From 33a627461343b8155c1ec2309d4b1dba4e51c1ec Mon Sep 17 00:00:00 2001
From: "Manu Mtz.-Almeida" <manu.mtza@gmail.com>
Date: Fri, 2 Mar 2018 16:04:27 +0100
Subject: [PATCH] perf(scroll): watchdog + simplification

---
 packages/core/src/components.d.ts             |  17 +-
 packages/core/src/components/app/app.tsx      |  29 +-
 packages/core/src/components/app/readme.md    |   8 -
 .../core/src/components/content/content.tsx   |   8 +-
 .../core/src/components/content/readme.md     |  10 +
 .../components/content/test/basic/index.html  |   7 +
 .../infinite-scroll/infinite-scroll.tsx       |  13 +-
 .../input-shims/hacks/input-blurring.ts       |  14 +-
 .../components/input-shims/input-shims.tsx    |   5 +-
 .../core/src/components/input-shims/readme.md |  14 -
 packages/core/src/components/scroll/readme.md |  31 +-
 .../core/src/components/scroll/scroll.scss    |   3 -
 .../core/src/components/scroll/scroll.tsx     | 276 ++++++++----------
 packages/core/src/index.d.ts                  |   2 +-
 packages/core/src/utils/helpers.ts            |  11 +-
 15 files changed, 183 insertions(+), 265 deletions(-)

diff --git a/packages/core/src/components.d.ts b/packages/core/src/components.d.ts
index b1ce4ccebc..4ba1aafc56 100644
--- a/packages/core/src/components.d.ts
+++ b/packages/core/src/components.d.ts
@@ -42,20 +42,16 @@ import {
   GestureCallback,
   GestureDetail,
 } from './components/gesture/gesture';
-import {
-  App,
-  FrameworkDelegate as FrameworkDelegate2,
-} from '.';
 import {
   PickerButton,
   PickerColumn as PickerColumn2,
 } from './components/picker/picker';
-import {
-  ScrollCallback,
-} from './components/scroll/scroll';
 import {
   SelectPopoverOption,
 } from './components/select-popover/select-popover';
+import {
+  FrameworkDelegate as FrameworkDelegate2,
+} from '.';
 import {
   DomRenderFn,
   HeaderFn,
@@ -770,6 +766,7 @@ declare global {
     export interface IonContentAttributes extends HTMLAttributes {
       forceOverscroll?: boolean;
       fullscreen?: boolean;
+      scrollEvents?: boolean;
     }
   }
 }
@@ -1240,7 +1237,7 @@ declare global {
   }
   namespace JSXElements {
     export interface IonInputShimsAttributes extends HTMLAttributes {
-      app?: App;
+      
     }
   }
 }
@@ -2706,9 +2703,7 @@ declare global {
     export interface IonScrollAttributes extends HTMLAttributes {
       forceOverscroll?: boolean;
       mode?: string;
-      onionScroll?: ScrollCallback;
-      onionScrollEnd?: ScrollCallback;
-      onionScrollStart?: ScrollCallback;
+      scrollEvents?: boolean;
     }
   }
 }
diff --git a/packages/core/src/components/app/app.tsx b/packages/core/src/components/app/app.tsx
index a7b51e70a4..6b4a8870d1 100644
--- a/packages/core/src/components/app/app.tsx
+++ b/packages/core/src/components/app/app.tsx
@@ -4,7 +4,6 @@ import { Config, NavEvent, OverlayController, PublicNav, PublicViewController }
 import { getOrAppendElement } from '../../utils/helpers';
 
 const rootNavs = new Map<number, HTMLIonNavElement>();
-const ACTIVE_SCROLLING_TIME = 100;
 let backButtonActions: BackButtonAction[] = [];
 
 @Component({
@@ -21,11 +20,9 @@ export class App {
 
   private isDevice = false;
   private deviceHacks = false;
-  private scrollTime = 0;
 
   externalNavPromise: void | Promise<any> = null;
   externalNavOccuring = false;
-  didScroll = false;
 
   @Element() element: HTMLElement;
   @Event() exitApp: EventEmitter<ExitAppEventDetail>;
@@ -102,29 +99,6 @@ export class App {
     return true;
   }
 
