i'm implementing drag and drop in collection view by using UICollectionViewDragDelegate and tried to hiding the drag preview while dragging
i manage to hide it by using this line of codes after following this thread Custom View for UICollectionViewCell Drag Preview:
public func collectionView(_ collectionView: UICollectionView, itemsForBeginning session: UIDragSession, at indexPath: IndexPath) -> [UIDragItem] {
let dragItem = UIDragItem(itemProvider: NSItemProvider())
dragItem.previewProvider = {
return nil
}
}
but the drag preview is still displayed when it's being lifted and the only method that allow me to modify the drag preview during lifting is
public func collectionView(_ collectionView: UICollectionView, dragPreviewParametersForItemAt indexPath: IndexPath) -> UIDragPreviewParameters? {
let previewParameters = UIDragPreviewParameters()
previewParameters.visiblePath = UIBezierPath(roundedRect: CGRect(x: 0, y: 0, width: 50, height: 50), cornerRadius: 0)
previewParameters.backgroundColor = UIColor.clear
return previewParameters
}
but it only allows me to set the background color not hide the drag preview
and the second way that i tried is by checking the cell state
public override func dragStateDidChange(_ dragState: UICollectionViewCell.DragState) {
switch dragState {
case .none:
self.layer.opacity = 1
case .lifting:
self.layer.opacity = 0
case .dragging:
self.layer.opacity = 1
}
}
but it's also not working
do any of you know how to hide this? or atleast hide the border and the shadow is also could solve this problem
here is the lifted cell
finally i found out the solution, the drag preview actually is named as _UIPlatterView (after debug hierarchy it) and it's child view named as _UIPortalView that block the cell during long press / lifting
and as solution of this post, just subclass the collection view and remove the the child view of _UIPlatterView
How to hide shadows in UITableViewCell when cell is dragging
public class CustomCollectionView: UICollectionView {
override public func didAddSubview(_ subview: UIView) {
super.didAddSubview(subview)
if "\(type(of: subview))" == "_UIPlatterView" {
subview.subviews.forEach({ $0.removeFromSuperview() })
}
}
}
but it's not the end, the above solution still shows the drag preview in just a little second and i add this code to clean it
extension ExampleViewController: UICollectionViewDragDelegate {
public func collectionView(_ collectionView: UICollectionView, dragPreviewParametersForItemAt indexPath: IndexPath) -> UIDragPreviewParameters? {
guard let currentCell: MUICalendarCollectionViewCell = collectionView.cellForItem(at: indexPath) as? MUICalendarCollectionViewCell else { return nil }
let previewParameters = UIDragPreviewParameters()
let path = UIBezierPath(rect: CGRect.zero)
previewParameters.visiblePath = path
previewParameters.backgroundColor = MUIColor.clear
return previewParameters
}
}
Related
I have a collectionView that has custom cells. Each custom cell has an image in it and other components. The problem I'm having is that when I scroll, the images changes to another images from other cells on the collectionView. I'm gonna put the code that I'm using so it's more clear to see how I'm implementing the collectionView and it's cell.
func registerCell() {
collectionView.register(PageCell.self, forCellWithReuseIdentifier: cellId)
}
func scrollViewWillEndDragging(_ scrollView: UIScrollView, withVelocity velocity: CGPoint, targetContentOffset: UnsafeMutablePointer<CGPoint>) {
let pageNumber = Int(targetContentOffset.pointee.x / view.frame.width)
pageControl.currentPage = pageNumber
}
func collectionView(_ collectionView: UICollectionView, numberOfItemsInSection section: Int) -> Int {
return feedTitles.count
}
func collectionView(_ collectionView: UICollectionView, cellForItemAt indexPath: IndexPath) -> UICollectionViewCell {
let cell = collectionView.dequeueReusableCell(withReuseIdentifier: cellId, for: indexPath) as! PageCell
cell.titleLabel.text = self.feedTitles[indexPath.item].uppercased()
cell.textView.text = self.feedDescription[indexPath.item]
guard let imageURL = URL(string: self.feedImages[indexPath.item]) else {return cell}
cell.imageView.load(url: imageURL)
return cell
}
func collectionView(_ collectionView: UICollectionView, layout collectionViewLayout: UICollectionViewLayout, sizeForItemAt indexPath: IndexPath) -> CGSize {
return CGSize(width: collectionView.frame.width, height: collectionView.frame.height)
}
func collectionView(_ collectionView: UICollectionView, layout collectionViewLayout: UICollectionViewLayout, minimumLineSpacingForSectionAt section: Int) -> CGFloat {
return 0
}
This is the code for getting the images from a server:
var feeds: [Feed]?
func getFeed(){
feedService.getFeed() { [weak self](feedRes) in
switch feedRes {
case .success(let successResponse):
print("Success Feed")
if successResponse.data.count < 1 {
} else{
self?.feeds = successResponse.data
self?.feedTitles = self?.feeds?.compactMap({ $0.title
}) ?? ["Titulo Noticia"]
self?.feedImages = self?.feeds?.compactMap({ $0.image
}) ?? ["Imagen"]
self?.feedDescription = self?.feeds?.compactMap({ $0.content
}) ?? ["Descripcion Imagen"]
self?.collectionView.reloadData()
self?.pageControl.numberOfPages = (self?.feedTitles.count)!
