無限滾動

在工作中有時候我們會遇到無限滾動，比如相冊，廣告等，今天就用UICollectionView來實現無限滾動，話不多說，都在代碼中了！

#define ID @"photo"
#define KMaxSections 100

@interface PhotoViewController () <UICollectionViewDataSource, UICollectionViewDelegate>
@property (weak, nonatomic) IBOutlet UICollectionView *collectionView;
@property (nonatomic, weak) IBOutlet UIPageControl *pageContol;
@property (nonatomic, strong) NSArray *photoes;
@property (nonatomic, strong) NSTimer *timer;
@end

@implementation PhotoViewController


- (NSArray *)photos
{

if (_photoes == nil) {
        
        NSMutableArray * tmpArray = [NSMutableArray array];
        
        for (NSInteger i = 0; i < 20; i ++) {
            
            UIImage * image = [UIImage imageNamed:[NSString stringWithFormat:@"%ld.jpg",i + 1]];
        
            [tmpArray addObject:image];
        }
        
        _photoes = tmpArray;
    }
    
    return _photoes;

}

- (void)viewDidLoad
{
    [super viewDidLoad];
    
    // 註冊cell
    [self.collectionView registerNib:[UINib nibWithNibName:@"photo" bundle:nil] forCellWithReuseIdentifier:ID];
    
    // 默認顯示最中間的那組
    [self.collectionView scrollToItemAtIndexPath:[NSIndexPath indexPathForItem:0 inSection:KMaxSections/2] atScrollPosition:UICollectionViewScrollPositionLeft animated:NO];
    
    // 添加定時器
    [self addTimer];
}

/**
 *  添加定時器
 */
- (void)addTimer
{
    NSTimer *timer = [NSTimer scheduledTimerWithTimeInterval:2.0 target:self selector:@selector(nextPage) userInfo:nil repeats:YES];
    [[NSRunLoop mainRunLoop] addTimer:timer forMode:NSRunLoopCommonModes];
    self.timer = timer;
}

/**
 *  移除定時器
 */
- (void)removeTimer
{
    // 停止定時器
    [self.timer invalidate];
    self.timer = nil;
}

- (NSIndexPath *)resetIndexPath
{
    // 當前正在展示的位置
    NSIndexPath *currentIndexPath = [[self.collectionView indexPathsForVisibleItems] lastObject];
    // 馬上顯示回最中間那組的數據
    NSIndexPath *currentIndexPathReset = [NSIndexPath indexPathForItem:currentIndexPath.item inSection:KMaxSections/2];

    [self.collectionView scrollToItemAtIndexPath:currentIndexPathReset atScrollPosition:UICollectionViewScrollPositionLeft animated:NO];
    return currentIndexPathReset;
}

/**
 *  下一頁
 */
- (void)nextPage
{
    // 1.馬上顯示回最中間那組的數據
    NSIndexPath *currentIndexPathReset = [self resetIndexPath];
    
    // 2.計算出下一個需要展示的位置
    NSInteger nextItem = currentIndexPathReset.item + 1;
    NSInteger nextSection = currentIndexPathReset.section;
    if (nextItem == self.photoes.count) {
        nextItem = 0;
        nextSection++;
    }
    NSIndexPath *nextIndexPath = [NSIndexPath indexPathForItem:nextItem inSection:nextSection];
    
    // 3.通過動畫滾動到下一個位置
    [self.collectionView scrollToItemAtIndexPath:nextIndexPath atScrollPosition:UICollectionViewScrollPositionLeft animated:YES];
}

#pragma mark - UICollectionViewDataSource
- (NSInteger)collectionView:(UICollectionView *)collectionView numberOfItemsInSection:(NSInteger)section
{
    return self.photoes.count;
}

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

- (UICollectionViewCell *)collectionView:(UICollectionView *)collectionView cellForItemAtIndexPath:(NSIndexPath *)indexPath
{
    PhotoCell *cell = [collectionView dequeueReusableCellWithReuseIdentifier:ID forIndexPath:indexPath];
    
    cell.photo = self.photoes[indexPath.item];
    
    return cell;
}

#pragma mark  - UICollectionViewDelegate
/**
 *  當用戶即將開始拖拽的時候就調用
 */
- (void)scrollViewWillBeginDragging:(UIScrollView *)scrollView
{
    [self removeTimer];
}

/**
 *  當用戶停止拖拽的時候就調用
 */
- (void)scrollViewDidEndDragging:(UIScrollView *)scrollView willDecelerate:(BOOL)decelerate
{
    [self addTimer];
}

- (void)scrollViewDidScroll:(UIScrollView *)scrollView
{
    int page = (int)(scrollView.contentOffset.x / scrollView.bounds.size.width + 0.5) % self.newses.count;
    self.pageContol.currentPage = page;
}