Collectionview Cell Delegate method not called on scrollToItemAtIndexPath? - ios

I use scrollToItemAtIndexPath in the viewDidLayoutSubviews part of my controller to allow me to scroll a user to a specific cell.
My cells are supposed to make network calls for their params when they are loaded as part of the cellForItemAtIndexPath
The issue is that putting a print statement in cellForItemAtIndexPath it appears it's never called? What's the cause of the conflict here and the solution to make it work? Code as follows:
- (void)viewDidLayoutSubviews {
CDCChannelCollectionView *scrollView;
if (self.view == [self mixMonitorView]) {
scrollView = [[self mixMonitorView] scrollView];
NSIndexPath *indexPath = [NSIndexPath indexPathForItem:0 inSection: self.selectedChanIndex];
[scrollView scrollToItemAtIndexPath:indexPath atScrollPosition:UICollectionViewScrollPositionCenteredHorizontally animated:YES];
}
}
-(UICollectionViewCell *)collectionView:(UICollectionView *)collectionView cellForItemAtIndexPath:(NSIndexPath *)indexPath {
CDCChannelStrip *cell = [collectionView dequeueReusableCellWithReuseIdentifier:#"cell" forIndexPath:indexPath];
if (self.view == self.mixMonitorView) {
NSInteger chanInt = indexPath.section;
NSNumber *chanNum = [NSNumber numberWithInteger:chanInt];
NSNumber *chanNameNumber = [NSNumber numberWithInteger:chanInt + 1];
NSString *chanName = [NSString stringWithFormat:#"CH %#", chanNameNumber];
cell.channelNumber = chanInt;
cell.channelName.text = chanName;
[self getParameters:(chanNum)];
[self.mixMonitorView setChannelsStripToType:(cell)];
cell.clipsToBounds = YES;
return cell;
}
}

scrollToItemAtIndexPath will not call datasource methods unless it is scrolling to a collectionViewCell which is not visible before scrolling. You need to call reloadData if it is scrolling to an already visible cell

Datasource method will call on reloading the collection view.
Try below
[yourCollectionViewName reloadData];

Related

Executing network request once cellForItemAtIndexPath is completed for CollectionView?

I have a collection view that in which a cell is populated based on an array of int's.
After the cells are created I want to check for parameters for each cell via network request.
At the moment I carry out the network request for each cell in this method and it causes a cell to be created before its parameters are assigned due to inconsistent networks.
Is there a method to run a network function (sendGetPar:) on completion of laying out the cells? Obviously this can then be reused when the user scrolls etc.
-(UICollectionViewCell *)collectionView:(UICollectionView *)collectionView cellForItemAtIndexPath:(NSIndexPath *)indexPath {
BOOL isFocusOn = [_userDefault boolForKey:#"mixFocusOn"];
if (isFocusOn == TRUE) {
CDCChannelStrip *cell = [collectionView dequeueReusableCellWithReuseIdentifier:#"cell" forIndexPath:indexPath];
NSNumber *setChan = [self.focusChannels objectAtIndex:indexPath.section];
NSInteger chanInt = [setChan intValue] +1;
cell.clipsToBounds = YES;
[cell initData:(chanInt)];
[self.mixMonitorView setChannelsStripToType:(cell)];
[self.mixMonitorView sendGetPar:chanInt];
return cell;
}
I believe the Apple's documentation would be helpful
probably the methods:
collectionView:didEndDisplayingCell:forItemAtIndexPath:
Tells the delegate that the specified cell was removed from the collection view.
collectionView:didUpdateFocusInContext:withAnimationCoordinator:
Tells the delegate that a focus update occurred.
Something like this:
-(UICollectionViewCell *)collectionView:(UICollectionView *)collectionView cellForItemAtIndexPath:(NSIndexPath *)indexPath {
BOOL isFocusOn = [_userDefault boolForKey:#"mixFocusOn"];
if (isFocusOn == TRUE) {
CDCChannelStrip *cell = [collectionView dequeueReusableCellWithReuseIdentifier:#"cell" forIndexPath:indexPath];
NSInteger chanInt = indexPath.row +1;
cell.clipsToBounds = YES;
[cell initData:(chanInt)];
[self.mixMonitorView setChannelsStripToType:(cell)];
[self.mixMonitorView sendGetPar:chanInt];
return cell;
}
-(void) sendGetPar:(NSInteger)index // you could return the parameter via this method.
{
NSLog(#"Parameter:%ld", (long)index);
}

How to keep the didSelect action in UICollectionView in Objective-c?

