将scrollOffset保存到didEndDisplayingCell中的UITableView数据源，而不是正确出列

Question

I've got a UITableView with a custom cell defined, and in each custom cell, is a UICollectionView. I'm populating the data from a top-level array of data objects, with each containing an inner array of data objects for the collection view. In JSON, this looks something like:

[
    {
        "collectionViewData": [
            {
                "name": "subobject1"
            },
            {
                "name": "subobject2"
            },
            {
                "name": "subobject3"
            }
        ],
        "name": "object1"
    },
    {
        "collectionViewData": [
            {
                "name": "subobject1"
            },
            {
                "name": "subobject2"
            }
        ],
        "name": "object2"
    },
    {
        "collectionViewData": [
            {
                "name": "subobject1"
            },
            {
                "name": "subobject2"
            },
            {
                "name": "subobject3"
            },
            {
                "name": "subobject4"
            }
        ],
        "name": "object3"
    }
]

This works fine, and behaves as expected. What I'm trying to do now is save the contentOffset on didEndDisplayingCell into my datasource so that the next time the tableview cell is redrawn, the scroll location is restored.

This seems to work just fine - except something is goofing in the dequeuing mechanism because every other cell is loading the previously dequeued cell's offset - eg:

• 0 = offset of {170, 0}
• 1 = offset of {0, 0}
• 2 then loads with 0's offset of {170, 0}

What's going wrong? Why am I seeing this behavior and how do I fix it?

- (MyTableViewCell *)tableView:(UITableView *)tableView cellForRowAtIndexPath:(NSIndexPath *)indexPath
{
    static NSString *cellIdentifier = @"Identifier";

    MyTableViewCell *row = (MyTableViewCell *) [tableView dequeueReusableCellWithIdentifier:cellIdentifier];

    if (row == nil)
    {
        row = [[MyTableViewCell alloc] initWithStyle:UITableViewCellStyleDefault reuseIdentifier:cellIdentifier];
    }

    NSInteger idx = (indexPath.row % [MUTABLE_DATASOURCE_ARRAY count]);
    MyCustomCollectionObjectContainer *collectionViewObjectContainer = [MUTABLE_DATASOURCE_ARRAY objectAtIndex:idx];

    [row.collectionView setContentOffset:collectionViewObjectContainer.lastContentOffset animated:NO];
    DDLogDebug(@"ROW %ld %@: cellForRow actual: %@, want: %@", idx, collectionViewObjectContainer.collectionName, NSStringFromCGPoint(row.collectionView.contentOffset), NSStringFromCGPoint(collectionViewObjectContainer.lastContentOffset));
    row.collectionName = collectionViewObjectContainer.collectionName;
    row.collectionViewObjectContainer = collectionViewObjectContainer.innerDataArray;
    row.labelcollectionViewObjectContainer.text = [collectionViewObjectContainer.collectionName uppercaseString];
    [row.labelcollectionViewObjectContainer setNeedsDisplay];

    return row;
}

- (void)tableView:(UITableView *)tableView didEndDisplayingCell:(MyTableViewCell *)row forRowAtIndexPath:(NSIndexPath *)indexPath
{
    NSInteger idx = (indexPath.row % [MUTABLE_DATASOURCE_ARRAY count]);
    MyCustomCollectionObjectContainer *collectionViewObjectContainer = [MUTABLE_DATASOURCE_ARRAY objectAtIndex:idx];
    collectionViewObjectContainer.lastContentOffset = row.collectionView.contentOffset;
    MUTABLE_DATASOURCE_ARRAY[idx] = collectionViewObjectContainer;
    DDLogDebug(@"ROW %ld %@: didEndDisplayingCell set: %@", idx, collectionViewObjectContainer.collectionName, NSStringFromCGPoint(collectionViewObjectContainer.lastContentOffset));
}

Update: In this log output, sometimes the "actual" is wrong, and the "want" is correct, even though in the previous line, I'm clearly setting the content offset. Is there a quick and dirty way to force the content offset to take?

[row.collectionView setContentOffset:collectionViewObjectContainer.lastContentOffset animated:NO];
DDLogDebug(@"ROW %ld %@: cellForRow actual: %@, want: %@", idx, collectionViewObjectContainer.collectionName, NSStringFromCGPoint(row.collectionView.contentOffset), NSStringFromCGPoint(collectionViewObjectContainer.lastContentOffset));

Further to this, I forgot (slap💥!) that it wasn't working at all in the first place, so I overrode setContentOffset in my collection view subclass:

- (void)setContentOffset:(CGPoint)contentOffset animated:(BOOL)animated
{

}

Doh! That doesn't do ANYTHING. But if I remove it, now I'm plagued with my original problem in that when the new table view cell is initialized, the inner collection view does NOT scroll to the desired content offset even though the logging correctly logs out the content offset each time now. Maybe I need to call something other than setContentOffset altogether?

Answer 1

Super messed up, but it seems to work just fine when I put a 0.1s delay before calling setContentOffset :

dispatch_after(dispatch_time(DISPATCH_TIME_NOW, (int64_t)(0.1 * NSEC_PER_SEC)), dispatch_get_main_queue(), ^{
    [row.collectionView setContentOffset: collectionViewObjectContainer.lastContentOffset animated:NO];
});

Seems like this is more of a hack than a solution, but it is indeed working.

Update:

Tragic end to what could have been a cool problem to solve. Turns out I was setting the content offset in my UITableViewCell subclass's awakeFromNib to CGPointZero. Setting it to my saved content offset fixed it.