-  /**
-   * Boolean if the app is actively scrolling or not.
-   * @return {boolean} returns true or false
-   */
-  @Method()
-  isScrolling(): boolean {
-    const scrollTime = this.scrollTime;
-    if (scrollTime === 0) {
-      return false;
-    }
-    if (scrollTime < Date.now()) {
-      this.scrollTime = 0;
-      return false;
-    }
-    return true;
-  }
-
-  @Method()
-  setScrolling() {
-    this.scrollTime = Date.now() + ACTIVE_SCROLLING_TIME;
-    this.didScroll = true;
-  }
-
   @Method()
   getTopNavs(rootNavId = -1): PublicNav[] {
     return getTopNavsImpl(rootNavId);
@@ -142,7 +116,6 @@ export class App {
     return null;
   }
 
-
   /**
    * The back button event is triggered when the user presses the native
    * platform's back button, also referred to as the "hardware" back button.
@@ -244,7 +217,7 @@ export class App {
   render() {
     return [
       <ion-platform />,
-      this.deviceHacks && <ion-input-shims app={this} />,
+      this.deviceHacks && <ion-input-shims />,
       this.isDevice && <ion-tap-click />,
       this.isDevice && <ion-status-tap />,
       <slot></slot>
diff --git a/packages/core/src/components/app/readme.md b/packages/core/src/components/app/readme.md
index 527bea63f2..7faa1e312a 100644
--- a/packages/core/src/components/app/readme.md
+++ b/packages/core/src/components/app/readme.md
@@ -42,11 +42,6 @@ Returns an array of top level Navs
 Returns whether the application is enabled or not
 
 
-#### isScrolling()
-
-Boolean if the app is actively scrolling or not.
-
-
 #### registerBackButtonAction()
 
 The back button event is triggered when the user presses the native
@@ -69,9 +64,6 @@ This API is not meant for public usage and could
 change at any time
 
 
-#### setScrolling()
-
-
 #### updateExternalNavOccuring()
 
 Updates whether an external navigation event is occuring
diff --git a/packages/core/src/components/content/content.tsx b/packages/core/src/components/content/content.tsx
index cb88e9233b..eb57ef1852 100644
--- a/packages/core/src/components/content/content.tsx
+++ b/packages/core/src/components/content/content.tsx
@@ -35,6 +35,9 @@ export class Content {
    */
   @Prop() forceOverscroll: boolean;
 
+
+  @Prop() scrollEvents = false;
+
   @Listen('body:ionNavChanged')
   onNavChanged() {
     this.resize();
@@ -128,7 +131,10 @@ export class Content {
     this.resize();
 
     return [
-      <ion-scroll mode={this.mode} forceOverscroll={this.forceOverscroll}>
+      <ion-scroll
+        mode={this.mode}
+        scrollEvents={this.scrollEvents}
+        forceOverscroll={this.forceOverscroll}>
         <slot></slot>
       </ion-scroll>,
       <slot name='fixed'></slot>
diff --git a/packages/core/src/components/content/readme.md b/packages/core/src/components/content/readme.md
index b0a2e96a6c..4ef9272ad0 100644
--- a/packages/core/src/components/content/readme.md
+++ b/packages/core/src/components/content/readme.md
@@ -34,6 +34,11 @@ and footers. This effect can easily be seen by setting the toolbar
 to transparent.
 
 
+#### scrollEvents
+
+boolean
+
+
 ## Attributes
 
 #### force-overscroll
@@ -54,6 +59,11 @@ and footers. This effect can easily be seen by setting the toolbar
 to transparent.
 
 
+#### scroll-events
+
+boolean
+
+
 ## Methods
 
 #### scrollByPoint()
diff --git a/packages/core/src/components/content/test/basic/index.html b/packages/core/src/components/content/test/basic/index.html
index c22c965518..0962435be3 100644
--- a/packages/core/src/components/content/test/basic/index.html
+++ b/packages/core/src/components/content/test/basic/index.html
@@ -69,6 +69,13 @@
       </ion-footer>
 
