Is there any easy way of getting Item or index of item when UICollectionView stops scrolling?
This is how I found what item is available on center of the page when UICollectionView Stop Scrolling.
Based on this Answer. https://stackoverflow.com/a/24396643/2618600
- (void)scrollViewDidEndDecelerating:(UIScrollView *)scrollView
{
NSLog(#"Finished scrolling %#",scrollView);
if ([scrollView isKindOfClass:[UICollectionView class]])
{
UICollectionView *mainCollection = (UICollectionView *) scrollView;
CGRect visibleRect = (CGRect){.origin = mainCollection.contentOffset, .size = mainCollection.bounds.size};
CGPoint visiblePoint = CGPointMake(CGRectGetMidX(visibleRect), CGRectGetMidY(visibleRect));
NSIndexPath *visibleIndexPath = [mainCollection indexPathForItemAtPoint:visiblePoint];
NSLog(#"visibleIndexPath %ld",(long)visibleIndexPath.row);
}
You can use - (NSArray *)indexPathsForVisibleItems method of UICollectionView.
Related
EDIT: I have attached a simple demo project dropbox link at bottom
I have a UI with a UICollectionView at top and a scrollview at the bottom. I want the scrolling in collectionview to scroll the scrollview too in sync. I have disabled user-interaction in scrollview so only the collectionview can effect the scrolling in it.
Each collectionview item is 150px for this testing purpose.
The UIViews in the scrollview are screen width in size. So for every scroll of a collectionview item, I need to scroll the scrollview by screen width. To achieve this, I am calculating the distance the collectionview offset has changed by and then dividing it by the cell width (150) and multiplying it by scrollview's width.
I am trying to achieve the following UI:
Start:
Scroll collectionview to cell 1:
Scroll collectionview to cell 2:
This all works fine the first few times but as I scroll the collectionview back and forth a few times to longer distances (let's say cell 10 -> 0 -> 10 -> 0 and so on), the scrollview goes out of sync by "tiny" distances. To illustrate this, notice how there is the "yellow" color from the second UIView on the right edge of the scrollview:
I can see this issue by NSLogging the contentOffset of the scrollview too (notice how it starts getting out of sync by 0.5 after few times):
2018-11-25 19:24:28.273278-0500 ScrollViewMatchTest[19412:1203912] Finished: 0
2018-11-25 19:24:31.606521-0500 ScrollViewMatchTest[19412:1203912] Finished: 0.5
2018-11-25 19:24:55.173709-0500 ScrollViewMatchTest[19412:1203912] Finished: 1.5
2018-11-25 19:25:03.007528-0500 ScrollViewMatchTest[19412:1203912] Finished: 1.5
2018-11-25 19:25:07.841096-0500 ScrollViewMatchTest[19412:1203912] Finished: 2.5
2018-11-25 19:26:57.634429-0500
I am not really sure what's causing this problem and I have tried quite a few ways to fix it but in vain. I can sort of figure out a workaround (to reset the offset and bring it back in sync when scrolling finishes) but I would like to know why exactly this out of sync issue is caused.
