Using a button to scroll a uicollectionview - how do i check bounds? - ios

Question

I don't know how to figure out whether I get to the last IndexPath and when to "rewind" and scroll to the first IndexPath
This is some setup:
- (void)viewWillLayoutSubviews;
{
[super viewWillLayoutSubviews];
UICollectionViewFlowLayout *flowLayout = (id)self.collectionView.collectionViewLayout;
if (UIDeviceOrientationIsLandscape([[UIApplication sharedApplication] statusBarOrientation])) {
flowLayout.itemSize = CGSizeMake(1024.0f, 768.0f);
} else {
flowLayout.itemSize = CGSizeMake(1024.0f, 768.0f);
}
[flowLayout invalidateLayout]; //force the elements to get laid out again with the new size
visibleItems = [self.collectionView indexPathsForVisibleItems];
self.currentIndexPath = [visibleItems firstObject];
[self.collectionView.collectionViewLayout invalidateLayout];
}
This is my button code:
- (IBAction)addToUploadQueque:(id)sender {
NSLog(#"current: %#",self.currentIndexPath);
NSInteger section = [self numberOfSectionsInCollectionView:self.collectionView] - 1;
NSInteger item = [self collectionView:self.collectionView numberOfItemsInSection:section]-1;
NSIndexPath *lastIndexPath = [NSIndexPath indexPathForItem:item inSection:section];
NSIndexPath *firstIndexpath =[NSIndexPath indexPathForItem:0 inSection:0];
NSLog(#"current: %#",lastIndexPath);
if (self.currentIndexPath <= lastIndexPath) {
NSInteger newLast = [self.currentIndexPath indexAtPosition:self.currentIndexPath.length-1]+1;
self.currentIndexPath = [[self.currentIndexPath indexPathByRemovingLastIndex] indexPathByAddingIndex:newLast];
[self.collectionView scrollToItemAtIndexPath:self.currentIndexPath
atScrollPosition:UICollectionViewScrollPositionCenteredHorizontally
animated:YES];
}else{
self.currentIndexPath = [visibleItems firstObject];
[self.collectionView scrollToItemAtIndexPath:self.currentIndexPath
atScrollPosition:UICollectionViewScrollPositionCenteredHorizontally
animated:YES];
}
}
I'm making a button that iterates through each cell in the collection view and when it gets to the end (instead of going out of bounds) scroll back to the first cell.
Can anyone tell me what i'm doing wrong?

Answer 1

Judging from your comments, it doesn't sound like you want to just enumerate the visible cells in one pass, but rather want to enumerate through them manually. And it sounds like you're having an issue getting the next NSIndexPath. The problem with your code snippet is that you're incrementing the row/item (depending upon whether you're dealing with UITableView or UICollectionView), but not considering whether you've reached the end of a section, much less the end of the data source, before you try to use your incremented data source.
You could do something like:
NSInteger item = self.currentIndexPath.item;
NSInteger section = self.currentIndexPath.section;
item++; // go to next item
if (item >= [self.collectionView numberOfItemsInSection:section]) { // if you reached end of section ...
item = 0; // ... go to the start of the next section
section++;
if (section >= [self.collectionView numberOfSections]) { // if you reached the end of the data source ...
// all done, so set section to zero to go back to beginning, e.g. // ... then you're done
section = 0;
}
}
self.currentIndexPath = [NSIndexPath indexPathForItem:item inSection:section]; // otherwise, this is your new NSIndexPath
BTW, if we're going to focus on your code snippet, the another issue is that you're using the <= operator to compare two index paths. You cannot do that. You have to use the compare method of NSIndexPath.
But that if statement strikes me as unnecessary, as there is no index path in the data source after the last index path. If you're incrementing logic (above) correctly detects the end of the data source, then this if statement is unnecessary.
There are many, many issues in this code. But I'm wondering whether, rather than going through all of those details, whether a simpler approach is possible. If you just want to perform uploads for all of the visible rows, perhaps you could do something radically simpler, such as:
for (NSIndexPath *indexPath in self.collectionView.indexPathsForVisibleItems) {
// if the cell has some visual indication to reflect upload has been initiated,
// do that here
// do your asynchronous upload here, where the completion block dispatches
// updates to the cell/collectionView (to reflect that the individual upload
// is done)
}
Note, your code is scrolling to the cell (presumably when the upload is done). I might try to dissuade you from that approach, but instead just update the cell (e.g. set some flag that your cellForItemAtIndexPath method references, and then call reloadItemsAtIndexPaths for each row as that row finishes. Because uploads can be slow, you might not want the UI scrolling around as these asynchronous uploads finish.