     <script>
+      const content = document.getElementById('content');
+      content.scrollEvents = true;
+      content.addEventListener('ionScrollStart', () => console.log('scroll start'));
+      content.addEventListener('ionScroll', (ev) => console.log('scroll', ev.detail));
+      content.addEventListener('ionScrollEnd', () => console.log('scroll end'));
+
+
       function toggleFullscreen() {
         const content = document.getElementById('content');
         content.fullscreen = !content.fullscreen;
diff --git a/packages/core/src/components/infinite-scroll/infinite-scroll.tsx b/packages/core/src/components/infinite-scroll/infinite-scroll.tsx
index 82e4638272..0890fc8ab6 100644
--- a/packages/core/src/components/infinite-scroll/infinite-scroll.tsx
+++ b/packages/core/src/components/infinite-scroll/infinite-scroll.tsx
@@ -1,5 +1,5 @@
 import { Component, Element, Event, EventEmitter, EventListenerEnable, Listen, Method, Prop, State, Watch } from '@stencil/core';
-import { DomController, ScrollDetail } from '../../index';
+import { DomController } from '../../index';
 
 const enum Position {
   Top = 'top',
@@ -106,10 +106,9 @@ export class InfiniteScroll {
     this.scrollEl = null;
   }
 
-  @Listen('ionScroll', {enabled: false})
-  protected onScroll(ev: CustomEvent) {
+  @Listen('scroll', {enabled: false})
+  protected onScroll() {
     const scrollEl = this.scrollEl;
-    const detail = ev.detail as ScrollDetail;
     if (!scrollEl || !this.canStart()) {
       return 1;
     }
@@ -119,7 +118,7 @@ export class InfiniteScroll {
       // if there is no height of this element then do nothing
       return 2;
     }
-    const scrollTop = detail.scrollTop;
+    const scrollTop = scrollEl.scrollTop;
     const scrollHeight = scrollEl.scrollHeight;
     const height = scrollEl.offsetHeight;
     const threshold = this.thrPc ? (height * this.thrPc) : this.thrPx;
@@ -128,7 +127,7 @@ export class InfiniteScroll {
       ? scrollHeight - infiniteHeight - scrollTop - threshold - height
       : scrollTop - infiniteHeight - threshold;
 
-      if (distanceFromInfinite < 0) {
+    if (distanceFromInfinite < 0) {
       if (!this.didFire) {
         this.isLoading = true;
         this.didFire = true;
@@ -221,7 +220,7 @@ export class InfiniteScroll {
 
   private enableScrollEvents(shouldListen: boolean) {
     if (this.scrollEl) {
-      this.enableListener(this, 'ionScroll', shouldListen, this.scrollEl);
+      this.enableListener(this, 'scroll', shouldListen, this.scrollEl);
     }
   }
 
diff --git a/packages/core/src/components/input-shims/hacks/input-blurring.ts b/packages/core/src/components/input-shims/hacks/input-blurring.ts
index 84779ae3dc..30023fd389 100644
--- a/packages/core/src/components/input-shims/hacks/input-blurring.ts
+++ b/packages/core/src/components/input-shims/hacks/input-blurring.ts
@@ -1,11 +1,15 @@
-import { App } from '../../..';
 
 const SKIP_BLURRING = ['INPUT', 'TEXTAREA', 'ION-INPUT', 'ION-TEXTAREA'];
 
-export default function enableInputBlurring(app: App) {
+export default function enableInputBlurring() {
   console.debug('Input: enableInputBlurring');
 
   let focused = true;
+  let didScroll = false;
+
+  function onScroll() {
+    didScroll = true;
+  }
 
   function onFocusin() {
     focused = true;
@@ -13,8 +17,8 @@ export default function enableInputBlurring(app: App) {
 
   function onTouchend(ev: any) {
     // if app did scroll return early
-    if (app.didScroll) {
-      app.didScroll = false;
+    if (didScroll) {
+      didScroll = false;
       return;
     }
     const active = document.activeElement as HTMLElement;
@@ -49,10 +53,12 @@ export default function enableInputBlurring(app: App) {
     }, 50);
   }
 
