Removing empty space, if the section header is hid

问题:

I have two sections in UICollectionView. I want to show a section header in UICollectionView for only 1st section. Not in 0th section.

So I tried to return nil in viewForSupplementaryElementOfKind: method for section == 0 and returns view for the section == 1.

It crashes and shows below error:

Assertion failure in -[UICollectionView _createPreparedSupplementaryViewForElementOfKind:atIndexPath:withLayoutAttributes:applyAttributes]:

Here it is my code for the supplementary view.

- (UICollectionReusableView *)collectionView:(UICollectionView *)collectionView viewForSupplementaryElementOfKind:(NSString *)kind atIndexPath:(NSIndexPath *)indexPath
{
    UICollectionReusableView *sectionHeader = nil;
    if (kind == UICollectionElementKindSectionHeader && indexPath.section == 1) {
        sectionHeader = [collectionView dequeueReusableSupplementaryViewOfKind:kind withReuseIdentifier:@"EventSectionHeader" forIndexPath:indexPath];
        sectionHeader.layer.borderWidth = .5f;
        sectionHeader.layer.borderColor = [UIColor colorWithRed:221.0 / 255.0 green:223.0 / 255.0 blue:220.0 / 255.0 alpha:1.0].CGColor;
    }

    return sectionHeader;
}

I have found that returning nil in viewForSupplementaryElementOfKind: method crashing for others too. Other answers suggesting to remove that method.

But I want to show section header for specific sections only. How to achieve that returning view for only one section? Thanks. Any help would be appreciated.

EDIT:

As @san said, I have updated code to hide the section header. It works. It hides the header. But I am still seeing the empty space in the place of section header. Expected results is there should be no space for section header, if it is hidden.

updated code:

- (UICollectionReusableView *)collectionView:(UICollectionView *)collectionView viewForSupplementaryElementOfKind:(NSString *)kind atIndexPath:(NSIndexPath *)indexPath
{

    UICollectionReusableView *sectionHeader = nil;
    if (kind == UICollectionElementKindSectionHeader) {
        sectionHeader = [collectionView dequeueReusableSupplementaryViewOfKind:kind withReuseIdentifier:@"EventSectionHeader" forIndexPath:indexPath];
        sectionHeader.layer.borderWidth = .5f;
        sectionHeader.layer.borderColor = [UIColor colorWithRed:221.0 / 255.0 green:223.0 / 255.0 blue:220.0 / 255.0 alpha:1.0].CGColor;
        if (indexPath.section == 0) {
            sectionHeader.hidden = YES;

        }else {
            sectionHeader.hidden = NO;
        }
    }

    return sectionHeader;
}

I even tried setting the frame for sectionHeader as @san said. But no luck. same result.

回答1:

At last, I found an answer for my question. I have missed something. Anyway sorry for other fellow users.

I set the header height and width inside the below method till now as @san said.

- (UICollectionReusableView *)collectionView:(UICollectionView *)collectionView viewForSupplementaryElementOfKind:(NSString *)kind atIndexPath:(NSIndexPath *)indexPath

But It is not the correct method to set the frame size of supplementary views. Later I found another method inside the flowLayout, which helps me to set the header and footer sizes.

This really works well for me:

- (CGSize)collectionView:(UICollectionView *)collectionView layout:(UICollectionViewLayout*)collectionViewLayout referenceSizeForHeaderInSection:(NSInteger)section
{
    if (section == 0) {
        return CGSizeZero;
    }else {
        return CGSizeMake(CGRectGetWidth(collectionView.bounds), 135);
    }
}

UPDATE: Since someone questioned about my skill in comments, attaching Apple Documentation link for returning CGSizeZero in above method.

回答2:

The documentation for collectionView:viewForSupplementaryElementOfKind:atIndexPath: states:

This method must always return a valid view object. If you do not want a supplementary view in a particular case, your layout object should not create the attributes for that view. Alternatively, you can hide views by setting the hidden property of the corresponding attributes to YES or set the alpha property of the attributes to 0. To hide header and footer views in a flow layout, you can also set the width and height of those views to 0.

Considering you have already tried setting the height to zero and setting the view to be hidden, you should subclass UICollectionViewFlowLayout and implement layoutAttributesForSupplementaryViewOfKind:atIndexPath:

Check the indexPath (as you already do) and return nil if you don't want any layout attributes for that specific supplementary view.

- (UICollectionViewLayoutAttributes *)layoutAttributesForSupplementaryViewOfKind:(NSString *)kind atIndexPath:(NSIndexPath *)indexPath
{
    if([indexPath section] == 0)
    {
         return nil;
    }

    return [super layoutAttributesForSupplementaryViewOfKind:kind atIndexPath:indexPath];
}

回答3:

Documentation clearly says -

Return Value

A configured supplementary view object. You must not return nil from this method.

So you need follow -

This method must always return a valid view object. If you do not want a supplementary view in a particular case, your layout object should not create the attributes for that view. Alternatively, you can hide views by setting the hidden property of the corresponding attributes to YES or set the alpha property of the attributes to 0. To hide header and footer views in a flow layout, you can also set the width and height of those views to 0.

Coming to your code, below snippet should work for you:

- (UICollectionReusableView *)collectionView:(UICollectionView *)collectionView viewForSupplementaryElementOfKind:(NSString *)kind atIndexPath:(NSIndexPath *)indexPath
{
    UICollectionReusableView *sectionHeader = nil;
    if (kind == UICollectionElementKindSectionHeader) {
        sectionHeader = [collectionView dequeueReusableSupplementaryViewOfKind:kind withReuseIdentifier:@"EventSectionHeader" forIndexPath:indexPath];

        if(indexPath.section == 1)
          {
             sectionHeader.layer.borderWidth = .5f;
             sectionHeader.layer.borderColor = [UIColor colorWithRed:221.0 / 255.0 green:223.0 / 255.0 blue:220.0 / 255.0 alpha:1.0].CGColor;
          }
        else
        {
          sectionHeader.frame = CGRectZero;
          sectionHeader.hidden = YES;
        }
    }

    return sectionHeader;
}

回答4:

You can hide/show your reusable header section by adding UICollectionViewDelegateFlowLayout delegate and using below code

- (CGSize)collectionView:(UICollectionView *)collectionView layout:(UICollectionViewLayout*)collectionViewLayout referenceSizeForHeaderInSection:(NSInteger)section{
if (self.isForSearch) { //---> for hiding
    return CGSizeMake(0,0);
}
else{//---> for showing
    return ((UICollectionViewFlowLayout*)self.collectionChoosePlanView.collectionViewLayout).headerReferenceSize;
}
}

So you can hide/show it