args = selectors.map(fn => fn(state));
 
-    return projector.apply(null, args);
+    return memoizedProjector.memoized.apply(null, args);
   });
 
   function release() {
-    reset();
+    memoizedState.reset();
+    memoizedProjector.reset();
 
     memoizedSelectors.forEach(selector => selector.release());
   }
 
-  return Object.assign(memoized, { release });
+  return Object.assign(memoizedState.memoized, { release });
 }
 
 export function createFeatureSelector<T>(