封装了一个居左的iOS轮播视图

效果图

UI原理

设置ScrollView 居左，展示范围超出scrollview，并且自定义重用机制，进入屏幕范围，就添加视图，出去屏幕范围，就移除

核心代码

//
//  LBPointLeftBannerView.m
//  LBPointLeftBannerView
//
//  Created by liubo on 2021/8/29.
//

#import "LBPointLeftBannerView.h"

@interface LBPointLeftBannerView ()<UIScrollViewDelegate>

@property (nonatomic, strong, readwrite) LBPointLeftBannerViewBaseCell *currentCell;

@property (nonatomic, assign, readwrite) NSInteger currentSelectIndex;

// 实际的个数
@property (nonatomic, assign) NSInteger realCount;

// 显示的个数
@property (nonatomic, assign) NSInteger showCount;

// 定时器
@property (nonatomic, weak) NSTimer     *timer;
@property (nonatomic, assign) NSInteger timerIndex;

// 当前显示的cell大小
@property (nonatomic, assign) CGSize cellSize;

@property (nonatomic, strong) NSMutableDictionary *viewClsDict;
@property (nonatomic, strong) NSMutableArray *visibleCells;
@property (nonatomic, strong) NSMutableArray *reusableCells;
@property (nonatomic, assign) NSRange        visibleRange;

// 处理xib加载时导致的尺寸不准确问题
@property (nonatomic, assign) CGSize        originSize;

///右边间距
@property (nonatomic, assign) CGFloat rightMargin;

@end

@implementation LBPointLeftBannerView

#pragma mark - Life Cycle
- (instancetype)initWithFrame:(CGRect)frame {
    if (self = [super initWithFrame:frame]) {
        [self initialization];
    }
    return self;
}

- (instancetype)initWithCoder:(NSCoder *)aDecoder {
    if (self = [super initWithCoder:aDecoder]) {
        [self initialization];
    }
    return self;
}

- (void)layoutSubviews {
    [super layoutSubviews];
    
    if (CGSizeEqualToSize(self.originSize, CGSizeZero)) return;
    
    // 解决xib加载时导致的布局错误问题
    if (!CGSizeEqualToSize(self.bounds.size, self.originSize)) {
        [self updateScrollViewAndCellSize];
    }
}

// 解决当父视图释放时，当前视图因为NSTimer强引用而导致的不能释放
- (void)willMoveToSuperview:(UIView *)newSuperview {
    if (!newSuperview) {
        [self stopTimer];
    }
}

- (void)dealloc {
    [self stopTimer];
    self.scrollView.delegate = nil;
}

// 重新此方法是为了解决当cell超出UIScrollView时不能点击的问题
- (UIView *)hitTest:(CGPoint)point withEvent:(UIEvent *)event {
    if ([self pointInside:point withEvent:event]) {
        // 判断点击的点是否在cell上
        for (UIView *cell in self.scrollView.subviews) {
            // 将cell的frame转换到当前视图上
            CGRect convertFrame = CGRectZero;
            convertFrame.size = cell.frame.size;
            
            convertFrame.origin.x = cell.frame.origin.x + self.scrollView.frame.origin.x - self.scrollView.contentOffset.x;
            convertFrame.origin.y = self.scrollView.frame.origin.y + cell.frame.origin.y;
            
            // 判断点击的点是否在cell上
            if (CGRectContainsPoint(convertFrame, point)) {
                // 修复cell上添加其他点击事件无效的bug
                UIView *view = [super hitTest:point withEvent:event];
                if (view == self || view == cell || view == self.scrollView) return cell;
                return view;
            }
        }
        // 判断点击的点是否在UIScrollView上
        CGPoint newPoint = CGPointZero;
        newPoint.x = point.x - self.scrollView.frame.origin.x + self.scrollView.contentOffset.x;
        newPoint.y = point.y - self.scrollView.frame.origin.y + self.scrollView.contentOffset.y;
        if ([self.scrollView pointInside:newPoint withEvent:event]) {
            return [self.scrollView hitTest:newPoint withEvent:event];
        }
        // 系统处理
        return [super hitTest:point withEvent:event];
    }
    return nil;
}