self?.pageControl.isHidden = false
}
case .failure(let feedError):
print("Fail Feed \(feedError)")
print(feedError.localizedDescription)
self?.pageControl.isHidden = true
}
}
}
This is the "load" function:
func load(url: URL) {
DispatchQueue.global().async { [weak self] in
if let data = try? Data(contentsOf: url) {
if let image = UIImage(data: data) {
DispatchQueue.main.async {
self?.image = image
}
}
}
}
}
The issue is inside your cell.imageView.load(url: imageURL) implementation.
Downloading an image is an asynchronous operation. When you create a cell - you start an operation and you don't know when it's going to complete. Whenever it completes, it sets the image on your UIImageView instance.
The cell on the other hand is a reuseable component for collectionView and when you scroll, same cell instance that disappeared by going out of screen from top will also reappear by coming in from bottom of the screen. This means that one cell instance can trigger multiple image download requests and it doesn't cancel it's previous in-flight image download request. It also does not know which image download call completed - the last one (or the one before that etc.)
What you need to do is - add some additional protection in the imageView load method so that when a cell (and hence imageView) instance is reused - it only sets the image for last request (not the ones done prior to the last one).
Copy paste following code into your project.
import UIKit
import ObjectiveC.runtime
private var keyTagIdentifier: String?
public extension UIView {
var tagIdentifier: String? {
get { return objc_getAssociatedObject(self, &keyTagIdentifier) as? String }
set { objc_setAssociatedObject(self, &keyTagIdentifier, newValue, .OBJC_ASSOCIATION_RETAIN) }
}
}
Update your image load extension like this
public extension UIImageView {
func load(url: URL) {
// 1. Assign the current request identifier on this instance
let lastRequestIdentifier = url.absoluteString
self.tagIdentifier = lastRequestIdentifier
// 2. Download the image as you are doing today
// 3. After the image has been downloaded and you set the image
// Please check whether we are still interested in same image or not
// This will be written inside your image download completion block
if self.tagIdentifier == lastRequestIdentifier {
self.image = // the image you downloaded
}
}
}
I believe the issue is due to UICollectionView reusing cells. Meaning the cell that disappears from the top, is rendered at the bottom, usually with the same state as the cell that disappeared from the top and vice-versa for the cells at the bottom. You can avoid this behaviour by adding the following to you code to your PageCell,
override func prepareForReuse() {
super.prepareForReuse()
imageView?.image = nil
}
I implement a simple drag and drop sample.
import UIKit
class ViewController: UIViewController, UICollectionViewDelegate, UICollectionViewDataSource, UICollectionViewDelegateFlowLayout {
private var collectionView: UICollectionView?
var colors: [UIColor] = [
.link,
.systemGreen,
.systemBlue,
.red,
.systemOrange,
.black,
.systemPurple,
.systemYellow,
.systemPink,
.link,
.systemGreen,
.systemBlue,
.red,
.systemOrange,
.black,
.systemPurple,
.systemYellow,
.systemPink
]
override func viewDidLoad() {
super.viewDidLoad()
let layout = UICollectionViewFlowLayout()
layout.scrollDirection = .vertical
layout.itemSize = CGSize(width: view.frame.size.width/3.2, height: view.frame.size.width/3.2)
layout.sectionInset = UIEdgeInsets(top: 0, left: 0, bottom: 0, right: 0)
collectionView = UICollectionView(frame: .zero, collectionViewLayout: layout)
//collectionView?.register(UICollectionViewCell.self, forCellWithReuseIdentifier: "cell")
let customCollectionViewCellNib = CustomCollectionViewCell.getUINib()
collectionView?.register(customCollectionViewCellNib, forCellWithReuseIdentifier: "cell")
collectionView?.delegate = self
collectionView?.dataSource = self
collectionView?.backgroundColor = .white
view.addSubview(collectionView!)