+  document.addEventListener('ionScrollStart', onScroll);
   document.addEventListener('focusin', onFocusin, true);
   document.addEventListener('touchend', onTouchend, false);
 
   return () => {
+    document.removeEventListener('ionScrollStart', onScroll, true);
     document.removeEventListener('focusin', onFocusin, true);
     document.removeEventListener('touchend', onTouchend, false);
   };
diff --git a/packages/core/src/components/input-shims/input-shims.tsx b/packages/core/src/components/input-shims/input-shims.tsx
index 0e7e4e2db4..b6cc3a8172 100644
--- a/packages/core/src/components/input-shims/input-shims.tsx
+++ b/packages/core/src/components/input-shims/input-shims.tsx
@@ -1,5 +1,5 @@
 import { Component, Listen, Prop } from '@stencil/core';
-import { App, Config } from '../..';
+import { Config } from '../..';
 
 import enableHideCaretOnScroll from './hacks/hide-caret';
 import enableInputBlurring from './hacks/input-blurring';
@@ -24,7 +24,6 @@ export class InputShims {
   private scrollAssistMap = new WeakMap<HTMLElement, Function>();
 
   @Prop({context: 'config'}) config: Config;
-  @Prop() app: App;
 
   componentDidLoad() {
     this.keyboardHeight = this.config.getNumber('keyboardHeight', 290);
@@ -33,7 +32,7 @@ export class InputShims {
 
     const inputBlurring = this.config.getBoolean('inputBlurring', true);
     if (inputBlurring && INPUT_BLURRING) {
-      enableInputBlurring(this.app);
+      enableInputBlurring();
     }
 
     const scrollPadding = this.config.getBoolean('scrollPadding', true);
diff --git a/packages/core/src/components/input-shims/readme.md b/packages/core/src/components/input-shims/readme.md
index cc7f3f4b9d..a5f662f426 100644
--- a/packages/core/src/components/input-shims/readme.md
+++ b/packages/core/src/components/input-shims/readme.md
@@ -5,20 +5,6 @@
 <!-- Auto Generated Below -->
 
 
-## Properties
-
-#### app
-
-
-
-
-## Attributes
-
-#### app
-
-
-
-
 
 ----------------------------------------------
 
diff --git a/packages/core/src/components/scroll/readme.md b/packages/core/src/components/scroll/readme.md
index e95cb9286d..702d179116 100644
--- a/packages/core/src/components/scroll/readme.md
+++ b/packages/core/src/components/scroll/readme.md
@@ -21,19 +21,9 @@ Note, the does not disable the system bounce on iOS. That is an OS level setting
 string
 
 
-#### onionScroll
-
-
-
-
-#### onionScrollEnd
-
-
-
-
-#### onionScrollStart
-
+#### scrollEvents
 
+boolean
 
 
 ## Attributes
@@ -52,26 +42,17 @@ Note, the does not disable the system bounce on iOS. That is an OS level setting
 string
 
 
-#### onion-scroll
-
-
-
-
-#### onion-scroll-end
-
-
-
-
-#### onion-scroll-start
-
+#### scroll-events
 
+boolean
 
 
 ## Events
 
 #### ionScroll
 
-Emitted while scrolling.
+Emitted while scrolling. This event is disabled by default.
+Look at the property: `scrollEvents`
 
 
 #### ionScrollEnd
diff --git a/packages/core/src/components/scroll/scroll.scss b/packages/core/src/components/scroll/scroll.scss
index cfb9c11002..a0739acd50 100644
--- a/packages/core/src/components/scroll/scroll.scss
+++ b/packages/core/src/components/scroll/scroll.scss
@@ -14,9 +14,6 @@ ion-scroll {
 
   contain: size style layout;
 
-  margin: 0 !important; // scss-lint:disable all
-  padding: 0 !important; // scss-lint:disable all
-
   .focus-input {
     @include margin(0);
     @include padding(0);
diff --git a/packages/core/src/components/scroll/scroll.tsx b/packages/core/src/components/scroll/scroll.tsx
index bb1b56a725..9b5dfa44d2 100644
--- a/packages/core/src/components/scroll/scroll.tsx
+++ b/packages/core/src/components/scroll/scroll.tsx
@@ -1,6 +1,5 @@
-import { Component, Element, Event, EventEmitter, EventListenerEnable, Listen, Method, Prop } from '@stencil/core';
-import { Config, DomController, GestureDelegate, GestureDetail } from '../../index';
-
+import { Component, Element, Event, EventEmitter, Listen, Method, Prop } from '@stencil/core';
+import { Config, DomController, GestureDetail } from '../../index';
 