#pragma mark - Public Methods
- (void)reloadData {
    // 移除所有cell
    [self.scrollView.subviews makeObjectsPerformSelector:@selector(removeFromSuperview)];
    
    // 停止定时器
    [self stopTimer];
    
    // 加载数据
    if (self.dataSource && [self.dataSource respondsToSelector:@selector(numberOfCellsInCycleScrollView:)]) {
        // 实际个数
        self.realCount = [self.dataSource numberOfCellsInCycleScrollView:self];
        
        // 展示个数
        if (self.isInfiniteLoop) {
            self.showCount = self.realCount == 1 ? 1 : self.realCount * 3;
        }else {
            self.showCount = self.realCount;
        }
    }
    
    // 清除原始数据
    [self.visibleCells removeAllObjects];
    [self.reusableCells removeAllObjects];
    self.visibleRange = NSMakeRange(0, 0);
    
    //cell数量为1或defaultSelectIndex超过了当前数量
    if(self.defaultSelectIndex >= self.realCount) {
        self.defaultSelectIndex = 0;
    }
    if(self.realCount == 1) {
        self.timerIndex = 0;
    }
    
    for (NSInteger i = 0; i < self.showCount; i++){
        [self.visibleCells addObject:[NSNull null]];
    }
    
    __weak __typeof(self) weakSelf = self;
    [self refreshSizeCompletion:^{
        [weakSelf initialScrollViewAndCellSize];
    }];
}

- (void)refreshSizeCompletion:(void(^)(void))completion {
    if (self.bounds.size.width == 0 || self.bounds.size.height == 0) {
        [self layoutIfNeeded];
        // 此处做延时处理是为了解决使用Masonry布局时导致的view的大小不能及时更新的bug
        dispatch_after(dispatch_time(DISPATCH_TIME_NOW, (int64_t)(0.0f * NSEC_PER_SEC)), dispatch_get_main_queue(), ^{
            if (self.bounds.size.width == 0 || self.bounds.size.height == 0) {
                [self refreshSizeCompletion:completion];
            }else {
                !completion ? : completion();
            }
        });
    }else {
        !completion ? : completion();
    }
}


- (void)registerClass:(nonnull Class)cellClass forViewReuseIdentifier:(NSString *)identifier
{
    [self.viewClsDict setObject:NSStringFromClass(cellClass) forKey:identifier];
}

- (__kindof LBPointLeftBannerViewBaseCell *)dequeueReusableCellWithIdentifier:(NSString *)identifier
{
    
    LBPointLeftBannerViewBaseCell *cell;
    for (LBPointLeftBannerViewBaseCell *cellReusable in self.reusableCells)
    {
        if ([cellReusable.reuseIdentifier isEqualToString:identifier]) {
            cell = cellReusable;
        }
    }
    if (!cell) {
        Class cellCls = NSClassFromString(self.viewClsDict[identifier]);
        cell = [[cellCls alloc] initWithReuseIdentifier:identifier];
        cell.userInteractionEnabled = NO;
    } else {
        [self.reusableCells removeObject:cell];
    }
    return cell;
}


- (void)scrollToCellAtIndex:(NSInteger)index animated:(BOOL)animated {
    if (index < self.realCount) {
        [self stopTimer];
        
        if (self.isInfiniteLoop) {
            self.timerIndex = self.realCount + index;
            [NSObject cancelPreviousPerformRequestsWithTarget:self selector:@selector(startTimer) object:nil];
            [self performSelector:@selector(startTimer) withObject:nil afterDelay:0.5];
        } else {
            self.timerIndex = index;
        }
        
        [self.scrollView setContentOffset:CGPointMake(self.cellSize.width * self.timerIndex, 0) animated:animated];
           
        [self setupCellsWithContentOffset:self.scrollView.contentOffset];
        [self updateVisibleCellAppearance];
    }
}

- (void)adjustCurrentCell {
    if (self.isAutoScroll && self.realCount > 0) {
        self.scrollView.contentOffset = CGPointMake(self.cellSize.width * self.timerIndex, 0);
    }
}

- (void)startTimer {
    if (self.realCount > 1 && self.isAutoScroll) {
        [self stopTimer];
        
