Invalid update: invalid number of items on UICollectionView

Question

I am stumped on this. Here is my scenario. In my Appdelegate, I am creating

1. An instance of a view controller that will be presented modally to collect two pi

Answer 1

1. As the collectionView reload data asynchronously, when you call [collectionView reloadData] method, the dataSource may be reloaded after a while. you can call [collectionView layoutIfNeeded] to force the collectionView reload at once.
2. And when you call [collectionView insertItemsAtIndexPaths:@[indexPaths...]] upon [collectionView reloadData], sometimes the number Of items maybe not equal, you can call [collectionView numberOfItemsInSection:] as a trick before call [collectionView insertItemsAtIndexPaths:@[indexPaths...]].
3. when the dataSource is empty, the insertion will crash, you should call [collectionView reloadData] instead of [collectionView numberOfItemsInSection:]

As the above, to avoid this issue, you can do this:

1. when reload collectionView:

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

2. when invoke the batch updates:

    [self.collectionView performBatchUpdates:^{
        ...
        [self.dataSource addDataSource:moreDatas];

        NSMutableArray *addIndexPaths;
        ...

        if (self.dataSource.count == 1 ||
            [self.collectionView numberOfItemsInSection:0] == self.dataSource.count){
            [self.collectionView reloadData];
        }else{
            [self.collectionView insertItemsAtIndexPaths:addIndexPaths];
        }
    } completion:^(BOOL finished) {}];