 @Component({
   tag: 'ion-scroll',
@@ -14,19 +13,14 @@ import { Config, DomController, GestureDelegate, GestureDetail } from '../../ind
 })
 export class Scroll {
 
-  private gesture: GestureDelegate;
-  private positions: number[] = [];
-  private tmr: any;
-  private queued = false;
-  private app: HTMLIonAppElement;
+  private watchDog: any;
   private isScrolling = false;
-  private detail: ScrollDetail = {};
-
+  private lastScroll = 0;
+  private detail: ScrollDetail;
+  private queued = false;
   @Element() private el: HTMLElement;
 
-  @Prop({ connect: 'ion-gesture-controller'}) gestureCtrl: HTMLIonGestureControllerElement;
   @Prop({ context: 'config'}) config: Config;
-  @Prop({ context: 'enableListener'}) enableListener: EventListenerEnable;
   @Prop({ context: 'dom' }) dom: DomController;
   @Prop({ context: 'isServer' }) isServer: boolean;
 
@@ -40,69 +34,76 @@ export class Scroll {
    */
   @Prop({mutable: true}) forceOverscroll: boolean;
 
-  @Prop() onionScrollStart: ScrollCallback;
-  @Prop() onionScroll: ScrollCallback;
-  @Prop() onionScrollEnd: ScrollCallback;
+  @Prop() scrollEvents = false;
 
   /**
    * Emitted when the scroll has started.
    */
-  @Event() ionScrollStart: EventEmitter;
+  @Event() ionScrollStart: EventEmitter<ScrollBaseDetail>;
 
   /**
-   * Emitted while scrolling.
+   * Emitted while scrolling. This event is disabled by default.
+   * Look at the property: `scrollEvents`
    */
-  @Event({bubbles: false}) ionScroll: EventEmitter;
+  @Event({bubbles: false}) ionScroll: EventEmitter<ScrollDetail>;
 
   /**
    * Emitted when the scroll has ended.
    */
-  @Event() ionScrollEnd: EventEmitter;
+  @Event() ionScrollEnd: EventEmitter<ScrollBaseDetail>;
 
-  componentWillLoad() {
-    return this.gestureCtrl.create({
-      name: 'scroll',
-      priority: 100,
-      disableScroll: false,
-    }).then((gesture) => {
-      this.gesture = gesture;
-    });
+  constructor() {
+    // Detail is used in a hot loop in the scroll event, by allocating it here
+    // V8 will be able to inline any read/write to it since it's a monomorphic class.
+    // https://mrale.ph/blog/2015/01/11/whats-up-with-monomorphism.html
+    this.detail = {
+      positions: [],
+      scrollTop: 0,
+      scrollLeft: 0,
+      type: 'scroll',
+      event: undefined,
+      startX: 0,
+      startY: 0,
+      startTimeStamp: 0,
+      currentX: 0,
+      currentY: 0,
+      velocityX: 0,
+      velocityY: 0,
+      deltaX: 0,
+      deltaY: 0,
+      timeStamp: 0,
+      data: undefined
+    };
   }
 
-  componentDidLoad() {
+  componentWillLoad() {
     if (this.isServer) {
       return;
     }
     if (this.forceOverscroll === undefined) {
       this.forceOverscroll = this.mode === 'ios' && ('ontouchstart' in window);
     }
-    this.app = this.el.closest('ion-app');
   }
 
-  componentDidUnload() {
-    this.gesture && this.gesture.destroy();
-    this.gesture = this.detail = this.detail.event = null;
-  }
-
-  // Native Scroll *************************
-
   @Listen('scroll', { passive: true })
-  onNativeScroll() {
-    if (!this.queued) {
+  onScroll(ev: UIEvent) {
+    const timeStamp = Date.now();
+    const didStart = !this.isScrolling;
+    this.lastScroll = timeStamp;
+    if (didStart) {
+      this.onScrollStart();
+    }
+    if (!this.queued && this.scrollEvents) {
       this.queued = true;
-
       this.dom.read(timeStamp => {
         this.queued = false;
-        this.onScroll(timeStamp);
+        this.detail.event = ev;
+        updateScrollDetail(this.detail, this.el, timeStamp, didStart);
+        this.ionScroll.emit(this.detail);
       });
     }
   }
 
