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new feature to support smooth bi-directional content loading
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Summary:
== Problem / Background ==

Most lists paginate in a single direction (standard infinite list), but some paginate in both directions. Most common example is a chat thread where new messages show up on the bottom, and old content can be loaded by scrolling up. Comment threads are another example.

Right now, adding content to the bottom of a scroll view is smooth - the content doesn't jump. But when adding to the top of the scrollview, the content gets pushed down, which is jarring (note this may appear reversed because of inverting the list which is common for chat applications).

== Approach ==

The basic idea is simple - we set a flag in JS, then for every uimanager transaction, we record which is the first eligible and visible view in the ScrollView, and compare it's new origin to the old one. If it has changed, we update the contentOffset of the ScrollView to compensate.

This is done by observing `willPerformMounting` directly (only from scrollviews that have this new property set), and then observing the prev state with prependUIBlock and making the update synchronously in addUIBlock to avoid any flicker.

There is also a way to skip views that we don't care about, like a spinner at the top of the view that we don't want to stay in place - we actually want it to get pushed up by the new content, replaced visually in the viewport.

== Notes ==

Most chat applications will probably want to do a scrollToTop when new content comes in and the user is already scrolled at or near the bottom.

This is glitchy if visible children are re-ordered, which could be fixed with additional logic, but it doesn't come up in the type of applications we're targetting here so punting on that.

== Test Plan ==

https://youtu.be/4GcqDGz9eOE

Reviewed By: shergin

Differential Revision: D6696921

fbshipit-source-id: 822e7dfcb207006cd1ba098356324ea81f619428
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@sahrens @facebook-github-bot
sahrens authored and facebook-github-bot committed Jan 13, 2018
1 parent b815eb5 commit cae7179
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13 changes: 13 additions & 0 deletions Libraries/Components/ScrollView/ScrollView.js
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Original file line number Diff line number Diff line change
Expand Up @@ -233,6 +233,19 @@ const ScrollView = createReactClass({
* - `true`, deprecated, use 'always' instead
*/
keyboardShouldPersistTaps: PropTypes.oneOf(['always', 'never', 'handled', false, true]),
/**
* When non-null, the scroll view will adjust the scroll position so that the content at or
* beyond the specified index that is currently visible will not change position. This is useful
* for lists that are loading content in both directions, e.g. a chat thread, where new messages
* coming in might otherwise cause the scroll position to jump. A value of 1 can be used to skip
* a spinner that does not need to maintain position. The default value is null.
*
* Caveat: reordering elements in the scrollview with this enabled will probably cause jumpiness
* and jank. It can be fixed, but there are currently no plans to do so.
*
* @platform ios
*/
maintainPositionAtOrBeyondIndex: PropTypes.number,
/**
* The maximum allowed zoom scale. The default value is 1.0.
* @platform ios
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225 changes: 177 additions & 48 deletions RNTester/js/ScrollViewExample.js
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Original file line number Diff line number Diff line change
Expand Up @@ -12,10 +12,13 @@
*/
'use strict';

var React = require('react');
var ReactNative = require('react-native');
var {
Platform,
import type {StyleObj} from 'StyleSheetTypes';

