UICollectionView cell selection and cell reuse
Upon cell selection, I want to handle changing the cell appearance. I figured the delegate method collectionView:didSelectItemAtIndexPath: & collectionView:didDeselectItemAtIndexPath: is where I should edit the cell.
-(void)collectionView:(UICollectionView *)collectionView
didSelectItemAtIndexPath:(NSIndexPath *)indexPath {
DatasetCell *datasetCell =
(DatasetCell *)[collectionView cellForItemAtIndexPath:indexPath];
[datasetCell replaceHeaderGradientWith:[UIColor skyBlueHeaderGradient]];
datasetCell.backgroundColor = [UIColor skyBlueColor];
}
and
-(void)collectionView:(UICollectionView *)collectionView
didDeselectItemAtIndexPath:(NSIndexPath *)indexPath {
DatasetCell *datasetCell =
(DatasetCell *)[collectionView cellForItemAtIndexPath:indexPath];
[datasetCell replaceHeaderGradientWith:[UIColor grayGradient]];
datasetCell.backgroundColor = [UIColor myDarkGrayColor];
}
This works fine, except when the cell gets reused. If I select cell at index (0, 0), it changes the appearance but when I scroll down, there is another cell in the selected state.
I believe I should use the UICollectionViewCell method -(void)prepareForReuse to prep the cell for resuse (ie, set the cell appearance to non selected state) but its giving me difficulties.
-(void)prepareForReuse {
if ( self.selected ) {
[self replaceHeaderGradientWith:[UIColor skyBlueHeaderGradient]];
self.backgroundColor = [UIColor skyBlueColor];
} else {
[self replaceHeaderGradientWith:[UIColor grayGradient]];
self.backgroundColor = [UIColor myDarkGrayColor];
}
}
When I scroll back to the top, the cell at index (0, 0) is in the deselected state.
When I just used the cell.backgroundView property, to prevent this from happening was to:
-(void)prepareForReuse {
self.selected = FALSE;
}
and the selection state worked as intended.
Any ideas?
Your observation is correct. This behavior is happening due to the reuse of cells. But you dont have to do any thing with the prepareForReuse. Instead do your check in cellForItem and set the properties accordingly. Some thing like..
- (UICollectionViewCell *)collectionView:(UICollectionView *)collectionView cellForItemAtIndexPath:(NSIndexPath *)indexPath
{
UICollectionViewCell *cell = [collectionView dequeueReusableCellWithReuseIdentifier:@"cvCell" forIndexPath:indexPath];
if (cell.selected) {
cell.backgroundColor = [UIColor blueColor]; // highlight selection
}
else
{
cell.backgroundColor = [UIColor redColor]; // Default color
}
return cell;
}
-(void)collectionView:(UICollectionView *)collectionView didSelectItemAtIndexPath:(NSIndexPath *)indexPath {
UICollectionViewCell *datasetCell =[collectionView cellForItemAtIndexPath:indexPath];
datasetCell.backgroundColor = [UIColor blueColor]; // highlight selection
}
-(void)collectionView:(UICollectionView *)collectionView didDeselectItemAtIndexPath:(NSIndexPath *)indexPath {
UICollectionViewCell *datasetCell =[collectionView cellForItemAtIndexPath:indexPath];
datasetCell.backgroundColor = [UIColor redColor]; // Default color
}
Framework will handle switching the views for you once you setup your cell's backgroundView and selectedBackgroundView, see example from Managing the Visual State for Selections and Highlights:
UIView* backgroundView = [[UIView alloc] initWithFrame:self.bounds];
backgroundView.backgroundColor = [UIColor redColor];
self.backgroundView = backgroundView;
UIView* selectedBGView = [[UIView alloc] initWithFrame:self.bounds];
selectedBGView.backgroundColor = [UIColor whiteColor];
self.selectedBackgroundView = selectedBGView;
you only need in your class that implements UICollectionViewDelegate enable cells to be highlighted and selected like this:
- (BOOL)collectionView:(UICollectionView *)collectionView
shouldHighlightItemAtIndexPath:(NSIndexPath *)indexPath
{
return YES;
}
- (BOOL)collectionView:(UICollectionView *)collectionView
shouldSelectItemAtIndexPath:(NSIndexPath *)indexPath;
{
return YES;
}
This works me.
UICollectionView has changed in iOS 10 introducing some problems to solutions above.