-  @Listen('touchend', {passive: true, capture: true })
-  onTouchEnd(_ev: TouchEvent) {
-    this.refocus();
-  }
-
   @Method()
   scrollToTop(duration: number): Promise<void> {
     return this.scrollToPoint(0, 0, duration);
@@ -212,115 +213,29 @@ export class Scroll {
     return promise;
   }
 
-  private onScroll(timeStamp: number) {
-    const detail = this.detail;
-    const positions = this.positions;
-    const el = this.el;
-    if (this.app) {
-      this.app.setScrolling();
-    }
 
-    detail.timeStamp = timeStamp;
+  private onScrollStart() {
+    this.isScrolling = true;
+    this.ionScrollStart.emit({
+      isScrolling: true
+    });
 
-    // get the current scrollTop
-    // ******** DOM READ ****************
-    detail.scrollTop = el.scrollTop;
-
-    // get the current scrollLeft
-    // ******** DOM READ ****************
-    detail.scrollLeft = el.scrollLeft;
-
-
-    if (!this.isScrolling) {
-      // currently not scrolling, so this is a scroll start
-      this.isScrolling = true;
-
-      // remember the start positions
-      detail.startY = detail.scrollTop;
-      detail.startX = detail.scrollLeft;
-
-      // new scroll, so do some resets
-      detail.velocityY = detail.velocityX = detail.deltaY = detail.deltaX = positions.length = 0;
-
-      // emit only on the first scroll event
-      if (this.onionScrollStart) {
-        this.onionScrollStart(detail);
+    // watchdog
+    this.watchDog = setInterval(() => {
+      if (this.lastScroll < Date.now() - 120) {
+        this.onScrollEnd();
       }
-      this.gesture.capture();
-      this.ionScrollStart.emit(detail);
-    }
-    detail.deltaY = (detail.scrollTop - detail.startY);
-    detail.deltaX = (detail.scrollLeft - detail.startX);
-
-    // actively scrolling
-    positions.push(detail.scrollTop, detail.scrollLeft, detail.timeStamp);
-
-    // move pointer to position measured 100ms ago
-    const timeRange = timeStamp - 100;
-    let startPos = positions.length - 1;
-
-    while (startPos > 0 && positions[startPos] > timeRange) {
-      startPos -= 3;
-    }
-
-    if (startPos > 3) {
-      // compute relative movement between these two points
-      const frequency = 1 / (positions[startPos] - timeStamp);
-      const movedY = positions[startPos - 1] - detail.scrollLeft;
-      const movedX = positions[startPos - 2] - detail.scrollTop;
-
-      // based on XXms compute the movement to apply for each render step
-      // velocity = space/time = s*(1/t) = s*frequency
-      detail.velocityX = movedX * frequency;
-      detail.velocityY = movedY * frequency;
-    } else {
-      detail.velocityX = 0;
-      detail.velocityY = 0;
-    }
-
-    clearTimeout(this.tmr);
-    this.tmr = setTimeout(() => {
-
-      // haven't scrolled in a while, so it's a scrollend
-      this.isScrolling = false;
-
-      this.dom.read(timeStamp => {
-        if (!this.isScrolling) {
-          this.onEnd(timeStamp);
-        }
-      });
-    }, 80);
-
-    // emit on each scroll event
-    if (this.onionScroll) {
-      this.onionScroll(detail);
-    } else {
-      this.ionScroll.emit(detail);
-    }
+    }, 100);
   }
 
-  private onEnd(timeStamp: number) {
-    const detail = this.detail;
+  private onScrollEnd() {
 
-    detail.timeStamp = timeStamp;
-
-    // emit that the scroll has ended
-    this.gesture.release();
-
-    if (this.onionScrollEnd) {
-      this.onionScrollEnd(detail);
-    }
-    this.ionScrollEnd.emit(detail);
-    this.refocus();
-  }
-
-  private refocus() {
-    // TODO: renable when desktop testing is done
-    // setTimeout(() => {
-    //   if (document.activeElement === document.body) {
-    //     (this.el.querySelector('.focus-input') as HTMLElement).focus();
-    //   }
-    // });
+    clearInterval(this.watchDog);
+    this.watchDog = null;
+    this.isScrolling = false;
+    this.ionScrollEnd.emit({
+      isScrolling: false
+    });
   }
 
   hostData() {
@@ -339,18 +254,67 @@ export class Scroll {
       </div>
     ];
   }
-
 }
 