let gesture = UILongPressGestureRecognizer(target: self, action: #selector(handleLongPressGesture))
collectionView?.addGestureRecognizer(gesture)
}
#objc func handleLongPressGesture(_ gesture: UILongPressGestureRecognizer) {
guard let collectionView = collectionView else {
return
}
switch gesture.state {
case .began:
guard let targetIndexPath = collectionView.indexPathForItem(at: gesture.location(in: self.collectionView)) else {
return
}
collectionView.beginInteractiveMovementForItem(at: targetIndexPath)
case .changed:
collectionView.updateInteractiveMovementTargetPosition(gesture.location(in: collectionView))
case .ended:
collectionView.endInteractiveMovement()
default:
collectionView.cancelInteractiveMovement()
}
}
override func viewDidLayoutSubviews() {
super.viewDidLayoutSubviews()
collectionView?.frame = view.bounds
}
func collectionView(_ collectionView: UICollectionView, numberOfItemsInSection section: Int) -> Int {
return colors.count
}
func collectionView(_ collectionView: UICollectionView, cellForItemAt indexPath: IndexPath) -> UICollectionViewCell {
let cell = collectionView.dequeueReusableCell(withReuseIdentifier: "cell", for: indexPath)
cell.backgroundColor = colors[indexPath.row]
return cell
}
func collectionView(_ collectionView: UICollectionView, layout collectionViewLayout: UICollectionViewLayout, sizeForItemAt indexPath: IndexPath) -> CGSize {
return CGSize(width: view.frame.size.width/3.2, height: view.frame.size.width/3.2)
}
func collectionView(_ collectionView: UICollectionView, canMoveItemAt indexPath: IndexPath) -> Bool {
return true
}
func collectionView(_ collectionView: UICollectionView, moveItemAt sourceIndexPath: IndexPath, to destinationIndexPath: IndexPath) {
let item = colors.remove(at: sourceIndexPath.row)
colors.insert(item, at: destinationIndexPath.row)
}
}
However, I notice that, if my UICollectionViewCell is created with XIB, it will randomly exhibit flickering behaviour, during drag and drop.
The CustomCollectionViewCell is a pretty straightforward code.
CustomCollectionViewCell.swift
import UIKit
extension UIView {
static func instanceFromNib() -> Self {
return getUINib().instantiate(withOwner: self, options: nil)[0] as! Self
}
static func getUINib() -> UINib {
return UINib(nibName: String(describing: self), bundle: nil)
}
}
class CustomCollectionViewCell: UICollectionViewCell {
override func awakeFromNib() {
super.awakeFromNib()
// Initialization code
}
}
Flickering
By using the following code
let customCollectionViewCellNib = CustomCollectionViewCell.getUINib()
collectionView?.register(customCollectionViewCellNib, forCellWithReuseIdentifier: "cell")
It will have the following random flickering behaviour - https://youtu.be/CbcUAHlRJKI
No flickering
However, if the following code is used instead
collectionView?.register(UICollectionViewCell.self, forCellWithReuseIdentifier: "cell")
Things work fine. There are no flickering behaviour - https://youtu.be/QkV2HlIrXK8
May I know why it is so? How can I avoid the flickering behaviour, when my custom UICollectionView is created from XIB?
Please note that, the flickering behaviour doesn't happen all the time. It happens randomly. It is easier to reproduce the problem using real iPhone device, than simulator.
Here's the complete sample code - https://github.com/yccheok/xib-view-cell-cause-flickering
While we are rearranging cells in UICollectionView (gesture is active), it handles all of the cell movements for us (without having us to worry about changing dataSource while the rearrange is in flight).
At the end of this rearrange gesture, UICollectionView rightfully expects that we will reflect the change in our dataSource as well which you are doing correctly here.
func collectionView(_ collectionView: UICollectionView, moveItemAt sourceIndexPath: IndexPath, to destinationIndexPath: IndexPath) {
let item = colors.remove(at: sourceIndexPath.row)
colors.insert(item, at: destinationIndexPath.row)
}
Since UICollectionView expects a dataSource update from our side, it performs following steps -
Call our collectionView(_:, moveItemAt:, to:) implementation to provide us a chance to reflect the changes in dataSource.
Call our collectionView(_:, cellForItemAt:) implementation for the destinationIndexPath value from call #1, to re-create a new cell at that indexPath from scratch.
Okay, but why would it perform step 2 even if this is the correct cell to be at that indexPath?
It's because UICollectionView doesn't know for sure whether you actually made those dataSource changes or not. What happens if you don't make those changes? - now your dataSource & UI are out of sync.
In order to make sure that your dataSource changes are correctly reflected in the UI, it has to do this step.
Now when the cell is being re-created, you sometimes see the flicker. Let the UI reload the first time, put a breakpoint in the cellForItemAt: implementation at the first line and rearrange a cell. Right after rearrange completes, your program will pause at that breakpoint and you can see following on the screen.
Why does it not happen with UICollectionViewCell class (not XIB)?
It does (as noted by others) - it's less frequent. Using the above steps by putting a breakpoint, you can catch it in that state.
How to solve this?
Get a reference to the cell that's currently being dragged.
Return this instance from cellForItemAt: implementation.
var currentlyBeingDraggedCell: UICollectionViewCell?
var willRecreateCellAtDraggedIndexPath: Bool = false
#objc func handleLongPressGesture(_ gesture: UILongPressGestureRecognizer) {
guard let cv = collectionView else { return }
let location = gesture.location(in: cv)
switch gesture.state {
case .began:
guard let targetIndexPath = cv.indexPathForItem(at: location) else { return }
currentlyBeingDraggedCell = cv.cellForItem(at: targetIndexPath)
cv.beginInteractiveMovementForItem(at: targetIndexPath)
case .changed:
cv.updateInteractiveMovementTargetPosition(location)
case .ended:
willRecreateCellAtDraggedIndexPath = true
cv.endInteractiveMovement()
default:
cv.cancelInteractiveMovement()
}
}
func collectionView(_ collectionView: UICollectionView, cellForItemAt indexPath: IndexPath) -> UICollectionViewCell {
if willRecreateCellAtDraggedIndexPath,
let currentlyBeingDraggedCell = currentlyBeingDraggedCell {
self.willRecreateCellAtDraggedIndexPath = false
self.currentlyBeingDraggedCell = nil
return currentlyBeingDraggedCell
}
let cell = collectionView.dequeueReusableCell(withReuseIdentifier: "cell", for: indexPath)
cell.contentView.backgroundColor = colors[indexPath.item]
return cell
}
Will this solve the problem 100%?