I have a UICollectionView and I add from my cellForItemAtIndexPath 12 cells. When I do scroll on this to see down cells, all functions is preserved, but when I do scroll to go up again, some cells don't execute the function loaded in didSelectItemAtIndexPath.
I set disabled some rows. But not the row that I clicked Why could be this? Could be a bad prepare for reuse cell?
I'm trying the reuse function, but this only affect drawing the cell wrong or on the another position and the function added in didSelectItemAtIndexPath not work:
[self.myCollectionView reloadData];
[self.myCollectionView layoutIfNeeded];
NSArray *visibleItems = [self.myCollectionView indexPathsForVisibleItems];
NSIndexPath *currentItem = [visibleItems objectAtIndex:0];
NSIndexPath *nextItem = [NSIndexPath indexPathForItem:currentItem.item + 1 inSection:currentItem.section];
[self.myCollectionView scrollToItemAtIndexPath:nextItem atScrollPosition:UICollectionViewScrollPositionTop animated:YES];
WHen I do scroll and the do click to one cell, this not open my secondViewController, I think that this cell is gettin a wrong index that was disabled.
- (UICollectionViewCell *)collectionView:(UICollectionView *)collectionView cellForItemAtIndexPath:(NSIndexPath *)indexPath
{
MyViewController *cell = [collectionView dequeueReusableCellWithReuseIdentifier:#"myCell" forIndexPath:indexPath];
switch ([indexPath row]) {
case 0:
titleCell= #"Title0";
detailCell=#"Detail0";
if([indexPath row]==2){
[cell setUserInteractionEnabled:NO];//Here I set disable Could be the problem caused by this??
}
[cell.image setHidden:YES];
cell.image.contentMode = UIViewContentModeScaleAspectFit;
break;
//Here other cases with the same structure
return cell;
}
- (void)collectionView:(UICollectionView *)collectionView didSelectItemAtIndexPath:(NSIndexPath *)indexPath
{
[collectionView deselectItemAtIndexPath:indexPath animated:NO];
MySecondClass *secondClass = [self.storyboard instantiateViewControllerWithIdentifier:#"myVC"];
[self.navigationController pushViewController:secondClass animated:YES];
}
-(void)collectionView:(UICollectionView *)collectionView willDisplayCell:(UICollectionViewCell *)cell forItemAtIndexPath:(NSIndexPath *)indexPath
{
//Here assign yellow background color to first row and white color to the second row
}
And in my cell class I added the prepareForReuse but this not work...
- (void)prepareForReuse {
[super prepareForReuse];
[self removeFromSuperview];
[self setNeedsLayout];
}
The problem lies on this piece of code:
- (UICollectionViewCell *)collectionView:(UICollectionView *)collectionView cellForItemAtIndexPath:(NSIndexPath *)indexPath
{
MyViewController *cell = [collectionView dequeueReusableCellWithReuseIdentifier:#"myCell" forIndexPath:indexPath];
switch ([indexPath row]) {
case 0:
titleCell= #"Title0";
detailCell=#"Detail0";
if([indexPath row]==2){
[cell setUserInteractionEnabled:NO];//Here I set disable Could be the problem caused by this??
}
[cell.image setHidden:YES];
cell.image.contentMode = UIViewContentModeScaleAspectFit;
break;
return cell;
}
Because cells are reusable. So the cell that you disabled earlier (For example cell0) will continue to be reused. When you scroll, that cell0 will become cell11 for example. However, its setting is still disabled.
You need to add a else statement to remove the disabled setting like this.
if([indexPath row]==2){
[cell setUserInteractionEnabled:NO];//Here I set disable Could be the problem caused by this??
}
else{
[cell setUserInteractionEnabled:YES];
}
When a cell scrolls off the screen it is potentially deallocated or (more likely) recycled, so any state you have in that cell is not persisted once its off screen. The solution is that you should check if a cell is selected and update its appearance in cellForRowAtIndexPath. You can just have didSelectItemAtIndexPath and didDeselectItemAtIndexPath call reloadRowsAtIndexPath and cellForRowAtIndexPath will handle all of the highlighting appearance logic in one place.

