chore(scrollTo): add scroll completed callback as option

src/util/scroll-view.ts

@@ -1,4 +1,4 @@
-import { CSS, pointerCoord, nativeRaf, cancelRaf } from '../util/dom';
+import { CSS, pointerCoord, nativeRaf, rafFrames, cancelRaf } from '../util/dom';
 
 
 export class ScrollView {
@@ -35,25 +35,35 @@ export class ScrollView {
     }
   }
 
-  scrollTo(x: number, y: number, duration: number): Promise<any> {
+  scrollTo(x: number, y: number, duration: number, done?: Function): Promise<any> {
     // scroll animation loop w/ easing
     // credit https://gist.github.com/dezinezync/5487119
-    let self = this;
 
+    let promise: Promise<any>;
+    if (done === undefined) {
+      // only create a promise if a done callback wasn't provided
+      // done can be a null, which avoids any functions
+      promise = new Promise((res, rej) => {
+        done = res;
+        done = rej;
+      });
+    }
+
+    const self = this;
     if (!self._el) {
       // invalid element
-      return Promise.resolve();
+      done();
+      return promise;
     }
 
     x = x || 0;
     y = y || 0;
 
-    let fromY = self._el.scrollTop;
+    const fromY = self._el.scrollTop;
-    let fromX = self._el.scrollLeft;
+    const fromX = self._el.scrollLeft;
 
-    let maxAttempts = (duration / 16) + 100;
+    const maxAttempts = (duration / 16) + 100;
 
-    return new Promise(resolve => {
     let startTime: number;
     let attempts = 0;
 
@@ -63,7 +73,8 @@ export class ScrollView {
 
       if (!self._el || !self.isPlaying || attempts > maxAttempts) {
         self.isPlaying = false;
-          resolve();
+        self._el.style.transform = ``;
+        done();
         return;
       }
 
@@ -85,8 +96,8 @@ export class ScrollView {
         nativeRaf(step);
 
       } else {
-          // done
+        self._el.style.transform = ``;
-          resolve();
+        done();
       }
     }
 
@@ -94,12 +105,12 @@ export class ScrollView {
     self.isPlaying = true;
 
     // chill out for a frame first
-      nativeRaf(() => {
+    rafFrames(2, () => {
       startTime = Date.now();
-        nativeRaf(step);
+      step();
     });
 
-    });
+    return promise;
   }
 
   scrollToTop(duration: number): Promise<any> {