From 3d717f5535950e7b28ea14e2e7d65444017c6f5e Mon Sep 17 00:00:00 2001
From: Jannick Garthen <jannick.garthen@gmail.com>
Date: Sun, 28 Oct 2018 15:03:19 +0100
Subject: [PATCH] feat(ViewportProvider): implement experimental scheduler to
 prioritize updates

---
 examples/index.tsx       |  6 ++-
 lib/ObserveViewport.tsx  |  6 ++-
 lib/ViewportProvider.tsx | 95 +++++++++++++++++++++++++++++++++++-----
 lib/hooks.ts             |  7 +--
 lib/types.ts             |  3 ++
 5 files changed, 101 insertions(+), 16 deletions(-)

diff --git a/examples/index.tsx b/examples/index.tsx
index dff959e..e552ee2 100644
--- a/examples/index.tsx
+++ b/examples/index.tsx
@@ -25,7 +25,9 @@ const ViewportHeader = connectViewport({ omit: ['scroll'] })<{ a: string }>(
 );
 
 const DisplayScroll = () => {
-  const { x, y } = useScroll();
+  const { x, y } = useScroll({
+    priority: 'low',
+  });
   return (
     <>
       x: {x}, y: {y}
@@ -116,7 +118,7 @@ class Example extends React.PureComponent<{}, { disabled: boolean }> {
 }
 
 render(
-  <ViewportProvider>
+  <ViewportProvider experimentalSchedulerEnabled>
     <main role="main">
       <Example />
       <Placeholder />
diff --git a/lib/ObserveViewport.tsx b/lib/ObserveViewport.tsx
index 4b2ed29..ad03aa9 100644
--- a/lib/ObserveViewport.tsx
+++ b/lib/ObserveViewport.tsx
@@ -12,6 +12,7 @@ import {
   IViewport,
   TViewportChangeHandler,
   IViewportChangeOptions,
+  PriorityType,
 } from './types';
 import { warnNoContextAvailable } from './utils';
 
@@ -29,6 +30,7 @@ interface IProps {
   disableScrollUpdates: boolean;
   disableDimensionsUpdates: boolean;
   deferUpdateUntilIdle: boolean;
+  priority: PriorityType;
 }
 
 interface IContext {
@@ -54,10 +56,11 @@ export default class ObserveViewport extends React.Component<IProps, IState> {
 
   private tickId: NodeJS.Timer;
 
-  static defaultProps = {
+  static defaultProps: IProps = {
     disableScrollUpdates: false,
     disableDimensionsUpdates: false,
     deferUpdateUntilIdle: false,
+    priority: 'normal',
   };
 
   constructor(props: IProps) {
@@ -134,6 +137,7 @@ export default class ObserveViewport extends React.Component<IProps, IState> {
       notifyScroll: () => !this.props.disableScrollUpdates,
       notifyDimensions: () => !this.props.disableDimensionsUpdates,
       notifyOnlyWhenIdle: () => this.props.deferUpdateUntilIdle,
+      priority: () => this.props.priority,
       recalculateLayoutBeforeUpdate: (viewport: IViewport) => {
         if (this.props.recalculateLayoutBeforeUpdate) {
           return this.props.recalculateLayoutBeforeUpdate(viewport);
diff --git a/lib/ViewportProvider.tsx b/lib/ViewportProvider.tsx
index 211dcdb..b353286 100644
--- a/lib/ViewportProvider.tsx
+++ b/lib/ViewportProvider.tsx
@@ -8,8 +8,15 @@ import {
 } from './types';
 import ViewportCollector from './ViewportCollector';
 
+interface IProps {
+  experimentalSchedulerEnabled?: boolean;
+}
+
 interface IListener extends IViewportChangeOptions {
   handler: TViewportChangeHandler;
+  iterations: number;
+  averageExecutionCost: number;
+  skippedIterations: number;
 }
 
 export const ViewportContext = React.createContext({
@@ -22,14 +29,51 @@ export const ViewportContext = React.createContext({
   version: '__VERSION__',
 });
 
+const maxIterations = (priority: 'highest' | 'high' | 'normal' | 'low') => {
+  switch (priority) {
+    case 'highest':
+      return 0;
+    case 'high':
+      return 4;
+    case 'normal':
+      return 16;
+    case 'low':
+      return 64;
+  }
+};
+
+const shouldSkipIteration = (
+  { priority: getPriority, averageExecutionCost, skippedIterations }: IListener,
+  budget: number,
+): boolean => {
+  const priority = getPriority();
+  if (priority === 'highest') {
+    return false;
+  }
+  if (priority !== 'low' && averageExecutionCost <= budget) {
+    return false;
+  }
+  if (averageExecutionCost <= budget / 10) {
+    return false;
+  }
+  const probability = skippedIterations / maxIterations(priority);
+  if (probability >= 1) {
+    return false;
+  }
+  return Math.random() > probability;
+};
+
 export default class ViewportProvider extends React.PureComponent<
-  {},
+  IProps,
   { hasListeners: boolean }
 > {
+  static defaultProps: {
+    experimentalSchedulerEnabled: false;
+  };
   private listeners: IListener[] = [];
   private updateListenersTick: NodeJS.Timer;
 