Workaround solution by resetting the contentOffset of scrollView to closes multiple of screen width:
-(void)scrollViewDidEndScrollingAnimation:(UIScrollView *)scrollView
{
[NSObject cancelPreviousPerformRequestsWithTarget:self];
if (scrollView==self.myCollectionView) {
NSLog(#"Finished: %g",self.myScrollView.contentOffset.x);
NSLog(#"Closest: %g",RoundTo(self.myScrollView.contentOffset.x, self.myScrollView.frame.size.width));
[self.myScrollView setContentOffset:CGPointMake(RoundTo(self.myScrollView.contentOffset.x, self.myScrollView.frame.size.width), self.myScrollView.contentOffset.y) animated:YES];
}
}
float RoundTo(float number, float to)
{
if (number >= 0) {
return to * floorf(number / to + 0.5f);
}
else {
return to * ceilf(number / to - 0.5f);
}
}
END OF WORKAROUND SOLUTION
I have attached a simple demo project to illustrate this issue as well (run the app and scroll back and forth aggresively on the top scrollview a few times): https://www.dropbox.com/s/e2bzgo6abq5wmgw/ScrollViewMatchTest.zip?dl=0
Here's the code:
#import "ViewController.h"
#define countOfItems 50
#interface ViewController (){
CGFloat previousOffset_Header;
CGFloat previousOffset_Scrollview;
}
#end
#implementation ViewController
- (void)viewDidLoad {
[super viewDidLoad];
[self.myCollectionView registerNib:[UINib nibWithNibName:#"MyCell" bundle:nil] forCellWithReuseIdentifier:#"cell"];
[self.myCollectionView reloadData];
for (NSInteger i=0; i<countOfItems; i++) {
UIView *myView = [[UIView alloc] initWithFrame:CGRectMake(i*self.myScrollView.frame.size.width, 0, self.myScrollView.frame.size.width, self.myScrollView.frame.size.height)];
myView.backgroundColor=i%2==0?[UIColor blueColor]:[UIColor yellowColor];
[self.myScrollView addSubview:myView];
}
self.myScrollView.contentSize=CGSizeMake(countOfItems*self.myScrollView.frame.size.width, self.myScrollView.frame.size.height);
}
- (NSInteger)collectionView:(UICollectionView *)collectionView numberOfItemsInSection:(NSInteger)section{
return countOfItems;
}
- (UICollectionViewCell *)collectionView:(UICollectionView *)collectionView cellForItemAtIndexPath:(NSIndexPath *)indexPath{
MyCollectionViewCell *cell = [collectionView dequeueReusableCellWithReuseIdentifier:#"cell" forIndexPath:indexPath];
cell.backgroundColor = indexPath.item%2==0?[UIColor blueColor]:[UIColor yellowColor];
cell.myLabel.text=[NSString stringWithFormat:#"%ld",indexPath.item];
return cell;
}
- (void)scrollViewWillBeginDragging:(UIScrollView *)scrollView{
if (scrollView==self.myCollectionView) {
previousOffset_Header = self.myCollectionView.contentOffset.x;
previousOffset_Scrollview = self.myScrollView.contentOffset.x;
}
}
- (void)scrollViewDidScroll:(UIScrollView *)scrollView{
if (scrollView==self.myCollectionView) {
[NSObject cancelPreviousPerformRequestsWithTarget:self];
[self performSelector:#selector(scrollViewDidEndScrollingAnimation:) withObject:scrollView afterDelay:0.1 inModes:#[NSRunLoopCommonModes]];
CGFloat offsetToMoveBy = (self.myCollectionView.contentOffset.x-previousOffset_Header)*(self.myScrollView.frame.size.width/150);
previousOffset_Scrollview = previousOffset_Scrollview +offsetToMoveBy;
[self.myScrollView setContentOffset:CGPointMake(previousOffset_Scrollview, self.myScrollView.contentOffset.y) animated:NO];
previousOffset_Header = self.myCollectionView.contentOffset.x;
}
}
-(void)scrollViewDidEndScrollingAnimation:(UIScrollView *)scrollView
{
[NSObject cancelPreviousPerformRequestsWithTarget:self];
NSLog(#"Finished: %g",self.myScrollView.contentOffset.x);
}
#end
Be aware in viewDidLoad method myScrollView frame is not really setted to the real device.
So in init of the view, the views's width and height might be wrong in your code.
