Made onStart synchronous

Summary: This makes it easier to aggregate the spring's state with other entities.  Otherwise, the 1 frame delay can make the system report that it's at rest, even when start() was called on the same frame that another object came to rest.

Reviewers: skevy

Reviewed By: skevy

Differential Revision: http://codereview.cc/D3325

src/__tests__/Springs-test.js

@@ -197,12 +197,26 @@ describe("Spring", () => {
     expect(spring._listeners).toEqual([]);
   });
 
-  it("notifies listeners that the spring has stopped when .stop() is called mid-simulation", () => {
+  it("synchronously notifies listeners that the spring has started when .start() is called", () => {
+    const onStartCallback = jest.fn();
+
     const spring = new Spring({
       toValue: 1
     });
 
+    spring.onStart(onStartCallback);
+
+    spring.start();
+    expect(onStartCallback).toHaveBeenCalledWith(spring);
+  });
+
+  it("synchronously notifies listeners that the spring has stopped when .stop() is called mid-simulation", () => {
     const onStopCallback = jest.fn();
+
+    const spring = new Spring({
+      toValue: 1
+    });
+
     spring
       .onStop(s => {
         onStopCallback(s);
@@ -213,8 +227,8 @@ describe("Spring", () => {
     jest.runTimersToTime(1000 / 60 * 10);
 
     spring.stop();
-    expect(spring.isAtRest).toBeFalsy();
     expect(onStopCallback).toHaveBeenCalledWith(spring);
+    expect(spring.isAtRest).toBeFalsy();
 
     jest.runTimersToTime(1000 / 60 * 1);
 

src/index.js

@@ -80,8 +80,8 @@ export class Spring {
       this._isAnimating = true;
 
       if (!this._currentAnimationStep) {
+        this._notifyListeners("onStart");
         this._currentAnimationStep = requestAnimationFrame((t: number) => {
-          this._notifyListeners("onStart");
           this._step(t);
         });
       }