NO. UICollectionView will still remove the cell from it's view hierarchy and ask us for a new cell - we are just providing it with an existing cell instance (that we know is going to be correct according to our own implementation).
You can still catch it in the state where it disappears from UI before appearing again. However this time there's almost no work to be done, so it will be significantly faster and you will see the flickering less often.
BONUS
iOS 15 seems to be working on similar problems via UICollectionView.reconfigureItems APIs. See an explanation in following Twitter thread.
Whether these improvements will land in rearrange or not, we will have to see.
Other Observations
Your UICollectionViewCell subclass' XIB looks like following
However it should look like following (1st one is missing contentView wrapper, you get this by default when you drag a Collection View Cell to the XIB from the View library OR create a UICollectionViewCell subclass with XIB).
And your implementation uses -
cell.backgroundColor = colors[indexPath.row]
You should use contentView to do all the UI customization, also note the indexPath.item(vs row) that better fits with cellForItemAt: terminology (There are no differences in these values though). cellForRowAt: & indexPath.row are more suited for UITableView instances.
cell.contentView.backgroundColor = colors[indexPath.item]
UPDATE
Should I use this workaround for my app in production?
NO.
As noted by OP in the comments below -
The proposed workaround has 2 shortcomings.
(1) Missing cell
(2) Wrong content cell.
This is clearly visible in https://www.youtube.com/watch?v=uDRgo0Jczuw Even if you perform explicit currentlyBeingDraggedCell.backgroundColor = colors[indexPath.item] within if block, wrong content cell issue is still there.
The flickering is caused by the cell being recreated at its new position. You can try holding to the cell.
(only the relevant code is shown)
// keeps a reference to the cell being dragged
private weak var draggedCell: UICollectionViewCell?
// the flag is set when the dragging completes
private var didInteractiveMovementEnd = false
#objc func handleLongPressGesture(_ gesture: UILongPressGestureRecognizer) {
switch gesture.state {
case .began:
// keep cell reference
draggedCell = collectionView.cellForItem(at: targetIndexPath)
collectionView.beginInteractiveMovementForItem(at: targetIndexPath)
case .ended:
// reuse the cell in `cellForItem`
didInteractiveMovementEnd = true
collectionView.performBatchUpdates {
collectionView.endInteractiveMovement()
} completion: { completed in
self.draggedCell = nil
self.didInteractiveMovementEnd = false
}
}
func collectionView(_ collectionView: UICollectionView, cellForItemAt indexPath: IndexPath) -> UICollectionViewCell {
// reuse the dragged cell
if didInteractiveMovementEnd, let draggedCell = draggedCell {
return draggedCell
}
let cell = collectionView.dequeueReusableCell(withReuseIdentifier: "cell", for: indexPath)
...
}
I am having a dataset displayed in a UICollectionView. The dataset is split into sections and each section has a header. Further, each cell has a detail view underneath it that is expanded when the cell is clicked.
For reference:
For simplicity, I have implemented the details cells as standard cells that are hidden (height: 0) by default and when the non-detail cell is clicked, the height is set to non-zero value. The cells are updates using invalidateItems(at indexPaths: [IndexPath]) instead of reloading cells in performBatchUpdates(_ updates: (() -> Void)?, completion: ((Bool) -> Void)? = nil) as the animations seems glitchy otherwise.
Now to the problem, the invalidateItems function obviously updates only cells, not supplementary views like the section header and therefore calling only this function will result in overflowing the section header:
After some time Googling, I found out that in order to update also the supplementary views, one has to call invalidateSupplementaryElements(ofKind elementKind: String, at indexPaths: [IndexPath]). This might recalculate the section header's bounds correctly, however results in the content not appearing:
This is most likely caused due to the fact that the func collectionView(_ collectionView: UICollectionView, viewForSupplementaryElementOfKind kind: String, at indexPath: IndexPath) -> UICollectionReusableView does not seem to be called.
I would be extremely grateful if somebody could tell me how to correctly invalidate supplementary views to the issues above do not happen.
