hpique hpique - 2 years ago 297
iOS Question

Where to highlight UICollectionViewCell: delegate or cell?

According to the Collection View Programming Guide one should handle the visual state of the cell highlights in the

. Like this:

- (void)collectionView:(PSUICollectionView *)collectionView didHighlightItemAtIndexPath:(NSIndexPath *)indexPath
MYCollectionViewCell *cell = (MYCollectionViewCell*)[collectionView cellForItemAtIndexPath:indexPath];
[cell highlight];

- (void)collectionView:(UICollectionView *)collectionView didUnhighlightItemAtIndexPath:(NSIndexPath *)indexPath
MYCollectionViewCell *cell = (MYCollectionViewCell*)[collectionView cellForItemAtIndexPath:indexPath];
[cell unhighlight];

What I don't like about this approach is that it adds logic to the delegate that is very specific to the cell. In fact,
manages its highlighted state independently, via the

Wouldn't overriding
be a cleaner solution, then?

- (void)setHighlighted:(BOOL)highlighted
[super setHighlighted:highlighted];
if (highlighted) {
[self highlight];
} else {
[self unhighlight];

Are there any disadvantages to this approach instead of the delegate approach?

Answer Source

As the documentation says, you can rely on highlighted property to be changed while the cell is highlighted. For example the following code will make the cell red when highlighted (not its subviews though):

- (void)setHighlighted:(BOOL)highlighted {
    [super setHighlighted:highlighted];
    [self setNeedsDisplay];

- (void)drawRect:(CGRect)rect {
    [super drawRect:rect];

    if (self.highlighted) {
        CGContextRef context = UIGraphicsGetCurrentContext();
        CGContextSetRGBFillColor(context, 1, 0, 0, 1);
        CGContextFillRect(context, self.bounds);

And if you add something like this the background will become purple (red + opaque blue):

- (void)collectionView:(UICollectionView *)colView didHighlightItemAtIndexPath:(NSIndexPath *)indexPath {
    UICollectionViewCell *cell = [colView cellForItemAtIndexPath:indexPath];
    cell.contentView.backgroundColor = [UIColor colorWithRed:0 green:0 blue:1 alpha:0.5];

- (void)collectionView:(UICollectionView *)colView didUnhighlightItemAtIndexPath:(NSIndexPath *)indexPath {
    UICollectionViewCell *cell = [colView cellForItemAtIndexPath:indexPath];
    cell.contentView.backgroundColor = nil;

So you can use both together (not necessarily both changing the cell appearance). The difference is that with delegate methods you also have indexPath. It might be used to create multi-selection (you will use this methods together with selection delegate methods), to show some preview while the cell is highlighted, to show some animation with other views... There's quite a few appliance for this delegate methods in my opinion.

As a conclusion, I would leave the cell appearance to be handled by the cell itself and use delegate methods to let controller make something cool in the same time.

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