Here is a good guide: https://littlebitesofcocoa.com/241-uicollectionview-cell-pre-fetching
Cells now stay around for a bit after going off-screen. Which means that sometimes we might not be able to get hold of a cell in didDeselectItemAt indexPath in order to adjust it. It can then show up on screen un-updated and un-recycled. prepareForReuse does not help this corner case.
The easiest solution is disabling the new scrolling by setting isPrefetchingEnabled to false. With this, managing the cell's display with
cellForItemAt didSelect didDeselect works as it used to.
However, if you'd rather keep the new smooth scrolling behaviour it's better to use willDisplay :
func collectionView(_ collectionView: UICollectionView, willDisplay cell: UICollectionViewCell, forItemAt indexPath: IndexPath) {
let customCell = cell as! CustomCell
if customCell.isSelected {
customCell.select()
} else {
customCell.unselect()
}
}
func collectionView(_ collectionView: UICollectionView, cellForItemAt indexPath: IndexPath) -> UICollectionViewCell {
let cell = collectionView.dequeueReusableCell(withReuseIdentifier: "CustomCell", for: indexPath) as! CustomCell
//Don't even need to set selection-specific things here as recycled cells will also go through willDisplay
return cell
}
func collectionView(_ collectionView: UICollectionView, didSelectItemAt indexPath: IndexPath) {
let cell = collectionView.cellForItem(at: indexPath) as? CustomCell
cell?.select()
}
func collectionView(_ collectionView: UICollectionView, didDeselectItemAt indexPath: IndexPath) {
let cell = collectionView.cellForItem(at: indexPath) as? CustomCell
cell?.unselect() // <----- this can be null here, and the cell can still come back on screen!
}
With the above you control the cell when it's selected, unselected on-screen, recycled, and just re-displayed.
Anil was on the right track (his solution looks like it should work, I developed this solution independently of his). I still used the prepareForReuse: method to set the cell's selected to FALSE, then in the cellForItemAtIndexPath I check to see if the cell's index is in `collectionView.indexPathsForSelectedItems', if so, highlight it.
In the custom cell:
-(void)prepareForReuse {
self.selected = FALSE;
}
In cellForItemAtIndexPath: to handle highlighting and dehighlighting reuse cells:
if ([collectionView.indexPathsForSelectedItems containsObject:indexPath]) {
[collectionView selectItemAtIndexPath:indexPath animated:FALSE scrollPosition:UICollectionViewScrollPositionNone];
// Select Cell
}
else {
// Set cell to non-highlight
}
And then handle cell highlighting and dehighlighting in the didDeselectItemAtIndexPath: and didSelectItemAtIndexPath:
This works like a charm for me.
I had a horizontal scrolling collection view (I use collection view in Tableview) and I too faced problems withcell reuse, whenever I select one item and scroll towards right, some other cells in the next visible set gets select automatically. Trying to solve this using any custom cell properties like "selected", highlighted etc didnt help me so I came up with the below solution and this worked for me.
Step1:
Create a variable in the collectionView to store the selected index, here I have used a class level variable called selectedIndex
- (UICollectionViewCell *)collectionView:(UICollectionView *)collectionView cellForItemAtIndexPath:(NSIndexPath *)indexPath
{
MyCVCell *cell = (MyCVCell*)[collectionView dequeueReusableCellWithReuseIdentifier:@"MyCVCell" forIndexPath:indexPath];
// When scrolling happens, set the selection status only if the index matches the selected Index
if (selectedIndex == indexPath.row) {
cell.layer.borderWidth = 1.0;
cell.layer.borderColor = [[UIColor redColor] CGColor];
}
else
{
// Turn off the selection
cell.layer.borderWidth = 0.0;
}
return cell;
}
- (void)collectionView:(UICollectionView *)collectionView didSelectItemAtIndexPath:(NSIndexPath *)indexPath
{
MyCVCell *cell = (MyCVCell *)[collectionView cellForItemAtIndexPath:indexPath];
// Set the index once user taps on a cell
selectedIndex = indexPath.row;
// Set the selection here so that selection of cell is shown to ur user immediately
cell.layer.borderWidth = 1.0;
cell.layer.borderColor = [[UIColor redColor] CGColor];
[cell setNeedsDisplay];
}
- (void)collectionView:(UICollectionView *)collectionView didDeselectItemAtIndexPath:(NSIndexPath *)indexPath
{
MyCVCell *cell = (MyCVCell *)[collectionView cellForItemAtIndexPath:indexPath];
// Set the index to an invalid value so that the cells get deselected
selectedIndex = -1;
cell.layer.borderWidth = 0.0;
[cell setNeedsDisplay];
}
-anoop