-export interface ScrollDetail extends GestureDetail {
+// ******** DOM READ ****************
+function updateScrollDetail(
+  detail: ScrollDetail,
+  el: HTMLElement,
+  timeStamp: number,
+  didStart: boolean
+) {
+  const scrollTop = el.scrollTop;
+  const scrollLeft = el.scrollLeft;
+  const positions = detail.positions;
+  if (didStart) {
+    // remember the start positions
+    detail.startTimeStamp = timeStamp;
+    detail.startY = scrollTop;
+    detail.startX = scrollLeft;
+    positions.length = 0;
+  }
+
+  detail.timeStamp = timeStamp;
+  detail.currentY = detail.scrollTop = scrollTop;
+  detail.currentX = detail.scrollLeft = scrollLeft;
+  detail.deltaY = scrollTop - detail.startY;
+  detail.deltaX = scrollLeft - detail.startX;
+
+  // actively scrolling
+  positions.push(scrollTop, scrollLeft, timeStamp);
+
+  // move pointer to position measured 100ms ago
+  const timeRange = timeStamp - 100;
+  let startPos = positions.length - 1;
+
+  while (startPos > 0 && positions[startPos] > timeRange) {
+    startPos -= 3;
+  }
+
+  if (startPos > 3) {
+    // compute relative movement between these two points
+    const frequency = 1 / (positions[startPos] - timeStamp);
+    const movedX = positions[startPos - 1] - scrollLeft;
+    const movedY = positions[startPos - 2] - scrollTop;
+
+    // based on XXms compute the movement to apply for each render step
+    // velocity = space/time = s*(1/t) = s*frequency
+    detail.velocityX = movedX * frequency;
+    detail.velocityY = movedY * frequency;
+  } else {
+    detail.velocityX = 0;
+    detail.velocityY = 0;
+  }
+}
+
+export interface ScrollDetail extends GestureDetail, ScrollBaseDetail {
+  positions?: number[];
   scrollTop?: number;
   scrollLeft?: number;
-  scrollHeight?: number;
-  scrollWidth?: number;
-  contentHeight?: number;
-  contentWidth?: number;
-  contentTop?: number;
-  contentBottom?: number;
+}
+
+export interface ScrollBaseDetail {
+  isScrolling?: boolean;
 }
 
 export interface ScrollCallback {
diff --git a/packages/core/src/index.d.ts b/packages/core/src/index.d.ts
index b8e50be0c6..655f6f5cae 100644
--- a/packages/core/src/index.d.ts
+++ b/packages/core/src/index.d.ts
@@ -80,7 +80,7 @@ export {
 } from './components/router/router-utils';
 export { Row } from './components/row/row';
 export { Reorder } from './components/reorder/reorder';
-export { Scroll, ScrollCallback, ScrollDetail } from './components/scroll/scroll';
+export { Scroll } from './components/scroll/scroll';
 export { Searchbar } from './components/searchbar/searchbar';
 export { Segment } from './components/segment/segment';
 export { SegmentButton } from './components/segment-button/segment-button';
diff --git a/packages/core/src/utils/helpers.ts b/packages/core/src/utils/helpers.ts
index 7ea8f2b47e..ec2704abe2 100644
--- a/packages/core/src/utils/helpers.ts
+++ b/packages/core/src/utils/helpers.ts
@@ -44,9 +44,12 @@ export function isCheckedProperty(a: any, b: any): boolean {
   return (a == b); // tslint:disable-line
 }
 
-export function assert(bool: boolean, msg: string) {
-  if (!bool) {
-    console.error(msg);
+export function assert(actual: any, reason: string) {
+  if (!actual) {
+    const message = 'ASSERT: ' + reason;
+    console.error(message);
+    debugger; // tslint:disable-line
+    throw new Error(message);
   }
 }
 
@@ -143,7 +146,7 @@ export function getPageElement(el: HTMLElement) {
   if (tabs) {
     return tabs;
   }
-  const page = el.closest('ion-page,.ion-page,page-inner');
+  const page = el.closest('ion-app,ion-page,.ion-page,page-inner');
   if (page) {
     return page;
   }