CALayer addSublayer在didSelectItemAtIndexPath中不起作用

Question

我想实现在CollectionView中单击单元格时可以变成红色的效果，所以我使用了CAlayer，但它不起作用。当我使用target-action在单元内部实现时，它可以完美地工作。代码如下：

- (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

您需要使用单元的contentView来完成此操作。此外，请记住，单元格是缓存，因此您不能使用存储在单元格中的状态。你得把它放在别的地方。并且您不应该使用didSelectItemAtIndexPath来更新UI。而是更改那里的状态，然后请求并更新相关单元。

这里有一个很好的例子来说明。在此示例中，您可以选择多个单元格，它们都将具有红色背景。单元格的选择状态存储在控制器的字典中。如果你只想在给定的时刻选择一个单元格，你可以很容易地改变这一点。然后，您还需要更新变为未选中的单元格，但这是另一个示例。

#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