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Android ScrollView fix for pagingEnabled
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Summary:
The snapToOffsets changes improved the flinging algorithm for snapToInterval/snapToOffsets but actually broke it for pagingEnabled because it's meant to only scroll one page at a time.

First, I just brough back the old algorithm, but noticed that it has a bunch of issues (e.g. facebook#20155). So, I tried to improve the algorithm to make sure it uses the proper target offset prediction using the same physics model that Android uses for it's normal scrolling but still be limited to one page scrolls.

This resolves facebook#21116.

Reviewed By: shergin

Differential Revision: D9945017

fbshipit-source-id: be7d4dfd1140f4c4d32bad93a03812dc80286069
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@olegbl @grabbou
olegbl authored and grabbou committed Oct 2, 2018
1 parent aae1e29 commit 0869e54
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Showing 2 changed files with 148 additions and 36 deletions.
92 changes: 74 additions & 18 deletions ReactAndroid/src/main/java/com/facebook/react/views/scroll/ReactHorizontalScrollView.java
View file
Original file line number Diff line number Diff line change
Expand Up @@ -277,7 +277,7 @@ public boolean onTouchEvent(MotionEvent ev) {
@Override
public void fling(int velocityX) {
if (mPagingEnabled) {
smoothScrollAndSnap(velocityX);
flingAndSnap(velocityX);
} else if (mScroller != null) {
// FB SCROLLVIEW CHANGE

Expand Down Expand Up @@ -455,7 +455,7 @@ public void run() {
// Only if we have pagingEnabled and we have not snapped to the page do we
// need to continue checking for the scroll. And we cause that scroll by asking for it
mSnappingToPage = true;
smoothScrollAndSnap(0);
flingAndSnap(0);
ViewCompat.postOnAnimationDelayed(ReactHorizontalScrollView.this,
this,
ReactScrollViewHelper.MOMENTUM_DELAY);
Expand All @@ -474,28 +474,15 @@ public void run() {
ReactScrollViewHelper.MOMENTUM_DELAY);
}

/**
* This will smooth scroll us to the nearest snap offset point
* It currently just looks at where the content is and slides to the nearest point.
* It is intended to be run after we are done scrolling, and handling any momentum scrolling.
*/
private void smoothScrollAndSnap(int velocityX) {
if (getChildCount() <= 0) {
return;
}

int maximumOffset = Math.max(0, computeHorizontalScrollRange() - getWidth());
int targetOffset = 0;
int smallerOffset = 0;
int largerOffset = maximumOffset;

private int predictFinalScrollPosition(int velocityX) {
// ScrollView can *only* scroll for 250ms when using smoothScrollTo and there's
// no way to customize the scroll duration. So, we create a temporary OverScroller
// so we can predict where a fling would land and snap to nearby that point.
OverScroller scroller = new OverScroller(getContext());
scroller.setFriction(1.0f - mDecelerationRate);

// predict where a fling would end up so we can scroll to the nearest snap offset
int maximumOffset = Math.max(0, computeHorizontalScrollRange() - getWidth());
int width = getWidth() - getPaddingStart() - getPaddingEnd();
scroller.fling(
getScrollX(), // startX
Expand All @@ -509,7 +496,76 @@ private void smoothScrollAndSnap(int velocityX) {
width/2, // overX
0 // overY
);
targetOffset = scroller.getFinalX();
return scroller.getFinalX();
}

/**
* This will smooth scroll us to the nearest snap offset point
* It currently just looks at where the content is and slides to the nearest point.
* It is intended to be run after we are done scrolling, and handling any momentum scrolling.
*/
private void smoothScrollAndSnap(int velocity) {
double interval = (double) getSnapInterval();
double currentOffset = (double) getScrollX();
double targetOffset = (double) predictFinalScrollPosition(velocity);

int previousPage = (int) Math.floor(currentOffset / interval);
int nextPage = (int) Math.ceil(currentOffset / interval);
int currentPage = (int) Math.round(currentOffset / interval);
int targetPage = (int) Math.round(targetOffset / interval);

if (velocity > 0 && nextPage == previousPage) {
nextPage ++;
} else if (velocity < 0 && previousPage == nextPage) {
previousPage --;
}

if (
// if scrolling towards next page
velocity > 0 &&
// and the middle of the page hasn't been crossed already
currentPage < nextPage &&
// and it would have been crossed after flinging
targetPage > previousPage
) {
currentPage = nextPage;
}
else if (
// if scrolling towards previous page
velocity < 0 &&
// and the middle of the page hasn't been crossed already
currentPage > previousPage &&
// and it would have been crossed after flinging
targetPage < nextPage
) {
currentPage = previousPage;
}

targetOffset = currentPage * interval;
if (targetOffset != currentOffset) {
mActivelyScrolling = true;
smoothScrollTo((int) targetOffset, getScrollY());
}
}

private void flingAndSnap(int velocityX) {
if (getChildCount() <= 0) {
return;
}

// pagingEnabled only allows snapping one interval at a time
if (mSnapInterval == 0 && mSnapOffsets == null) {
smoothScrollAndSnap(velocityX);
return;
}

int maximumOffset = Math.max(0, computeHorizontalScrollRange() - getWidth());
int targetOffset = predictFinalScrollPosition(velocityX);
int smallerOffset = 0;
int largerOffset = maximumOffset;
int firstOffset = 0;
int lastOffset = maximumOffset;
int width = getWidth() - getPaddingStart() - getPaddingEnd();