Code:
override func numberOfSections(in collectionView: UICollectionView) -> Int {
return dataManager.getSectionCount()
}
override func collectionView(_ collectionView: UICollectionView, numberOfItemsInSection section: Int) -> Int {
let count = dataManager.getSectionItemCount(section: section)
reminder = count % itemsPerWidth
return count * 2
}
override func collectionView(_ collectionView: UICollectionView, cellForItemAt indexPath: IndexPath) -> UICollectionViewCell {
if isDetailCell(indexPath: indexPath) {
let cell = collectionView.dequeueReusableCell(withReuseIdentifier: Reusable.CELL_SERVICE, for: indexPath) as! ServiceCollectionViewCell
cell.lblName.text = "Americano detail"
cell.layer.borderWidth = 0.5
cell.layer.borderColor = UIColor(hexString: "#999999").cgColor
return cell
} else {
let item = indexPath.item > itemsPerWidth ? indexPath.item - (((indexPath.item / itemsPerWidth) / 2) * itemsPerWidth) : indexPath.item
let product = dataManager.getItem(index: item, section: indexPath.section)
let cell = collectionView.dequeueReusableCell(withReuseIdentifier: Reusable.CELL_SERVICE, for: indexPath) as! ServiceCollectionViewCell
cell.lblName.text = product.name
cell.layer.borderWidth = 0.5
cell.layer.borderColor = UIColor(hexString: "#999999").cgColor
return cell
}
}
override func collectionView(_ collectionView: UICollectionView, viewForSupplementaryElementOfKind kind: String, at indexPath: IndexPath) -> UICollectionReusableView {
switch kind {
case UICollectionElementKindSectionHeader:
if indexPath.section == 0 {
let header = collectionView.dequeueReusableSupplementaryView(ofKind: UICollectionElementKindSectionHeader, withReuseIdentifier: Reusable.CELL_SERVICE_HEADER_ROOT, for: indexPath) as! ServiceCollectionViewHeaderRoot
header.lblCategoryName.text = "Section Header"
header.imgCategoryBackground.af_imageDownloader = imageDownloader
header.imgCategoryBackground.af_setImage(withURLRequest: ImageHelper.getURL(file: category.backgroundFile!))
return header
} else {
let header = collectionView.dequeueReusableSupplementaryView(ofKind: UICollectionElementKindSectionHeader, withReuseIdentifier: Reusable.CELL_SERVICE_HEADER, for: indexPath) as! ServiceCollectionViewHeader
header.lblCategoryName.text = "Section Header"
return header
}
default:
assert(false, "Unexpected element kind")
}
}
// MARK: UICollectionViewDelegate
func collectionView(_ collectionView: UICollectionView, layout collectionViewLayout: UICollectionViewLayout, sizeForItemAt indexPath: IndexPath) -> CGSize {
let width = collectionView.frame.size.width / CGFloat(itemsPerWidth)
if isDetailCell(indexPath: indexPath) {
if expandedCell == indexPath {
return CGSize(width: collectionView.frame.size.width, height: width)
} else {
return CGSize(width: collectionView.frame.size.width, height: 0)
}
} else {
return CGSize(width: width, height: width)
}
}
func collectionView(_ collectionView: UICollectionView, layout collectionViewLayout: UICollectionViewLayout, referenceSizeForHeaderInSection section: Int) -> CGSize {
if section == 0 {
return CGSize(width: collectionView.frame.width, height: collectionView.frame.height / 3)
} else {
return CGSize(width: collectionView.frame.width, height: heightHeader)
}
}
override func collectionView(_ collectionView: UICollectionView, didSelectItemAt indexPath: IndexPath) {
if isDetailCell(indexPath: indexPath) {
return
}
var offset = itemsPerWidth
if isLastRow(indexPath: indexPath) {
offset = reminder
}
let detailPath = IndexPath(item: indexPath.item + offset, section: indexPath.section)
let context = UICollectionViewFlowLayoutInvalidationContext()
let maxItem = collectionView.numberOfItems(inSection: 0) - 1
var minItem = detailPath.item
if let expandedCell = expandedCell {
minItem = min(minItem, expandedCell.item)
}
// TODO: optimize this
var cellIndexPaths = (0 ... maxItem).map { IndexPath(item: $0, section: 0) }
var supplementaryIndexPaths = (0..<collectionView.numberOfSections).map { IndexPath(item: 0, section: $0)}
for i in indexPath.section..<collectionView.numberOfSections {
cellIndexPaths.append(contentsOf: (0 ... collectionView.numberOfItems(inSection: i) - 1).map { IndexPath(item: $0, section: i) })
//supplementaryIndexPaths.append(IndexPath(item: 0, section: i))
}
context.invalidateSupplementaryElements(ofKind: UICollectionElementKindSectionHeader, at: supplementaryIndexPaths)
context.invalidateItems(at: cellIndexPaths)
if detailPath == expandedCell {
expandedCell = nil
} else {
expandedCell = detailPath
}
UIView.animate(withDuration: 0.25) {
collectionView.collectionViewLayout.invalidateLayout(with: context)
collectionView.layoutIfNeeded()
}
}
EDIT:
Minimalistic project demonstrating this issue: https://github.com/vongrad/so-expandable-collectionview
You should use an Invalidation Context. It's a bit complex, but here's a rundown:
First, you need to create a custom subclass of UICollectionViewLayoutInvalidationContext since the default one used by most collection views will just refresh everything. There may be situations where you DO want to refresh everything though; in my instance, if the width of the collection view changes it has to layout all the cells again, so my solution looks like this:
class CustomInvalidationContext: UICollectionViewLayoutInvalidationContext {
var justHeaders: Bool = false
override var invalidateEverything: Bool { return !justHeaders }
override var invalidateDataSourceCounts: Bool { return false }
}
Now you need to tell the layout to use this context instead of the default:
override class var invalidationContextClass: AnyClass {
return CustomInvalidationContext.self
}
This won't trigger if we don't tell the layout it needs to update upon scrolling, so:
override func shouldInvalidateLayout(forBoundsChange newBounds: CGRect) -> Bool {
return true
}
I'm passing true here because there will always be something to update when the user scrolls the collection view, even if it's only the header frames. We'll determine exactly what gets changed when in the next section.