UITableView ScrollToRowAtIndexPath method not working

I have a UITableView as a subview of a View. In the ViewController when the table is being populated I'm highlighting one of the rows and keeping a record of the indexPath for that row. I'm doing this in the cellforRowAtIndexPath method.
-(UITableViewCell *) tableView:(UITableView *)tableView cellForRowAtIndexPath:(NSIndexPath *)indexPath{
favouriteCellView *cell = [tableView dequeueReusableCellWithIdentifier:#"reuseID"];
QBCOCustomObject *favouriteObject = [_favouriteResults objectAtIndex:indexPath.row];
if (favouriteObject.userID == self.user.ID) {
UIView *bgColor = [[UIView alloc] init];
bgColor.backgroundColor = [UIColor colorWithRed:173.0f/255.0f green:146.0f/255.0f blue:237.0f/255.0f alpha:.5];
self.highlightedRowIndex = indexPath;
[cell setSelectedBackgroundView:bgColor];
[tableView selectRowAtIndexPath:indexPath animated:NO scrollPosition:UITableViewScrollPositionMiddle];
}
return cell;
}
Then in the viewDidAppear Method I want the table to scroll to the highlighted cell.
-(void)viewDidAppear:(BOOL)animated{
[self.tableView scrollToRowAtIndexPath:self.highlightedRowIndex atScrollPosition:UITableViewScrollPositionTop animated:YES];
}
However I've double checked that the method is being hit with a breakpoint, but unfortunately the highlighted row is not being scrolled to the top of the table as I'd expected. AM I misunderstanding the scrollToRowAtIndexPath method? Or have I left something out of the above code.
If the row is not on screen, it will not yet be loaded by the table view. This means your cellForRowAtIndexPath for that cell will not yet be called. You'll want to choose this index in a way that does not depend on the view loading. Try this before you call scrollToRowAtIndexPath:
NSInteger row = 0;
for (QBCOCustomObject *object in _favouriteResults) {
if (object.userID == self.user.ID) break;
row++;
}
self.highlightedRowIndex = [NSIndexPath indexPathForRow:row inSection:0];

