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feat: consider both velocity and position while calculating index (#802)
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Motivation
Previously evaluating next index was based only on position OR velocity. It wasn't considering BOTH of them which appears to natural behavior.

I believe that the expected scenario is that we can calculate is component left without snapping will rather stop decaying on the current or next/previous page.
I figured out (calculation attached) that final position (actually delta of current and final extrapolated) is ~ to v^2 and knowing that I implemented a more physical-based model for evaluating next index.

Changes
I changed the implementation of calculating the next index. Also, I had to drop swipeVelocityThreshold and swipeDistanceThreshold and then I replaced it with swipeVelocityImpact. I think it's a good time for this move bc it's not very breaking and this version is not yet by default in react-navigation.

swipeVelocityImpact is a factor in delta pos = a * v^2
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@osdnk
osdnk committed Jun 12, 2019
1 parent cc09bc1 commit e7f832c
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Showing 4 changed files with 41 additions and 67 deletions.
8 changes: 2 additions & 6 deletions README.md
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Original file line number Diff line number Diff line change
Expand Up @@ -287,13 +287,9 @@ String indicating whether the keyboard gets dismissed in response to a drag gest

Boolean indicating whether to enable swipe gestures. Swipe gestures are enabled by default. Passing `false` will disable swipe gestures, but the user can still switch tabs by pressing the tab bar.

##### `swipeDistanceThreshold`
##### `swipeVelocityImpact`

Minimum swipe distance which triggers a tab switch. By default, this is automatically determined based on the screen width.

##### `swipeVelocityThreshold`

Minimum swipe velocity which triggers a tab switch. Defaults to `1200`.
Determines how relevant is a velocity while calculating next position while swiping. Defaults to `0.01`.

##### `onSwipeStart`

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91 changes: 36 additions & 55 deletions src/Pager.tsx
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Original file line number Diff line number Diff line change
Expand Up @@ -44,7 +44,6 @@ const {
onChange,
abs,
add,
and,
block,
call,
ceil,
Expand All @@ -60,7 +59,6 @@ const {
min,
multiply,
neq,
or,
not,
round,
set,
Expand All @@ -80,9 +78,8 @@ const DIRECTION_LEFT = 1;
const DIRECTION_RIGHT = -1;

const SWIPE_DISTANCE_MINIMUM = 20;
const SWIPE_DISTANCE_MULTIPLIER = 1 / 1.75;

const SWIPE_VELOCITY_THRESHOLD_DEFAULT = 800;
const SWIPE_VELOCITY_IMPACT = 800;

const SPRING_CONFIG = {
stiffness: 1000,
Expand All @@ -100,15 +97,14 @@ const TIMING_CONFIG = {

export default class Pager<T extends Route> extends React.Component<Props<T>> {
static defaultProps = {
swipeVelocityThreshold: SWIPE_VELOCITY_THRESHOLD_DEFAULT,
swipeVelocityImpact: SWIPE_VELOCITY_IMPACT,
};

componentDidUpdate(prevProps: Props<T>) {
const {
navigationState,
layout,
swipeDistanceThreshold,
swipeVelocityThreshold,
swipeVelocityImpact,
springConfig,
timingConfig,
} = this.props;
Expand Down Expand Up @@ -139,23 +135,11 @@ export default class Pager<T extends Route> extends React.Component<Props<T>> {
this.layoutWidth.setValue(layout.width);
}

if (swipeDistanceThreshold != null) {
if (prevProps.swipeDistanceThreshold !== swipeDistanceThreshold) {
this.swipeDistanceThreshold.setValue(swipeDistanceThreshold);
}
} else {
if (prevProps.layout.width !== layout.width) {
this.swipeDistanceThreshold.setValue(
layout.width * SWIPE_DISTANCE_MULTIPLIER
);
}
}

if (prevProps.swipeVelocityThreshold !== swipeVelocityThreshold) {
this.swipeVelocityThreshold.setValue(
swipeVelocityThreshold != null
? swipeVelocityThreshold
: SWIPE_VELOCITY_THRESHOLD_DEFAULT
if (prevProps.swipeVelocityImpact !== swipeVelocityImpact) {
this.swipeVelocityImpact.setValue(
swipeVelocityImpact != null
? swipeVelocityImpact
: SWIPE_VELOCITY_IMPACT
);
}

Expand Down Expand Up @@ -204,7 +188,7 @@ export default class Pager<T extends Route> extends React.Component<Props<T>> {
// Current state of the gesture
private velocityX = new Value(0);
private gestureX = new Value(0);
private gestureState = new Value(State.UNDETERMINED);
private gestureState = new Value(State.END);
private offsetX = new Value(0);

// Current progress of the page (translateX value)
Expand Down Expand Up @@ -235,11 +219,10 @@ export default class Pager<T extends Route> extends React.Component<Props<T>> {
private routesLength = new Value(this.props.navigationState.routes.length);
private layoutWidth = new Value(this.props.layout.width);