Now that it is always updating when the bounds change, we need to provide it with information about which parts should be invalidated and which should not. To make this easier, I have a function called getVisibleSections(in: CGRect) that returns an optional array of integers representing which sections overlap the given bounds rectangle. I won't detail this here as yours will be different. I'm also caching the content size of the collection view as _contentSize since this only changes when a full layout occurs.
With a small number of sections you could probably just invalidate all of them. Be that as it may, we now need to tell the layout how to set up its invalidation context when the bounds changes.
Note: make sure you're calling super to get the context rather than just creating one yourself; this is the proper way to do things.
override func invalidationContext(forBoundsChange newBounds: CGRect) -> UICollectionViewLayoutInvalidationContext {
let context = super.invalidationContext(forBoundsChange: newBounds) as! CustomInvalidationContext
// If we can't determine visible sections or the width has changed,
// we need to do a full layout - just return the default.
guard newBounds.width == _contentSize.width,
let visibleSections = getVisibleSections(in: newBounds)
else { return context }
// Determine which headers need a frame change.
context.justHeaders = true
let sectionIndices = visibleSections.map { IndexPath(item: 0, section: $0) }
context.invalidateSupplementaryElements(ofKind: "Header", at: sectionIndices)
return context
}
Note that I'm assuming your supplementary view kind is "Header"; change that if you need to. Now, provided that you've properly implemented layoutAttributesForSupplementaryView to return a suitable frame, your headers (and only your headers) should update as you scroll vertically.
Keep in mind that prepare() will NOT be called unless you do a full invalidation, so if you need to do any recalculations, override invalidateLayout(with:) as well, calling super at some point. Personally I do the calculations for shifting the header frames in layoutAttributesForSupplementaryView as it's simpler and just as performant.
Oh, and one last small tip: on the layout attributes for your headers, don't forget to set zIndex to a higher value than the one in your cells so that they definitely appear in front. The default is 0, I use 1 for my headers.
What I suggest is to create a separate subclass of a UICollectionFlowView
and set it up respectivel look at this example:
import UIKit
class StickyHeadersCollectionViewFlowLayout: UICollectionViewFlowLayout {
// MARK: - Collection View Flow Layout Methods
override func shouldInvalidateLayout(forBoundsChange newBounds: CGRect) -> Bool {
return true
}
override func layoutAttributesForElements(in rect: CGRect) -> [UICollectionViewLayoutAttributes]? {
guard let layoutAttributes = super.layoutAttributesForElements(in: rect) else { return nil }
// Helpers
let sectionsToAdd = NSMutableIndexSet()
var newLayoutAttributes = [UICollectionViewLayoutAttributes]()
for layoutAttributesSet in layoutAttributes {
if layoutAttributesSet.representedElementCategory == .cell {
// Add Layout Attributes
newLayoutAttributes.append(layoutAttributesSet)
// Update Sections to Add
sectionsToAdd.add(layoutAttributesSet.indexPath.section)
} else if layoutAttributesSet.representedElementCategory == .supplementaryView {
// Update Sections to Add
sectionsToAdd.add(layoutAttributesSet.indexPath.section)
}
}
for section in sectionsToAdd {
let indexPath = IndexPath(item: 0, section: section)
if let sectionAttributes = self.layoutAttributesForSupplementaryView(ofKind: UICollectionElementKindSectionHeader, at: indexPath) {
newLayoutAttributes.append(sectionAttributes)
}
}
return newLayoutAttributes
}
override func layoutAttributesForSupplementaryView(ofKind elementKind: String, at indexPath: IndexPath) -> UICollectionViewLayoutAttributes? {
guard let layoutAttributes = super.layoutAttributesForSupplementaryView(ofKind: elementKind, at: indexPath) else { return nil }
guard let boundaries = boundaries(forSection: indexPath.section) else { return layoutAttributes }
guard let collectionView = collectionView else { return layoutAttributes }
// Helpers
let contentOffsetY = collectionView.contentOffset.y
var frameForSupplementaryView = layoutAttributes.frame
let minimum = boundaries.minimum - frameForSupplementaryView.height
let maximum = boundaries.maximum - frameForSupplementaryView.height
if contentOffsetY < minimum {
frameForSupplementaryView.origin.y = minimum
} else if contentOffsetY > maximum {
frameForSupplementaryView.origin.y = maximum
} else {
frameForSupplementaryView.origin.y = contentOffsetY
}
layoutAttributes.frame = frameForSupplementaryView
return layoutAttributes
}
// MARK: - Helper Methods
func boundaries(forSection section: Int) -> (minimum: CGFloat, maximum: CGFloat)? {
// Helpers
var result = (minimum: CGFloat(0.0), maximum: CGFloat(0.0))
// Exit Early
guard let collectionView = collectionView else { return result }
// Fetch Number of Items for Section
let numberOfItems = collectionView.numberOfItems(inSection: section)
// Exit Early
guard numberOfItems > 0 else { return result }
if let firstItem = layoutAttributesForItem(at: IndexPath(item: 0, section: section)),
let lastItem = layoutAttributesForItem(at: IndexPath(item: (numberOfItems - 1), section: section)) {
result.minimum = firstItem.frame.minY
result.maximum = lastItem.frame.maxY
// Take Header Size Into Account
result.minimum -= headerReferenceSize.height
result.maximum -= headerReferenceSize.height
// Take Section Inset Into Account
result.minimum -= sectionInset.top
result.maximum += (sectionInset.top + sectionInset.bottom)
}
return result
}
}
then add your collection view to your view controller and this way you will implement the invalidation methods which currently are not getting triggered.