UICollectionView allowsMultipleSelelections not working

I am trying to make some photoPicker with CollectionView.
Have
allowsMultipleSelection = YES
Using following method
- (void)collectionView:(UICollectionView *)collectionView didSelectItemAtIndexPath:(NSIndexPath *)indexPath {
selectedPictures = [NSMutableArray array];
[selectedPictures addObject:[imagesArray objectAtIndex:indexPath.item]];
NSLog(#"Selected list:\n %#", selectedPictures);
NSLog(#"Objects in Array %i", selectedPictures.count);
}
While I am selecting cells, it's always adding to MutableArray only one object according it's indexPath. What could be an issue?
Why don't u keep the selectedPictures as a member variable
in your code
- (void)collectionView:(UICollectionView *)collectionView didSelectItemAtIndexPath:(NSIndexPath *)indexPath {
selectedPictures = [NSMutableArray array]; //keep on creation the new array on each selection
[selectedPictures addObject:[imagesArray objectAtIndex:indexPath.item]]; //adding the selected images means single image
NSLog(#"Selected list:\n %#", selectedPictures);
NSLog(#"Objects in Array %i", selectedPictures.count);
}
try this
put his in viewDidLoad
- (void)viewDidLoad
{
selectedPictures = [[NSMutableArray alloc]init]; //initilise hear
}
- (void)collectionView:(UICollectionView *)collectionView didSelectItemAtIndexPath:(NSIndexPath *)indexPath {
// selectedPictures = [NSMutableArray array]; //keep on creation the new array on each selection
[selectedPictures addObject:[imagesArray objectAtIndex:indexPath.item]]; //adding the selected images means single image to already initialised array
NSLog(#"Selected list:\n %#", selectedPictures);
NSLog(#"Objects in Array %i", selectedPictures.count);
}
Hope this helps u .. :)
it may be caused by not calling super. While the documentation for UICollectionReusableView fails to mention this, the documentation for UITableViewCell, which has the same method, does.
- (void)prepareForReuse
{
[super prepareForReuse]
// Your code here.
}
Old Answer:
This may be a bug with the UICollectionView.
What's happening is cells that were previously selected are being reused and maintain the selected state. The collection view isn't setting selected to "NO".
The solution is to reset the the selected state in prepareForReuse of the cell:
- (void)prepareForReuse
{
self.selected = NO;
}
If the reused cell is selected, the collection view will set selected to "YES" after prepareForReuse is called.
This is something the UICollectionView should be doing on it's own. Thankfully the solution is simple. Unfortunately I spent a ton of time working around this bug by tracking my own select state. I didn't realize why it was happening until I was working on another project with smaller cells.
Also Try this
I'm not seeing why this would take place. I do not believe the issue is the use of row vs item, though you really should use item. I can imagine, though, if your collection view has more than one section, that only looking at row/item but ignoring section would be a problem (i.e. it would select the same item number in every section).
To cut the Gordian knot, I'd suggest saving the NSIndexPath of the selected item, and then using that for the basis of comparison. That also makes it easy to render an optimization in didSelectItemAtIndexPath. Anyway, first define your property:
#property (nonatomic, strong) NSIndexPath *selectedItemIndexPath;
And then implement cellForItemAtIndexPath and didSelectItemAtIndexPath:
- (UICollectionViewCell *)collectionView:(UICollectionView *)collectionView cellForItemAtIndexPath:(NSIndexPath *)indexPath
{
static NSString *cellIdentifier = #"Cell";
CollectionCell *cell = [collectionView dequeueReusableCellWithReuseIdentifier:cellIdentifier forIndexPath:indexPath];
cell.imageView.image = ...
if (self.selectedItemIndexPath != nil && [indexPath compare:self.selectedItemIndexPath] == NSOrderedSame) {
cell.imageView.layer.borderColor = [[UIColor redColor] CGColor];
cell.imageView.layer.borderWidth = 4.0;
} else {
cell.imageView.layer.borderColor = nil;
cell.imageView.layer.borderWidth = 0.0;
}
return cell;
}
- (void)collectionView:(UICollectionView *)collectionView didSelectItemAtIndexPath:(NSIndexPath *)indexPath
{
// always reload the selected cell, so we will add the border to that cell
NSMutableArray *indexPaths = [NSMutableArray arrayWithObject:indexPath];
if (self.selectedItemIndexPath)
{
// if we had a previously selected cell
if ([indexPath compare:self.selectedItemIndexPath] == NSOrderedSame)
{
// if it's the same as the one we just tapped on, then we're unselecting it
self.selectedItemIndexPath = nil;
}
else
{
// if it's different, then add that old one to our list of cells to reload, and
// save the currently selected indexPath
[indexPaths addObject:self.selectedItemIndexPath];
self.selectedItemIndexPath = indexPath;
}
}
else
{
// else, we didn't have previously selected cell, so we only need to save this indexPath for future reference
self.selectedItemIndexPath = indexPath;
}
// and now only reload only the cells that need updating
[collectionView reloadItemsAtIndexPaths:indexPaths];
}
Check also this
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
}
I solved my issue;
The problem was very simple, I should have initialise MutableArray not in the Method didSelectItemAtIndexPath, but in the ViewDidLoad. Now it adding pictures one by one

