uicollectionview在reloaddata之后立即选择一个项目？

Question

After calling -[UICollectionView reloadData] it takes some time for cells to be displayed, so selecting an item immediately after calling reloadData does not work. Is there a way to select an item immediately after reloadData ?

Answer 1

Along the lines of this answer I found that calling layoutIfNeeded after reloadData seemed to effectively 'flush' the reload before I do other things to the collectionView:

[self.collectionView reloadData];
[self.collectionView layoutIfNeeded];
...

On the page I found this solution, some commenters indicated it didn't work for them on iOS 9, but it's been fine for me so your mileage may vary.

Answer 2

I'm handling selection of cells in collectionView: cellForItemAtIndexPath: . The problem I found was that if the cell didn't exist, simply calling selectItemAtIndexPath: animated: scrollPosition: wouldn't actually select the item.

Instead you have to do:

cell.selected = YES;
[m_collectionView selectItemAtIndexPath:indexPath animated:NO scrollPosition:UICollectionViewScrollPositionNone];

Answer 3

Don't use reloadData

Use - (void)performBatchUpdates:(void (^)(void))updates completion:(void (^)(BOOL finished))completion instead. The completion block is executed after animations for cell insertion/deletion etc. have completed. You can put the call to reloadData in the (void (^)(void))updates block

Answer 4

Apple says:

You should not call this method in the middle of animation blocks where items are being inserted or deleted. Insertions and deletions automatically cause the table's data to be updated appropriately.

In fact, you should not call this method in the middle of any animation (including UICollectionView in the scrolling).

So, you can use:

[self.collectionView setContentOffset:CGPointZero animated:NO];
[self.collectionView performSelectorOnMainThread:@selector(reloadData) withObject:nil waitUntilDone:NO];

or mark sure not any animation, and then call reloadData ; or

[self.collectionView performBatchUpdates:^{
//insert, delete, reload, or move operations
} completion:nil];

Hope this is helpful to you.

Answer 5

The Swift way:

let selected = collectionView.indexPathsForSelectedItems()
collectionView.performBatchUpdates({ [weak self] in
    self?.collectionView.reloadSections(NSIndexSet(index: 0))
    }) { completed -> Void in
        selected?.forEach { [weak self] indexPath in
            self?.collectionView.selectItemAtIndexPath(indexPath, animated: false, scrollPosition: [])
        }
}

Answer 6

Make sure you're calling reloadData on the main thread. That could be the cause for the delay in your cell updates.

Answer 7

I handled it on the willDisplayCell colelctionView delegate. The idea: A temp variable is needed to specify the initial scrolling has performed already or not (scrollIsRequired). When the last visible cell will display, than we can scroll to the required cell and set this variable to avoid scrolling again.

- (void)collectionView:(UICollectionView *)collectionView willDisplayCell:(UICollectionViewCell *)cell forItemAtIndexPath:(NSIndexPath *)indexPath{

    //Perform any configuration

    if (CGRectGetMaxX(collectionView.frame) <= CGRectGetMaxX(cell.frame)) {
        // Last visible cell
        if (self.scrollIsRequired) {
            [self.collectionView selectItemAtIndexPath:[NSIndexPath indexPathForItem:self.initiallySelectedRepresentativeVerse inSection:0] animated:YES scrollPosition:UICollectionViewScrollPositionLeft];
            self.scrollIsRequired = NO;
        }
    }
}

It has worked for me like a charm.

Answer 8

This is what worked for me:

I kept a reference of the selected index path and overide the reloadData function:

override func reloadData() {
    super.reloadData()
    self.selectItem(at: self.selectedIndexPath, animated: false, scrollPosition: UICollectionViewScrollPosition())
}

I tried doing it using indexPathForSelectedItems, but it was creating an infinite loop on collection view load.

Answer 9

创建一个执行选择的方法，并在调用reload后使用performSelector调用它;

[self performSelector:@selector(selectIt) withObject:self afterDelay:0.1];