source here
Do reloadLoad cells in performBatchUpdates(_:) make it seems glitchy.
Just pass nil like below to update your cell's height.
collectionView.performBatchUpdates(nil, completion: nil)
EDIT:
I have recently found that performBatchUpdates(_:) only shift the header along with cell new height returned from the sizeForItemAt function. If using collection view cell sizing, your supplementary view may overlaps the cells. Then collectionViewLayout.invalidateLayout will fix without showing the animation.
If you want to go with sizing animation after calling performBatchUpdates(_:), try to calculate (then cache) and return cell's size in sizeForItemAt. It works for me.
I created a collection view controller from story board, and set its custom class to ItemCollectionVC, the custom class of its cell to ItemCell, and set its reuse identifier to Cell
Here's my ItemCollectionVC class:
import UIKit
private let reuseIdentifier = "Cell"
class ItemCollectionVC: UICollectionViewController {
var dataSourceItems: [Items] = []
var counterBuildItems: [Items] {
let weaponItemArray = WeaponItems.weaponItems as [Items]
let defenseItemArray = DefenseItems.defenseItems as [Items]
return weaponItemArray + defenseItemArray
}
var freeBuildItems = WeaponItems.weaponItems as [Items]
var captureKrakenItems: [Items] {
let weaponItemArray = WeaponItems.weaponItems as [Items]
let abilityItemArray = AbilityItems.abilityItems as [Items]
return weaponItemArray + abilityItemArray
}
override func viewDidAppear(_ animated: Bool) {
switch self.presentingViewController!.title! {
case "CounterBuildVC":
dataSourceItems = counterBuildItems
case "FreeBuildVC":
dataSourceItems = freeBuildItems
case "CaptureKrakenVC":
dataSourceItems = captureKrakenItems
default:
break
}
}
override func numberOfSections(in collectionView: UICollectionView) -> Int {
return 1
}
override func collectionView(_ collectionView: UICollectionView, numberOfItemsInSection section: Int) -> Int {
return dataSourceItems.count
}
override func collectionView(_ collectionView: UICollectionView, cellForItemAt indexPath: IndexPath) -> UICollectionViewCell {
let cell = collectionView.dequeueReusableCell(withReuseIdentifier: reuseIdentifier, for: indexPath) as! ItemCell
cell.cellImage.image = dataSourceItems[indexPath.row].image
print(dataSourceItems.count)
return cell
}
}
When the collection view controller is presented, it's empty, what could cause the problem?
One of three things caused this problem, pretty much every time I have encountered it, in a TableView or CollectionView:
1) Your ViewController is not the dataSource of your UICollectionView
2) numberOfRows or numberOfSections method returns 0
3) The height of your cell is 0, either due to constraint problems, or a heightForCell method being not/improperly implemented.
It's impossible to say which of these is your problem, and it's always possible that you've encountered something strange. Make certain that none of these is your problems, before exploring less likely options.
If you are pretty sure that the dataSource of the collectionView is connected to the viewController (it should be by default), then you should reloadData() because the collectionView reading from dataSourceItems. To understand the case, add a break point in cellForItemAt and add another one in viewDidAppear and check which one is called first?
override func viewDidAppear(_ animated: Bool) {
switch self.presentingViewController!.title! {
case "CounterBuildVC":
dataSourceItems = counterBuildItems
case "FreeBuildVC":
dataSourceItems = freeBuildItems
case "CaptureKrakenVC":
dataSourceItems = captureKrakenItems
default:
break
}
collectionView.reloadData()
}
Hope that helped.
