ios - UICollectionView:必须使用非零布局参数进行初始化

Question

我通过代码添加了UICollectionView。
现在，应用程序崩溃并显示消息:UICollectionView must be initialized with a non-nil layout parameter。
您有解决办法吗？
CollectionCell 是 UICollectionViewCell 的自定义类。

@property (nonatomic, strong) UICollectionView* collectionView;

- (void)viewDidLoad
{
    [super viewDidLoad];

    self.collectionView = [[UICollectionView alloc]init];
    [self.collectionView registerClass:[CollectionCell class] forCellWithReuseIdentifier:@"cell"];
    UICollectionViewFlowLayout* flowLayout = [[UICollectionViewFlowLayout alloc]init];
    flowLayout.itemSize = CGSizeMake(100, 100);
    [flowLayout setScrollDirection:UICollectionViewScrollDirectionHorizontal];
    [self.collectionView setCollectionViewLayout:flowLayout];

    self.collectionView.frame = CGRectMake(0, 60, 320, 500);
    self.collectionView.backgroundColor = [UIColor whiteColor];
    self.collectionView.delegate = self;
    self.collectionView.dataSource = self;
    [self.view addSubview:self.eventCollection];
}

- (NSInteger)numberOfSectionsInCollectionView:(UICollectionView *)collectionView
{
    return 1;
}

- (NSInteger)collectionView:(UICollectionView *)collectionView numberOfItemsInSection:(NSInteger)section
{
    return 20;
}

- (UICollectionViewCell *)collectionView:(UICollectionView *)collectionView cellForItemAtIndexPath:(NSIndexPath *)indexPath
{
    CollectionCell* cell = [self.eventCollection dequeueReusableCellWithReuseIdentifier:@"cell" forIndexPath:indexPath];
    cell.label.text = [NSString stringWithFormat:@"%d", indexPath.item];
    return cell;
}

Answer 1

崩溃非常明确地告诉您出了什么问题:

UICollectionView must be initialized with a non-nil layout parameter.

如果你勾选the documentation for UICollectionView ，你会发现唯一的初始化器是 initWithFrame:collectionViewLayout:。此外，在该初始化程序的参数中，您将看到:

frame

The frame rectangle for the collection view, measured in points. The origin of the frame is relative to the superview in which you plan to add it. This frame is passed to the superclass during initialization.

layout

The layout object to use for organizing items. The collection view stores a strong reference to the specified object. Must not be nil.

我已将重要部分加粗。您必须使用 initWithFrame:collectionViewLayout: 来初始化您的 UICollectionView，并且您必须向它传递一个非零的 UICollectionViewLayout 对象。

---

然后，解决此问题的一种方法是简单地更改您执行的初始化顺序:

UICollectionViewFlowLayout* flowLayout = [[UICollectionViewFlowLayout alloc] init];
flowLayout.itemSize = CGSizeMake(100, 100);
[flowLayout setScrollDirection:UICollectionViewScrollDirectionHorizontal];
self.collectionView = [[UICollectionView alloc] initWithFrame:self.view.frame collectionViewLayout:flowLayout];
[self.collectionView registerClass:[UICollectionViewCell class] forCellWithReuseIdentifier:@"cell"];

请注意，在上面的示例中，我假设您要使用 self.view.frame 作为 self.collectionView 的框架。如果不是这种情况，请插入您想要的任何框架。