There are a couple of things you should be doing differently, the way you calculate the offset and not using the UIScrollViewDelegate callbacks scrollViewDidEndDragging:willDecelerate: and scrollViewDidEndDecelerating:. I've rewritten the relevant code:
- (void)scrollViewDidScroll:(UIScrollView *)scrollView{
if (scrollView==self.myCollectionView) {
[self calculateScrollviewOffset:self.myCollectionView];
}
}
- (void)scrollViewDidEndDragging:(UIScrollView *)scrollView willDecelerate:(BOOL)decelerate {
if (scrollView == self.myCollectionView && !decelerate) {
[self calculateEndPosition: self.myCollectionView];
}
}
- (void)scrollViewDidEndDecelerating:(UIScrollView *)scrollView {
if (scrollView == self.myCollectionView) {
[self calculateEndPosition: self.myCollectionView];
}
}
- (void) calculateScrollviewOffset:(UICollectionView *) collectionView {
CGFloat cellWidth = 150.0;
CGFloat percentageMoved = collectionView.contentOffset.x / cellWidth;
[self setScrollViewOffset:percentageMoved animated:false];
}
- (void) calculateEndPosition:(UICollectionView*) collectionView {
CGFloat cellWidth = 150.0;
// NOTE: Add 0.5 to play around with the end animation positioning
CGFloat percentageMoved = floor(collectionView.contentOffset.x / cellWidth);
CGFloat collectionViewFixedOffset = (percentageMoved * cellWidth);
[collectionView setContentOffset:CGPointMake(collectionViewFixedOffset, collectionView.contentOffset.y) animated:true];
[self setScrollViewOffset:percentageMoved animated:true];
}
- (void) setScrollViewOffset:(CGFloat) percentageMoved animated:(BOOL) animated {
CGFloat newOffsetX = percentageMoved * self.myScrollView.frame.size.width;
[self.myScrollView setContentOffset:CGPointMake(newOffsetX, self.myScrollView.contentOffset.y) animated: animated];
}
I have a UICollectionView which expands on clicking a cell and once the screen fills it becomes scrollable.
Now when I scroll down I need my header view to scroll down with it and for that I've implemented the logic in the layoutAttributesForSupplementaryViewOfKind method in my custom UICollectionViewLayout class.
This works fine but now the issue is that when I the content becomes scrollable and I scroll down few cells and immediately click on a cell to shrink the content back to one screen at that point the header view doesn't gets arranged, i.e it still remains in the last scrolled position.
But there after if I perform any other action like cell tap it gets arranged properly.
I've tried calling setNeedsLayout, setNeedsDisplay and layoutSubviews where I reload my UICollectionView but the header still doesn't updates to its proper position.
Below is the code for my layoutAttributesForSupplementaryViewOfKind method.
Any help is appreciated.
- (UICollectionViewLayoutAttributes *)layoutAttributesForSupplementaryViewOfKind:(NSString *)kind atIndexPath:(NSIndexPath *)indexPath {
if (![kind isEqualToString:[myGridHeaderView kind]]) {
return nil;
}
myGridHeaderPosition headerPosition = [[self collectionView] headerPositionAtIndexPath:indexPath];
CGRect cellRect = [[self delegate] getRectForHeaderAtIndex:indexPath headerPosition:headerPosition];
if (CGRectEqualToRect(cellRect, CGRectZero)) {
return nil;
}
myGridHeaderLayoutAttribute* attributes = [myGridHeaderLayoutAttribute layoutAttributesForSupplementaryViewOfKind:kind withIndexPath:indexPath];
CGPoint centerPoint = CGPointMake(CGRectGetMidX(cellRect), CGRectGetMidY(cellRect));
CGSize size = cellRect.size;
UICollectionView * const cv = self.collectionView;
NSInteger zIndex = 1;
CGPoint const contentOffset = cv.contentOffset;
if (contentOffset.x > 0)
{
if (headerPosition != myGridHeaderPositionColumn)
{
centerPoint.x += contentOffset.x;
}
zIndex = 1005;
}
if (contentOffset.y > 0)
{
if (headerPosition != myGridHeaderPositionRow)
{
centerPoint.y += contentOffset.y;
}
zIndex = 1005;
}
if (headerPosition == myGridHeaderPositionCommon) {
zIndex = 1024;
}
attributes.zIndex = zIndex;
attributes.headerPosition = headerPosition;
attributes.center = centerPoint;
attributes.size = size;
attributes.alpha = 1.0;
return attributes;
}
When you scroll up and down , header will be visible and hidden , for use this code.
- (void)scrollViewDidScroll:(UIScrollView *)scrollView
{
isScrollingStart=YES;
NSLog(#"scrollViewDidScroll %f , %f",scrollView.contentOffset.x,scrollView.contentOffset.y);
if (scrollView.contentOffset.y<=124) {
_img_top_header.alpha=scrollView.contentOffset.y/124;
}
else
{
_img_top_header.alpha=1.0;
}
}
must be set image in header.