Some things that you could easily miss if you use storyboard
1) Don't forget that content in cell must have connected Top, Bottom constraint and
content view must have height, by this cell will know to set height for cell. If you don't have these, cell height will be 0, and function cellForItemAt will never get called.
2) You can use cell layout to set dynamic cell and height for cell if you use this function:
func collectionView(_ collectionView: UICollectionView,layout collectionViewLayout:
UICollectionViewLayout, sizeForItemAt indexPath: IndexPath) ->
CGSize {return CGSize(width: 20.00, height: 20.00)}
I fixed my problem by initializing UICollectionView properly like the following:
fileprivate let collectionView = UICollectionView(frame: CGRect.zero, collectionViewLayout: UICollectionViewFlowLayout.init())
Check if you get the right number of sections in the numberOfItemsInSection method for the collection view.
If you are adding a flow layout to the collection view, remove the flow layout, and check if the collection view cells show now.
If they do, just adjust your collection view flow layout code it should look like this
let _flowLayout = UICollectionViewFlowLayout()
_flowLayout.sectionInset = UIEdgeInsets(top:0, left: 0, bottom: 0, right: 0)
_flowLayout.scrollDirection = .vertical
yourCollectionView.collectionViewLayout = _flowLayout
you can set the inset to fit your use.
You have to add your code in viewWillAppear, then it will work properly.
/*
override func viewDidAppear(_ animated: Bool) {
switch self.presentingViewController!.title! {
case "CounterBuildVC":
dataSourceItems = counterBuildItems
case "FreeBuildVC":
dataSourceItems = freeBuildItems
case "CaptureKrakenVC":
dataSourceItems = captureKrakenItems
default:
break
}
}
Like this :-
override func viewWillAppear(_ animated: Bool) {
switch self.presentingViewController!.title! {
case "CounterBuildVC":
dataSourceItems = counterBuildItems
case "FreeBuildVC":
dataSourceItems = freeBuildItems
case "CaptureKrakenVC":
dataSourceItems = captureKrakenItems
default:
break
}
}
*/
In my case there was a problem with the collection view contentInset, try adding below code in your collection view sub class.
override func layoutSubviews() {
super.layoutSubviews()
self.contentInset = UIEdgeInsetsMake(0, 0, 0, 0)
}
override func reloadData() {
DispatchQueue.main.async {
super.reloadData()
}
}
I am having issues with displaying a checkmark on the a custom cell in a UICollectionView. For the first few taps everything works as expected, but when I begin scrolling or tapping repeatedly or click on the already selected cell, the behavior becomes odd as shown in the gif. Perhaps I am going about this in an incorrect way? The .addCheck() and .removeCheck() are methods inside the custom UICollectionViewCell class I made and all they do is add a checkmark image or remove one from the cell view. The odd behavior shown here
override func collectionView(_ collectionView: UICollectionView, cellForItemAt indexPath: IndexPath) -> UICollectionViewCell {
let cell = collectionView.dequeueReusableCell(withReuseIdentifier: "cell", for: indexPath) as! ColorUICollectionViewCell
// Configure the cell
let color = colorList[(indexPath as NSIndexPath).row]
cell.delegate = self
cell.textLabel.text = color.name
cell.backgroundColor = color.color
if color.selected {
cell.addCheck()
}
else {
cell.removeCheck()
}
return cell
}
// user selects item
override func collectionView(_ collectionView: UICollectionView, didSelectItemAt indexPath: IndexPath) {
// set colors to false for selection
for color in colorList {
color.selected = false
}
// set selected color to true for selection
let color = colorList[indexPath.row]
color.selected = true
settings.backgroundColor = color.color
//userDefaults.set(selectedIndex, forKey: "selectedIndex")
collectionView.reloadData()
}
Below is what the addCheck() and removeCheck() functions in my custom cell look like.
func addCheck() {
// create check image
let checkImage = UIImage(named: "checkmark")
checkImageView = UIImageView(frame: CGRect(x: 0, y: 0, width: bounds.size.height / 4, height: bounds.size.height / 4))
checkImageView.image = checkImage!.withRenderingMode(UIImageRenderingMode.alwaysTemplate)
checkImageView.tintColor = UIColor.white()
// add the views
addSubview(checkImageView)
}
func removeCheck() {
if checkImageView != nil {
checkImageView.removeFromSuperview()
}
}
first off, you can simplify your didSelect a bit:
override func collectionView(collectionView: UICollectionView, didSelectItemAt indexPath: IndexPath) {
// set colors to false for selection
for (index, color) in colorList.enumerate() {
if index == indexPath.row {
color.selected = false
settings.backgroundColor = color.color
}
else {
color.selected = false
}
}
collectionView.reloadData()
}
Based on the language in your cellForItemAt method, I'm guessing you're adding a second check mark image when you tap on the same cell twice, and it's not being tracked properly so that cell just keeps getting rotated around overtime the collectionView's reloaded
Post your cell class, or at least the logic for addCheck and removeCheck and we might find the problem.
What I would recommend is permanently having an imageView with the check mark over the cell, when simple show/hide it based on the selection. This should speed up the collectionView as well.