[refactored] startWith to have shorthand signature

Summary: Part of #230

Reviewers: O2 Material Motion, O3 Material JavaScript platform reviewers, #material_motion, vietanh

Reviewed By: #material_motion, vietanh

Subscribers: vietanh

Tags: #material_motion

Differential Revision: http://codereview.cc/D3427

packages/core/src/interactions/Tossable.ts

@@ -230,7 +230,7 @@ export class Tossable {
     this.velocity$ = getVelocity$({
       // Since drag starts at rest, whenDragIsAtRest$ emits immediately.  Thus,
       // we start with { 0, 0 } to ensure velocity doesn't emit undefined.
-      value$: this.draggedLocation$.startWith({ value: { x: 0, y: 0 } }),
+      value$: this.draggedLocation$.startWith({ x: 0, y: 0 }),
       pulse$: whenDragIsAtRest$,
     });
 

packages/core/src/operators/__tests__/startWith.test.ts

@@ -81,5 +81,22 @@ describe('motionObservable.startWith',
         expect(listener).to.have.been.calledWith(20);
       }
     );
+
+    it('should have a shorthand signature',
+      () => {
+        stream.startWith(10).subscribe(listener);
+
+        expect(listener).to.have.been.calledWith(10);
+      }
+    );
+
+    it('should prefer the shorthand signature if there are an incorrect number of named arguments',
+      () => {
+        const expected = { value: 10, timestamp: 12345 };
+        stream.startWith(expected).subscribe(listener);
+
+        expect(listener).to.have.been.calledWith(expected);
+      }
+    );
   }
 );

packages/core/src/operators/startWith.ts

@@ -25,11 +25,16 @@ import {
   Observer,
 } from '../types';
 
+import {
+  isDefined,
+} from '../typeGuards';
+
 export type StartWithArgs<T> = {
   value: T,
 };
 
 export interface MotionSeedable<T> {
+  startWith(value: T): ObservableWithMotionOperators<T>;
   startWith(kwargs: StartWithArgs<T>): ObservableWithMotionOperators<T>;
 }
 
@@ -41,7 +46,16 @@ export function withStartWith<T, S extends Constructor<ObservableWithFoundationa
      * Returns a remembered stream, so each new observer will synchronously
      * receive the most recent value.
      */
-    startWith({ value }: StartWithArgs<T>): ObservableWithMotionOperators<T> {
+    startWith(value: T): ObservableWithMotionOperators<T>;
+    startWith(kwargs: StartWithArgs<T>): ObservableWithMotionOperators<T>;
+    startWith({ value }: StartWithArgs<T> & T): ObservableWithMotionOperators<T> {
+      const hasValue = isDefined(value);
+      // Only destructure if the supplied argument has the correct number of
+      // members.
+      if (!hasValue || (hasValue && Object.keys(arguments[0]).length > 1)) {
+        value = arguments[0];
+      }
+
       return new MotionObservable(
         (observer: Observer<T>) => {
           observer.next(value);

packages/demos-react/src/SwipeableDemo.tsx

@@ -113,7 +113,7 @@ class SwipeableCard extends React.Component<{}, {}> {
 
       combineStyleStreams({
         filter: 'invert()',
-        scale: swipeable.styleStreamsByTargetName.icon.scale$.startWith({ value: 0 }),
+        scale: swipeable.styleStreamsByTargetName.icon.scale$.startWith(0),
         willChange: swipeable.styleStreamsByTargetName.icon.willChange$,
       }).subscribe(this.iconStyle$);
     }