I have a collection view that scrolls horizontally and spans its parent view's full width. My cheap way to achieve paging on it is to set the cell widths to be equal to 1/3 of the collection view width, and to set that same amount of width as left and right content insets.
I disable scrolling in IB and replace with left and right swipe recognizers. My code almost works without setting contentInset, but setting the contentInset seems prevent any scrolling from happening
- (void)viewDidLayoutSubviews {
[super viewDidLayoutSubviews];
CGFloat itemWidth = self.collectionView.bounds.size.width/3.0;
NSInteger count = [self collectionView:self.collectionView numberOfItemsInSection:0];
self.collectionView.contentSize = (CGSize){ .width=itemWidth*count, .height=self.collectionView.bounds.size.height };
// uncomment this line, and the scroll code in the swipes below fails to work
//self.collectionView.contentInset = UIEdgeInsetsMake(0, itemWidth, 0, itemWidth);
self.collectionView.contentOffset = (CGPoint){ .x=self.collectionView.contentSize.width/2.0, .y=0 };
}
- (CGSize)collectionView:(UICollectionView *)collectionView layout:(UICollectionViewLayout *)collectionViewLayout sizeForItemAtIndexPath:(NSIndexPath *)indexPath {
CGFloat width = self.view.bounds.size.width/3.0;
return (CGSize){ .width=width, .height=collectionView.bounds.size.height };
}
This code handles the swipes...
- (NSIndexPath *)centerIndexPath {
CGRect visibleRect = (CGRect){.origin = self.collectionView.contentOffset, .size = self.collectionView.bounds.size};
CGPoint visiblePoint = CGPointMake(CGRectGetMidX(visibleRect), CGRectGetMidY(visibleRect));
return [self.collectionView indexPathForItemAtPoint:visiblePoint];
}
- (void)swipeLeft:(UISwipeGestureRecognizer *)gr {
NSIndexPath *centerIndexPath = [self centerIndexPath];
NSLog(#"at %#", centerIndexPath);
if (centerIndexPath.row < [self collectionView:self.collectionView numberOfItemsInSection:0]-1) {
[self.collectionView scrollToItemAtIndexPath:centerIndexPath atScrollPosition:UICollectionViewScrollPositionLeft animated:YES];
}
}
- (void)swipeRight:(UISwipeGestureRecognizer *)gr {
NSIndexPath *centerIndexPath = [self centerIndexPath];
NSLog(#"at %#", centerIndexPath);
if (centerIndexPath.row > 0) {
[self.collectionView scrollToItemAtIndexPath:centerIndexPath atScrollPosition:UICollectionViewScrollPositionRight animated:YES];
}
}
All of this works, except when I set the contentInsets in the setup above. Then, even though I reach the scrollToItemAtIndexPath: code in the debugger, no scrolling occurs.
It's important to have those insets, because I want user to understand that center item is the selected item.
Can somebody explain why contentInset spoils scrolling and how to fix?
It looks like UICollectionView has its own built-in way to handle insets:
https://developer.apple.com/library/content/documentation/WindowsViews/Conceptual/CollectionViewPGforIOS/UsingtheFlowLayout/UsingtheFlowLayout.html#//apple_ref/doc/uid/TP40012334-CH3-SW1
Using Section Insets to Tweak the Margins of Your Content
Section insets are a way to adjust the space available for laying out cells. You can use insets to insert space after a section’s header view and before its footer view. You can also use insets to insert space around the sides of the content. Figure 3-5 demonstrates how insets affect some content in a vertically scrolling flow layout.
Figure 3-5 Section insets change the available space for laying out cells
I'm trying to implement a single sticky header in a UICollectionView.
My sticky header behavior is a bit different than the usual one you can see e.g. in UITableView. I have 3 headers in the collection view and I want only one of them to be sticky and stick to the top when the content is scrolled.
My code works pretty well. However, when I scroll down, the sticky header disappears suddenly at some point. Scrolling back makes the header appear again. What am I doing wrong?
I am attaching a implementation of my custom layout. It's a subclass of UICollectionViewFlowLayout.