// Threshold values to determine when to trigger a swipe gesture
private swipeDistanceThreshold = new Value(
this.props.swipeDistanceThreshold || 180
// Determines how relevant is a velocity while calculating next position while swiping
private swipeVelocityImpact = new Value(
this.props.swipeVelocityImpact || SWIPE_VELOCITY_IMPACT
);
private swipeVelocityThreshold = new Value(this.props.swipeVelocityThreshold);

// The position value represent the position of the pager on a scale of 0 - routes.length-1
// It is calculated based on the translate value and layout width
Expand Down Expand Up @@ -427,6 +410,21 @@ export default class Pager<T extends Route> extends React.Component<Props<T>> {
},
]);

private velocitySignum = cond(
this.velocityX,
divide(abs(this.velocityX), this.velocityX),
0
);
private extrapolatedPosition = add(
this.gestureX,
multiply(
this.velocityX,
this.velocityX,
this.velocitySignum,
this.swipeVelocityImpact
)
);

private translateX = block([
onChange(
this.index,
Expand Down Expand Up @@ -516,16 +514,14 @@ export default class Pager<T extends Route> extends React.Component<Props<T>> {
// Stop animations while we're dragging
stopClock(this.clock),
],
[

cond(eq(this.gestureState, State.END), [
set(this.isSwiping, FALSE),
this.transitionTo(
cond(
and(
greaterThan(abs(this.gestureX), SWIPE_DISTANCE_MINIMUM),
or(
greaterThan(abs(this.gestureX), this.swipeDistanceThreshold),
greaterThan(abs(this.velocityX), this.swipeVelocityThreshold)
)
greaterThan(
abs(this.extrapolatedPosition),
divide(this.layoutWidth, 2)
),
// For swipe gesture, to calculate the index, determine direction and add to index
// When the user swipes towards the left, we transition to the next tab
Expand All @@ -537,24 +533,9 @@ export default class Pager<T extends Route> extends React.Component<Props<T>> {
sub(
this.index,
cond(
greaterThan(
// Gesture can be positive, or negative
// Get absolute for comparision
abs(this.gestureX),
this.swipeDistanceThreshold
),
// If gesture value exceeded the threshold, calculate direction from distance travelled
cond(
greaterThan(this.gestureX, 0),
I18nManager.isRTL ? DIRECTION_RIGHT : DIRECTION_LEFT,
I18nManager.isRTL ? DIRECTION_LEFT : DIRECTION_RIGHT
),
// Otherwise calculate direction from the gesture velocity
cond(
greaterThan(this.velocityX, 0),
I18nManager.isRTL ? DIRECTION_RIGHT : DIRECTION_LEFT,
I18nManager.isRTL ? DIRECTION_LEFT : DIRECTION_RIGHT
)
greaterThan(this.velocitySignum, 0),
I18nManager.isRTL ? DIRECTION_RIGHT : DIRECTION_LEFT,
I18nManager.isRTL ? DIRECTION_LEFT : DIRECTION_RIGHT
)
)
),
Expand All @@ -565,7 +546,7 @@ export default class Pager<T extends Route> extends React.Component<Props<T>> {
this.index
)
),
]
])
),
this.progress,
]);
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6 changes: 2 additions & 4 deletions src/TabView.tsx
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Original file line number Diff line number Diff line change
Expand Up @@ -107,8 +107,7 @@ export default class TabView<T extends Route> extends React.Component<
removeClippedSubviews,
keyboardDismissMode,
swipeEnabled,
swipeDistanceThreshold,
swipeVelocityThreshold,
swipeVelocityImpact,
timingConfig,
springConfig,
tabBarPosition,
Expand All @@ -128,8 +127,7 @@ export default class TabView<T extends Route> extends React.Component<
layout={layout}
keyboardDismissMode={keyboardDismissMode}
swipeEnabled={swipeEnabled}
swipeDistanceThreshold={swipeDistanceThreshold}
swipeVelocityThreshold={swipeVelocityThreshold}
swipeVelocityImpact={swipeVelocityImpact}
timingConfig={timingConfig}
springConfig={springConfig}
onSwipeStart={onSwipeStart}
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3 changes: 1 addition & 2 deletions src/types.tsx
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Original file line number Diff line number Diff line change
Expand Up @@ -39,8 +39,7 @@ export type EventEmitterProps = {
export type PagerCommonProps = {
keyboardDismissMode: 'none' | 'on-drag';
swipeEnabled: boolean;
swipeDistanceThreshold?: number;
swipeVelocityThreshold?: number;
swipeVelocityImpact?: number;
onSwipeStart?: () => void;
onSwipeEnd?: () => void;
springConfig: {
Expand Down

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