-  constructor(props: {}) {
+  constructor(props: IProps) {
     super(props);
     this.state = {
       hasListeners: false,
@@ -46,7 +90,7 @@ export default class ViewportProvider extends React.PureComponent<
     options?: { isIdle: boolean },
   ) => {
     const { isIdle } = Object.assign({ isIdle: false }, options);
-    const updatableListeners = this.listeners.filter(
+    let updatableListeners = this.listeners.filter(
       ({ notifyScroll, notifyDimensions, notifyOnlyWhenIdle }) => {
         if (notifyOnlyWhenIdle() && !isIdle) {
           return false;
@@ -56,18 +100,43 @@ export default class ViewportProvider extends React.PureComponent<
         return updateForScroll || updateForDimensions;
       },
     );
+    if (this.props.experimentalSchedulerEnabled) {
+      if (!isIdle) {
+        const budget = 16 / updatableListeners.length;
+        updatableListeners = updatableListeners.filter(listener => {
+          const skip = shouldSkipIteration(listener, budget);
+          if (skip) {
+            listener.skippedIterations++;
+            return false;
+          }
+          listener.skippedIterations = 0;
+          return true;
+        });
+      }
+    }
     const layouts = updatableListeners.map(
       ({ recalculateLayoutBeforeUpdate }) => {
         if (recalculateLayoutBeforeUpdate) {
-          return recalculateLayoutBeforeUpdate(state);
+          const start = performance.now();
+          const layoutState = recalculateLayoutBeforeUpdate(state);
+          return [layoutState, performance.now() - start];
         }
         return null;
       },
     );
 
-    updatableListeners.forEach(({ handler }, index) => {
-      const layout = layouts[index];
+    updatableListeners.forEach((listener, index) => {
+      const { handler, averageExecutionCost, iterations } = listener;
+      const [layout, layoutCost] = layouts[index] || [null, 0];
+
+      const start = performance.now();
       handler(state, layout);
+      const totalCost = layoutCost + performance.now() - start;
+      const diff = totalCost - averageExecutionCost;
+      const i = iterations + 1;
+
+      listener.averageExecutionCost = averageExecutionCost + diff / i;
+      listener.iterations = i;
     });
   };
 
@@ -75,16 +144,22 @@ export default class ViewportProvider extends React.PureComponent<
     handler: TViewportChangeHandler,
     options: IViewportChangeOptions,
   ) => {
-    this.listeners.push({ handler, ...options });
-    this.updateListenersLazy();
+    this.listeners.push({
+      handler,
+      iterations: 0,
+      averageExecutionCost: 0,
+      skippedIterations: 0,
+      ...options,
+    });
+    this.updateHasListenersState();
   };
 
   removeViewportChangeListener = (h: TViewportChangeHandler) => {
     this.listeners = this.listeners.filter(({ handler }) => handler !== h);
-    this.updateListenersLazy();
+    this.updateHasListenersState();
   };
 
-  updateListenersLazy() {
+  updateHasListenersState() {
     clearTimeout(this.updateListenersTick);
     this.updateListenersTick = setTimeout(() => {
       this.setState({
diff --git a/lib/hooks.ts b/lib/hooks.ts
index eb5c893..6589190 100644
--- a/lib/hooks.ts
+++ b/lib/hooks.ts
@@ -5,11 +5,10 @@ import {
   createInitScrollState,
   createInitDimensionsState,
 } from './ViewportCollector';
-import { IViewport, IScroll, IDimensions } from './types';
+import { IViewport, IScroll, IDimensions, PriorityType } from './types';
 import { warnNoContextAvailable } from './utils';
 
-interface IFullOptions {
-  recalculateLayoutBeforeUpdate?: (props: IViewport) => any;
+interface IFullOptions extends IOptions {
   disableScrollUpdates?: boolean;
   disableDimensionsUpdates?: boolean;
   deferUpdateUntilIdle?: boolean;
@@ -17,6 +16,7 @@ interface IFullOptions {
 
 interface IOptions {
   deferUpdateUntilIdle?: boolean;
+  priority?: PriorityType;
   recalculateLayoutBeforeUpdate?: (props: IViewport) => any;
 }
 
@@ -43,6 +43,7 @@ const useViewportEffect = (
         notifyScroll: () => !options.disableScrollUpdates,
         notifyDimensions: () => !options.disableDimensionsUpdates,
         notifyOnlyWhenIdle: () => Boolean(options.deferUpdateUntilIdle),
+        priority: () => options.priority || 'normal',
         recalculateLayoutBeforeUpdate: options.recalculateLayoutBeforeUpdate,
       });
       return () => removeViewportChangeListener(handleViewportChange);
diff --git a/lib/types.ts b/lib/types.ts
index 6512b90..1173af1 100644
--- a/lib/types.ts
+++ b/lib/types.ts
@@ -45,6 +45,7 @@ export interface IViewportChangeOptions {
   notifyScroll: () => boolean;
   notifyDimensions: () => boolean;
   notifyOnlyWhenIdle: () => boolean;
+  priority: () => PriorityType;
   recalculateLayoutBeforeUpdate?: (viewport: IViewport) => any;
 }
 
@@ -57,3 +58,5 @@ export type OnUpdateType = (
   props: IViewport,
   options: IViewportCollectorUpdateOptions,
 ) => void;
+
+export type PriorityType = 'highest' | 'high' | 'normal' | 'low'