#implementation CustomFlowLayout
- (NSArray *)layoutAttributesForElementsInRect:(CGRect)rect
{
NSMutableArray *attributes = [[super layoutAttributesForElementsInRect:rect] mutableCopy];
CGPoint const contentOffset = self.collectionView.contentOffset;
for (UICollectionViewLayoutAttributes *layoutAttributes in attributes)
{
// Adjust the sticky header frame.
if ([layoutAttributes.representedElementKind isEqualToString:UICollectionElementKindSectionHeader] &&
layoutAttributes.indexPath.section == SectionWithStickyHeadeIndex)
{
NSInteger numberOfItemsInSection = [self.collectionView numberOfItemsInSection:SectionWithStickyHeadeIndex];
NSIndexPath *firstObjectIndexPath = [NSIndexPath indexPathForItem:0
inSection:SectionWithStickyHeadeIndex];
UICollectionViewLayoutAttributes *firstObjectAttrs;
if (numberOfItemsInSection > 0)
{
firstObjectAttrs = [self layoutAttributesForItemAtIndexPath:firstObjectIndexPath];
}
else
{
firstObjectAttrs = [self layoutAttributesForSupplementaryViewOfKind:UICollectionElementKindSectionHeader
atIndexPath:firstObjectIndexPath];
}
CGPoint origin = layoutAttributes.frame.origin;
// Adjust the header origin so it sticks to the top.
origin.y = MAX(contentOffset.y + self.collectionView.contentInset.top,
CGRectGetMinY(firstObjectAttrs.frame) - CGRectGetHeight(layoutAttributes.frame));
layoutAttributes.zIndex = CGFLOAT_MAX;
layoutAttributes.frame = (CGRect)
{
.origin = origin,
.size = layoutAttributes.frame.size
};
break;
}
}
return attributes;
}
- (BOOL)shouldInvalidateLayoutForBoundsChange:(CGRect)newBound
{
return YES;
}
#end
I'm not 100% sure on this, but it looks like once you scrolled down far enough, the header's original position was no longer located inside the rect argument. This caused the header's layout attributes to not be included in the attributes array you iterated over in in the for loop, resulting in the layout position no longer being adjusted to its "sticky" position at the top of the screen.
Try adding these lines right before the for loop to add the sticky header's layout attributes to the attributes array if they are not already there:
NSIndexPath *stickyHeaderIndexPath = [NSIndexPath indexPathForItem:0 inSection:SectionWithStickyHeaderIndex];
UICollectionViewLayoutAttributes *layoutAttributes = [self layoutAttributesForSupplementaryViewOfKind:UICollectionElementKindSectionHeader
atIndexPath:stickyHeaderIndexPath];
if (![attributes containsObject:layoutAttributes])
{
[attributes addObject:layoutAttributes];
}
The answer in Swift 5
let headerIndexPath = IndexPath(item: 0, section: 0)
if let headerAttributes = layoutAttributesForSupplementaryView(ofKind: UICollectionView.elementKindSectionHeader, at: headerIndexPath) {
if !attributes.contains(headerAttributes) {
attributes.append(headerAttributes)
}
}
All thanks to #BrendanCain
I have an iPad App where I'm using a UICollectionView and each UICollectionViewCell contains just a single UIImage.
Currently I'm displaying per 9 UIImages (3 rows * 3 columns) per page, I have several pages.
I would like to use Pinch Gesture to zoom on the entire UICollectionView to increase/decrease the number of row/columns displayed per page and the best would be to have beautiful zoom animation during the Pinch gesture!
Currently, I have added a Pinch Gesture on my UICollectionView. I catch the Pinch Gesture event to compute the number of rows/columns using the scale factor, if it has changed then I update the full UICollectionView using:
[_theCollectionView performBatchUpdates:^{
[_theCollectionView deleteSections:[NSIndexSet indexSetWithIndex:0]];
[_theCollectionView insertSections:[NSIndexSet indexSetWithIndex:0]];
} completion:nil];
It works but I don't have smooth animation during the transition.
Any idea? UICollectionView inherits from UIScrollView, is there a possibility to re-use the UIScrollView Pinch gesture feature to reach my goal?