UICollectionView not redrawing cells when inserted at position where cells were deleted before

I am making a UICollectionView control which would look like (fig-1) :
I have added the ability to delete cell by swiping the cells to right.
My problem case - If I delete the last cell by swiping (fig-2) , which will call the following code.
- (void)removeTheCell:(AnyObject *)obj {
// remove the object
NSIndexPath *indexPath = [NSIndexPath indexPathForItem:[self.allObjects indexOfObject:obj] inSection:0];
[self.allObjects removeObjectAtIndex:indexPath.row];
[self.collectionView deleteItemsAtIndexPaths:#[indexPath]];
}
And then add a new cell with different color using following method (fig-4):
- (void)addNewObject:(NSNotification *)notification {
NSDictionary *dict = notification.userInfo;
NSArray *newObjects_a = [dict objectForKey:ALL_OBJECTS];
NSMutableArray *indexArrays = [[NSMutableArray alloc] init];
for (AnyObject *obj in newObjects_a) {
[self.allObjects addObject:obj];
[indexArrays addObject:[NSIndexPath indexPathForItem:[self.allObjects indexOfObject:obj] inSection:0]];
}
[self.collectionView performBatchUpdates:^{
[self.collectionView insertItemsAtIndexPaths:indexArrays];
} completion:nil];
}
The cell that is displayed still looks like the old deleted cell with its last state (fig-4). But i checked the data source it doesn't contain the deleted object. It contains the latest data.
(fig-5)If i change to list layout by selecting the segment control which call the following method:
- (IBAction)SwitchCellFrames:(id)sender {
int selection = ((UISegmentedControl *)sender).selectedSegmentIndex;
isGridView = selection == 0 ? YES : NO;
if (isGridView) {
[self.collectionView setCollectionViewLayout:gridFlowLayout animated:YES];
}else {
[self.collectionView setCollectionViewLayout:listFlowLayout animated:YES];
}
}
layout variables are defined as :
gridFlowLayout = [[UICollectionViewFlowLayout alloc] init];
[gridFlowLayout setItemSize:CGSizeMake(160, 155)];
[gridFlowLayout setMinimumInteritemSpacing:0.0f];
[gridFlowLayout setMinimumLineSpacing:0.0f];
[gridFlowLayout setScrollDirection:UICollectionViewScrollDirectionVertical];
listFlowLayout = [[UICollectionViewFlowLayout alloc] init];
[listFlowLayout setItemSize:CGSizeMake(320, 80)];
[listFlowLayout setMinimumInteritemSpacing:0.0f];
[listFlowLayout setMinimumLineSpacing:0.0f];
[listFlowLayout setScrollDirection:UICollectionViewScrollDirectionVertical];
The collectionView now updates the new cell with the right color (fig-5/fig-6).
I tried [self.collectionView setNeedsDisplay] / [self.collectionView setNeedsLayout] / [self.collectionView reloadData]. These are not causing the UI to redraw itself.
I don't know what is causing the UICollectionView to retain the deleted view. Please Help.
Found a work arround that is working for me in this situation :
I was creating and updating the ui under - (void)awakeFromNib() of my customCell class. So, when the the new cell is added at the location from where a cell was earlier deleted, - (void)awakeFromNib() was not getting called again and a previous copy of cell was being returned.
Therefore, i made the ui update method public and removed it from - (void)awakeFromNib(). Calling UI update method explicitly from cellForItemAtIndexPath solved the problem for me.
-(UICollectionViewCell *)collectionView:(UICollectionView *)collectionView cellForItemAtIndexPath:(NSIndexPath *)indexPath{
CustomViewCell *cell = [collectionView dequeueReusableCellWithReuseIdentifier:#"CollectionCell" forIndexPath:indexPath];
cell.obj = [self.allObjects objectAtIndex:indexPath.row];
[cell handleChangesForLayoutAndPosition];
[cell updateUI];
[cell resetScrollView];
cell.delegate = self;
return cell;
}

Resources