CALayer addSublayer does not work in didSelectItemAtIndexPath - objective-c

Question

I want to achieve an effect that can turn to red when a cell is clicked in CollectionView,so I use CAlayer,but it does not work. When I use target-action inside cell to achieve, it can work perfectly.
Here are the codes:
- (void)collectionView:(UICollectionView *)collectionView didSelectItemAtIndexPath:(NSIndexPath *)indexPath
{
videoCell *cell = [collectionView dequeueReusableCellWithReuseIdentifier:#"videoCell" forIndexPath:indexPath];
CALayer *testLayer = [[CALayer layer] init];
testLayer.frame = cell.bounds;
testLayer.backgroundColor = [UIColor redColor].CGColor;
[cell.layer addSublayer:testLayer];
}

Answer 1

You need to use the cell's contentView to accomplish this. Also, remember cells are caches and so you can not use state that you store inside a cell. You have to store it somewhere else. And you should not use didSelectItemAtIndexPath to update the UI. Rather change the state there and then request and update of the relevant cell.
Here is a nice example to illustrate. In this example you can select multiple cells and they all will have a red background. The selection state of a cell is stored inside a dictionary in the controller. You could easily change this if you e.g. only want to have a single cell selected at a given moment in time. Then you need to also updated the cell that became unselected but that is another example.
#import "CollectionViewController.h"
#interface CollectionViewController () < UICollectionViewDelegate, UICollectionViewDataSource >
#property (nonatomic,strong) NSMutableDictionary * selectedCells; // Key is integer row, value is boolean selected
#end
#implementation CollectionViewController
static NSString * const reuseIdentifier = #"Cell";
- (void)viewDidLoad {
[super viewDidLoad];
// Register cell classes
[self.collectionView registerClass:[UICollectionViewCell class] forCellWithReuseIdentifier:reuseIdentifier];
// Do any additional setup after loading the view.
self.collectionView.dataSource = self;
self.collectionView.delegate = self;
// Empty / nothing selected for now
self.selectedCells = NSMutableDictionary.dictionary;
}
#pragma mark <UICollectionViewDataSource>
- (NSInteger)numberOfSectionsInCollectionView:(UICollectionView *)collectionView {
return 1;
}
- (NSInteger)collectionView:(UICollectionView *)collectionView numberOfItemsInSection:(NSInteger)section {
return 5;
}
- (UICollectionViewCell *)collectionView:(UICollectionView *)collectionView cellForItemAtIndexPath:(NSIndexPath *)indexPath {
UICollectionViewCell *cell = [collectionView dequeueReusableCellWithReuseIdentifier:reuseIdentifier forIndexPath:indexPath];
// Configure the cell based on state
if ( [( NSNumber * )[self.selectedCells objectForKey:#( indexPath.row )] boolValue] ) {
cell.contentView.backgroundColor = UIColor.redColor;
} else {
cell.contentView.backgroundColor = UIColor.blueColor;
}
return cell;
}
#pragma mark <UICollectionViewDelegate>
- (void)collectionView:(UICollectionView *)collectionView didSelectItemAtIndexPath:(NSIndexPath *)indexPath {
// Flip selected state
[self.selectedCells setObject:#( ! [( NSNumber * )[self.selectedCells objectForKey:#( indexPath.row )] boolValue] )
forKey:#( indexPath.row )];
// Request an UI update to reflect the updated state
[collectionView reloadItemsAtIndexPaths:#[ indexPath ]];
}
#end