I'm assuming you're using the default UICollectionViewDelegateFlowLayout, right? Then make sure you respond accordingly to the delegate methods, and when the pinch gesture occurs, simply invalidate the layout.
For example, if I want to adjust the size of every item, while pinching:
#interface ViewController () <UICollectionViewDataSource, UICollectionViewDelegate, UICollectionViewDelegateFlowLayout>
#property (nonatomic,assign) CGFloat scale;
#property (nonatomic,weak) IBOutlet UICollectionView *collectionView;
#end
#implementation ViewController
- (void)viewDidLoad
{
[super viewDidLoad];
self.scale = 1.0;
[self.collectionView registerClass:[UICollectionViewCell class] forCellWithReuseIdentifier:#"cell"];
UIPinchGestureRecognizer *gesture = [[UIPinchGestureRecognizer alloc] initWithTarget:self action:#selector(didReceivePinchGesture:)];
[self.collectionView addGestureRecognizer:gesture];
}
- (CGSize)collectionView:(UICollectionView *)collectionView layout:(UICollectionViewLayout *)collectionViewLayout sizeForItemAtIndexPath:(NSIndexPath *)indexPath
{
return CGSizeMake(50*self.scale, 50*self.scale);
}
- (void)didReceivePinchGesture:(UIPinchGestureRecognizer*)gesture
{
static CGFloat scaleStart;
if (gesture.state == UIGestureRecognizerStateBegan)
{
scaleStart = self.scale;
}
else if (gesture.state == UIGestureRecognizerStateChanged)
{
self.scale = scaleStart * gesture.scale;
[self.collectionView.collectionViewLayout invalidateLayout];
}
}
The property self.scale is just for show, you can apply this same concept to any other attribute, this doesn't require a beginUpdates/endUpdates because the user himself is carrying the timing of the scale.
Here's a running project, in case you want to see it in action.
Sorry for my 2 cents question, I have found the solution, very simple.
In my PinchGesture callback I have just done the following:
void (^animateChangeWidth)() = ^() {
_theFlowLayout.itemSize = cellSize;
};
[UIView transitionWithView:self.theCollectionView
duration:0.1f
options:UIViewAnimationOptionCurveLinear
animations:animateChangeWidth
completion:nil];
All cells of my UICollectionView are successfully changed and with a nice transition.
For Xamarin.iOS developers I found this solution: add a UIScrollView element to the main view and add the UICollectionView as an element of the UIScrollView. Then create a zoom delegate for the UIScrollView.
MainScrollView = new UIScrollView(new CGRect(View.Frame.X, View.Frame.Y, size.Width, size.Height));
_cellReuseId = GenCellReuseId();
_contentScroll = new UICollectionView(new CGRect(View.Frame.X, View.Frame.Y, size.Width, size.Height), new InfiniteScrollCollectionLayout(size.Width, size.Height));
_contentScroll.AllowsSelection = true;
_contentScroll.ReloadData();
_contentScroll.Center = MainScrollView.Center;
_contentScroll.Frame = new CGRect(_contentScroll.Frame.X, _contentScroll.Frame.Y - 32, _contentScroll.Frame.Width, _contentScroll.Frame.Height);
MainScrollView.ContentSize = _contentScroll.ContentSize;
MainScrollView.AddSubview(_contentScroll);
MainScrollView.MaximumZoomScale = 4f;
MainScrollView.MinimumZoomScale = 1f;
MainScrollView.BouncesZoom = true;
MainScrollView.ViewForZoomingInScrollView += (UIScrollView sv) =>
{
if (_contentScroll.Frame.Height < sv.Frame.Height && _contentScroll.Frame.Width < sv.Frame.Width)
{
_contentScroll.Center = MainScrollView.Center;
_contentScroll.Frame = new CGRect(_contentScroll.Frame.X, _contentScroll.Frame.Y - 64, _contentScroll.Frame.Width, _contentScroll.Frame.Height);
_contentScroll.BouncesZoom = true;
_contentScroll.AlwaysBounceHorizontal = false;
}
return _contentScroll;
};