const ActivityIndicator = require('ActivityIndicator');
const Platform = require('Platform');
const React = require('react');
const ReactNative = require('react-native');
const {
ScrollView,
StyleSheet,
Text,
Expand All @@ -30,11 +33,11 @@ exports.description =
'Component that enables scrolling through child components';
exports.examples = [
{
title: '<ScrollView>',
title: '<ScrollView>\n',
description:
'To make content scrollable, wrap it within a <ScrollView> component',
render: function() {
var _scrollView: ScrollView;
let _scrollView: ScrollView;
return (
<View>
<ScrollView
Expand All @@ -49,41 +52,38 @@ exports.examples = [
style={styles.scrollView}>
{THUMB_URLS.map(createThumbRow)}
</ScrollView>
<TouchableOpacity
style={styles.button}
<Button
label="Scroll to top"
onPress={() => {
_scrollView.scrollTo({y: 0});
}}>
<Text>Scroll to top</Text>
</TouchableOpacity>
<TouchableOpacity
style={styles.button}
}}
/>
<Button
label="Scroll to bottom"
onPress={() => {
_scrollView.scrollToEnd({animated: true});
}}>
<Text>Scroll to bottom</Text>
</TouchableOpacity>
<TouchableOpacity
style={styles.button}
}}
/>
<Button
label="Flash scroll indicators"
onPress={() => {
_scrollView.flashScrollIndicators();
}}>
<Text>Flash scroll indicators</Text>
</TouchableOpacity>
}}
/>
</View>
);
},
},
{
title: '<ScrollView> (horizontal = true)',
title: '<ScrollView> (horizontal = true)\n',
description:
"You can display <ScrollView>'s child components horizontally rather than vertically",
render: function() {
function renderScrollView(
title: string,
addtionalStyles: typeof StyleSheet,
) {
var _scrollView: ScrollView;
let _scrollView: ScrollView;
return (
<View style={addtionalStyles}>
<Text style={styles.text}>{title}</Text>
Expand All @@ -96,27 +96,24 @@ exports.examples = [
style={[styles.scrollView, styles.horizontalScrollView]}>
{THUMB_URLS.map(createThumbRow)}
</ScrollView>
<TouchableOpacity
style={styles.button}
<Button
label="Scroll to start"
onPress={() => {
_scrollView.scrollTo({x: 0});
}}>
<Text>Scroll to start</Text>
</TouchableOpacity>
<TouchableOpacity
style={styles.button}
}}
/>
<Button
label="Scroll to end"
onPress={() => {
_scrollView.scrollToEnd({animated: true});
}}>
<Text>Scroll to end</Text>
</TouchableOpacity>
<TouchableOpacity
style={styles.button}
}}
/>
<Button
label="Flash scroll indicators"
onPress={() => {
_scrollView.flashScrollIndicators();
}}>
<Text>Flash scroll indicators</Text>
</TouchableOpacity>
}}
/>
</View>
);
}
Expand All @@ -130,22 +127,144 @@ exports.examples = [
},
},
];
if (Platform.OS === 'ios') {
exports.examples.push({
title: '<ScrollView> smooth bi-directional content loading\n',
description:
'The `maintainPositionAtOrBeyondIndex` prop allows insertions to either end of the content ' +
'without causing the visible content to jump. Re-ordering is not supported.',
render: function() {
let itemCount = 6;
class AppendingList extends React.Component<{}, *> {
state = {
items: [...Array(itemCount)].map((_, ii) => (
<Thumb msg={`Item ${ii}`} />
)),
};
render() {
return (
<View>
<ScrollView
automaticallyAdjustContentInsets={false}
maintainPositionAtOrBeyondIndex={1}
style={styles.scrollView}>
<ActivityIndicator style={{height: 40}} />
{this.state.items.map(item =>
React.cloneElement(item, {key: item.props.msg}),
)}
</ScrollView>
<ScrollView
horizontal={true}
automaticallyAdjustContentInsets={false}
maintainPositionAtOrBeyondIndex={1}
style={[styles.scrollView, styles.horizontalScrollView]}>
<ActivityIndicator style={{height: 40}} />
{this.state.items.map(item =>
React.cloneElement(item, {key: item.props.msg, style: null}),
)}
</ScrollView>
<View style={styles.row}>
<Button
label="Add to top"
onPress={() => {
this.setState(state => {
const idx = itemCount++;
return {
items: [
<Thumb
style={{paddingTop: idx * 5}}
msg={`Item ${idx}`}
/>,
].concat(state.items),
};
});
}}
/>
<Button
label="Remove top"
onPress={() => {
this.setState(state => ({
items: state.items.slice(1),
}));
}}
/>
<Button
label="Change height top"
onPress={() => {
this.setState(state => ({
items: [
React.cloneElement(state.items[0], {
style: {paddingBottom: Math.random() * 40},
}),
].concat(state.items.slice(1)),
}));
}}
/>
</View>
<View style={styles.row}>
<Button
label="Add to end"
onPress={() => {
this.setState(state => ({
items: state.items.concat(
<Thumb msg={`Item ${itemCount++}`} />,
),
}));
}}
/>
<Button
label="Remove end"
onPress={() => {
this.setState(state => ({
items: state.items.slice(0, -1),
}));
}}
/>
<Button
label="Change height end"
onPress={() => {
this.setState(state => ({
items: state.items.slice(0, -1).concat(
React.cloneElement(
state.items[state.items.length - 1],
{
style: {paddingBottom: Math.random() * 40},
},
),
),
}));
}}
/>
</View>
</View>
);
}
}
return <AppendingList />;
},
});
}

class Thumb extends React.Component<$FlowFixMeProps, $FlowFixMeState> {
shouldComponentUpdate(nextProps, nextState) {
return false;
}

class Thumb extends React.PureComponent<{|
source?: string | number,
msg?: string,
style?: StyleObj,
|}> {
render() {
const {source} = this.props;
return (
<View style={styles.thumb}>
<Image style={styles.img} source={this.props.source} />
<View style={[styles.thumb, this.props.style]}>
<Image
style={styles.img}
source={source == null ? THUMB_URLS[6] : source}
/>
<Text>{this.props.msg}</Text>
</View>
);
}
}

var THUMB_URLS = [
let THUMB_URLS = [
require('./Thumbnails/like.png'),
require('./Thumbnails/dislike.png'),
require('./Thumbnails/call.png'),
Expand All @@ -162,9 +281,15 @@ var THUMB_URLS = [

THUMB_URLS = THUMB_URLS.concat(THUMB_URLS); // double length of THUMB_URLS

var createThumbRow = (uri, i) => <Thumb key={i} source={uri} />;
const createThumbRow = (uri, i) => <Thumb key={i} source={uri} />;

var styles = StyleSheet.create({
const Button = ({label, onPress}) => (
<TouchableOpacity style={styles.button} onPress={onPress}>
<Text>{label}</Text>
</TouchableOpacity>
);

const styles = StyleSheet.create({
scrollView: {
backgroundColor: '#eeeeee',
height: 300,
Expand All @@ -184,6 +309,10 @@ var styles = StyleSheet.create({
backgroundColor: '#cccccc',
borderRadius: 3,
},
row: {
flexDirection: 'row',
justifyContent: 'space-around',
},
thumb: {
margin: 5,
padding: 5,
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1 change: 1 addition & 0 deletions React/Views/ScrollView/RCTScrollView.h
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Original file line number Diff line number Diff line change
Expand Up @@ -45,6 +45,7 @@
@property (nonatomic, assign) BOOL DEPRECATED_sendUpdatedChildFrames;
@property (nonatomic, assign) NSTimeInterval scrollEventThrottle;
@property (nonatomic, assign) BOOL centerContent;
@property (nonatomic, copy) NSNumber *maintainPositionAtOrBeyondIndex;
@property (nonatomic, assign) int snapToInterval;
@property (nonatomic, copy) NSString *snapToAlignment;

Expand Down
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2 comments on commit cae7179

@maheshpatidar
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@maheshpatidar maheshpatidar commented on cae7179 Jun 13, 2019 •
edited
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I was looking for this solution for months and thought that I got it now, however after putting the changed files to my local library, it hasn't changed anything, it has the same behaviour like before.
Is there anything I am missing on it?

@DaveLomber
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What is the solution for Android? How to do the same?

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