[added] createProperty

Summary:
As specified in https://material-motion.github.io/material-motion/starmap/specifications/streams/connections/ReactiveProperty-createProperty

I was a little torn on whether or not I should implement `constantProperty` in terms of `createProperty` or just remove it.  The difference is only in the signature (does it take named args or not), unless you wanted to throw in `constantProperty().write()`.

Removing `constantProperty` seemed like the simplest and most consistent solution.

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

Reviewed By: #material_motion, O2 Material Motion, featherless

Tags: #material_motion

Differential Revision: http://codereview.cc/D2499

packages/springs-adaptor-rebound/src/__tests__/springSource.test.ts

@@ -32,7 +32,7 @@ import {
 
 import {
   State,
-  constantProperty,
+  createProperty,
 } from 'material-motion-streams';
 
 import {
@@ -62,8 +62,8 @@ describe('springSource',
     it('transitions from initialValue to destination',
       () => {
         springSource({
-          initialValue: constantProperty(2),
-          destination: constantProperty(3),
+          initialValue: createProperty({ initialValue: 2 }),
+          destination: createProperty({ initialValue: 3 }),
         }).subscribe(listener);
 
         expect(listener.firstCall).to.have.been.calledWith(2);
@@ -77,8 +77,8 @@ describe('springSource',
         let firstNextTime;
 
         springSource({
-          initialValue: constantProperty(0),
-          destination: constantProperty(0),
+          initialValue: createProperty({ initialValue: 0 }),
+          destination: createProperty({ initialValue: 0 }),
         }).subscribe({
           next(value) {},
           state: listener
@@ -95,8 +95,8 @@ describe('springSource',
         let tested;
 
         const spring = springSource({
-          initialValue: constantProperty(0),
-          destination: constantProperty(1),
+          initialValue: createProperty({ initialValue: 0 }),
+          destination: createProperty({ initialValue: 1 }),
         });
 
         const subscription = spring.subscribe({
@@ -122,8 +122,8 @@ describe('springSource',
         let tested;
 
         const spring = springSource({
-          initialValue: constantProperty(0),
-          destination: constantProperty(1),
+          initialValue: createProperty({ initialValue: 0 }),
+          destination: createProperty({ initialValue: 1 }),
         });
 
         const subscription = spring.subscribe({

packages/springs-adaptor-rebound/src/springSource.ts

@@ -19,7 +19,7 @@ import {
   MotionObserver,
   SpringArgs,
   State,
-  constantProperty,
+  createProperty,
 } from 'material-motion-streams';
 
 import {
@@ -66,9 +66,9 @@ export function springSource<T extends number | NumericDict>({
   //
   // tension: property.startWith(defaultTension).read()
   initialVelocity,
-  threshold = constantProperty(Number.EPSILON),
-  tension = constantProperty(342),
-  friction = constantProperty(30),
+  threshold = createProperty({ initialValue: Number.EPSILON }),
+  tension = createProperty({ initialValue: 342 }),
+  friction = createProperty({ initialValue: 30 }),
 }: SpringArgs<T>) {
   const firstInitialValue = initialValue.read();
 
@@ -92,7 +92,7 @@ export default springSource;
 function numericSpringSource({
   destination,
   initialValue,
-  initialVelocity = constantProperty(0),
+  initialVelocity = createProperty({ initialValue: 0 }),
   threshold,
   tension,
   friction,

packages/streams/src/properties/constantProperty.ts → packages/streams/src/properties/createProperty.ts

@@ -14,13 +14,15 @@
  *  under the License.
  */
 
-import {
-  ScopedReadable,
-} from '../types';
+import ReactiveProperty from './ReactiveProperty';
 
-export function constantProperty<T>(value: T): ScopedReadable<T> {
-  return {
-    read: () => value,
-  };
-};
-export default constantProperty;
+export function createProperty<T>({ initialValue }: { initialValue?: T} = {}): ReactiveProperty<T> {
+  const result = new ReactiveProperty<T>();
+
+  if (initialValue !== undefined) {
+    result.write(initialValue);
+  }
+
+  return result;
+}
+export default createProperty;

packages/streams/src/properties/index.ts

@@ -17,7 +17,7 @@
 export * from './ReactiveProperty';
 export { default as ReactiveProperty } from './ReactiveProperty';
 
-export * from './constantProperty';
-export { default as constantProperty } from './constantProperty';
+export * from './createProperty';
+export { default as createProperty } from './createProperty';
 
 export default undefined;