// offsets are from the right edge in RTL layouts
boolean isRTL = TextUtilsCompat.getLayoutDirectionFromLocale(Locale.getDefault()) == ViewCompat.LAYOUT_DIRECTION_RTL;
Expand Down
92 changes: 74 additions & 18 deletions ReactAndroid/src/main/java/com/facebook/react/views/scroll/ReactScrollView.java
View file
Original file line number Diff line number Diff line change
Expand Up @@ -300,7 +300,7 @@ public void getClippingRect(Rect outClippingRect) {
@Override
public void fling(int velocityY) {
if (mPagingEnabled) {
smoothScrollAndSnap(velocityY);
flingAndSnap(velocityY);
} else if (mScroller != null) {
// FB SCROLLVIEW CHANGE

Expand Down Expand Up @@ -423,7 +423,7 @@ public void run() {
// Only if we have pagingEnabled and we have not snapped to the page do we
// need to continue checking for the scroll. And we cause that scroll by asking for it
mSnappingToPage = true;
smoothScrollAndSnap(0);
flingAndSnap(0);
ViewCompat.postOnAnimationDelayed(ReactScrollView.this,
this,
ReactScrollViewHelper.MOMENTUM_DELAY);
Expand All @@ -442,28 +442,15 @@ public void run() {
ReactScrollViewHelper.MOMENTUM_DELAY);
}

/**
* This will smooth scroll us to the nearest snap offset point
* It currently just looks at where the content is and slides to the nearest point.
* It is intended to be run after we are done scrolling, and handling any momentum scrolling.
*/
private void smoothScrollAndSnap(int velocityY) {
if (getChildCount() <= 0) {
return;
}

int maximumOffset = getMaxScrollY();
int targetOffset = 0;
int smallerOffset = 0;
int largerOffset = maximumOffset;

private int predictFinalScrollPosition(int velocityY) {
// ScrollView can *only* scroll for 250ms when using smoothScrollTo and there's
// no way to customize the scroll duration. So, we create a temporary OverScroller
// so we can predict where a fling would land and snap to nearby that point.
OverScroller scroller = new OverScroller(getContext());
scroller.setFriction(1.0f - mDecelerationRate);

// predict where a fling would end up so we can scroll to the nearest snap offset
int maximumOffset = getMaxScrollY();
int height = getHeight() - getPaddingBottom() - getPaddingTop();
scroller.fling(
getScrollX(), // startX
Expand All @@ -477,7 +464,76 @@ private void smoothScrollAndSnap(int velocityY) {
0, // overX
height/2 // overY
);
targetOffset = scroller.getFinalY();
return scroller.getFinalY();
}

/**
* This will smooth scroll us to the nearest snap offset point
* It currently just looks at where the content is and slides to the nearest point.
* It is intended to be run after we are done scrolling, and handling any momentum scrolling.
*/
private void smoothScrollAndSnap(int velocity) {
double interval = (double) getSnapInterval();
double currentOffset = (double) getScrollY();
double targetOffset = (double) predictFinalScrollPosition(velocity);

int previousPage = (int) Math.floor(currentOffset / interval);
int nextPage = (int) Math.ceil(currentOffset / interval);
int currentPage = (int) Math.round(currentOffset / interval);
int targetPage = (int) Math.round(targetOffset / interval);

if (velocity > 0 && nextPage == previousPage) {
nextPage ++;
} else if (velocity < 0 && previousPage == nextPage) {
previousPage --;
}

if (
// if scrolling towards next page
velocity > 0 &&
// and the middle of the page hasn't been crossed already
currentPage < nextPage &&
// and it would have been crossed after flinging
targetPage > previousPage
) {
currentPage = nextPage;
}
else if (
// if scrolling towards previous page
velocity < 0 &&
// and the middle of the page hasn't been crossed already
currentPage > previousPage &&
// and it would have been crossed after flinging
targetPage < nextPage
) {
currentPage = previousPage;
}

targetOffset = currentPage * interval;
if (targetOffset != currentOffset) {
mActivelyScrolling = true;
smoothScrollTo(getScrollX(), (int) targetOffset);
}
}

private void flingAndSnap(int velocityY) {
if (getChildCount() <= 0) {
return;
}

// pagingEnabled only allows snapping one interval at a time
if (mSnapInterval == 0 && mSnapOffsets == null) {
smoothScrollAndSnap(velocityY);
return;
}

int maximumOffset = getMaxScrollY();
int targetOffset = predictFinalScrollPosition(velocityY);
int smallerOffset = 0;
int largerOffset = maximumOffset;
int firstOffset = 0;
int lastOffset = maximumOffset;
int height = getHeight() - getPaddingBottom() - getPaddingTop();

// get the nearest snap points to the target offset
if (mSnapOffsets != null) {
Expand Down

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