//        NSTimer *timer = [NSTimer scheduledTimerWithTimeInterval:self.autoScrollTime target:self selector:@selector(timerUpdate) userInfo:nil repeats:YES];
//        [[NSRunLoop mainRunLoop] addTimer:timer forMode:NSRunLoopCommonModes];
      //  self.timer = timer;
    }
}

- (void)stopTimer {
    if (self.timer) {
        [self.timer invalidate];
        self.timer = nil;
    }
}

#pragma mark Private Methods
- (void)initialization {
    // 初始化默认数据
    self.clipsToBounds      = YES;
    self.isAutoScroll       = YES;
    self.isInfiniteLoop     = YES;
    self.minimumCellAlpha   = 1.0f;
    self.autoScrollTime     = 3.0f;
    
    // 添加scrollView
    [self addSubview:self.scrollView];
}

- (void)initialScrollViewAndCellSize {
    self.originSize = self.bounds.size;
    [self updateScrollViewAndCellSize];
    
    // 默认选中
    if (self.defaultSelectIndex >= 0 && self.defaultSelectIndex < self.realCount) {
        [self handleCellScrollWithIndex:self.defaultSelectIndex];
    }
}

- (void)updateScrollViewAndCellSize {
    if (self.bounds.size.width <= 0 || self.bounds.size.height <= 0) return;
    
    // 设置cell尺寸
    self.cellSize = CGSizeMake(self.bounds.size.width - self.leftMargin, self.bounds.size.height);
    if (self.delegate && [self.delegate respondsToSelector:@selector(sizeForCellInCycleScrollView:)]) {
        self.cellSize = [self.delegate sizeForCellInCycleScrollView:self];
    }
    self.rightMargin = self.bounds.size.width - self.leftMargin - self.cellSize.width;
    // 设置scrollView大小
    self.scrollView.frame = CGRectMake(self.leftMargin, 0, self.cellSize.width + self.itemSpace, self.cellSize.height);
    self.scrollView.contentSize = CGSizeMake((self.cellSize.width + self.itemSpace)* self.showCount,0);
    self.scrollView.contentInset = UIEdgeInsetsMake(0, 0, 0, - self.rightMargin);
   // self.scrollView.center = CGPointMake(CGRectGetMidX(self.bounds), CGRectGetMidY(self.bounds));
    
    if (self.realCount > 1) {
        CGPoint offset = CGPointZero;
        
        if (self.isInfiniteLoop) { // 开启无限轮播
            // 滚动到第二组
            offset = CGPointMake((self.cellSize.width + self.itemSpace)* (self.realCount + self.defaultSelectIndex), 0);
            self.timerIndex = self.realCount + self.defaultSelectIndex;
        }else {
            offset = CGPointMake((self.cellSize.width + self.itemSpace)* self.defaultSelectIndex, 0);
            self.timerIndex = self.defaultSelectIndex;
        }
        
        [self.scrollView setContentOffset:offset animated:NO];
        
        // 自动轮播
        if (self.isAutoScroll) {
            [self startTimer];
        }
    }
    
    // 根据当前scrollView的offset设置显示的cell
    [self setupCellsWithContentOffset:self.scrollView.contentOffset];
    
    // 更新可见cell的显示
    [self updateVisibleCellAppearance];
}

- (void)setupCellsWithContentOffset:(CGPoint)offset {
    if (self.showCount == 0) return;
    if (self.cellSize.width <= 0 || self.cellSize.height <= 0) return;
    //计算_visibleRange
    CGFloat originX = self.scrollView.frame.origin.x == 0 ? 0.01 : self.scrollView.frame.origin.x;
    CGFloat originY = self.scrollView.frame.origin.y == 0 ? 0.01 : self.scrollView.frame.origin.y;
    
    CGPoint startPoint = CGPointMake(offset.x - originX, offset.y - originY);
    
    CGPoint endPoint = CGPointMake(offset.x + self.scrollView.frame.size.width + self.rightMargin, offset.y + self.scrollView.frame.size.height + originY);
    
    NSInteger startIndex = 0;
    for (NSInteger i = 0; i < self.visibleCells.count; i++) {
        if ((self.cellSize.width + self.itemSpace)* (i + 1) > startPoint.x) {
            startIndex = i;
            break;
        }
    }
    
    NSInteger endIndex = startIndex;
    for (NSInteger i = self.visibleCells.count - 1; i >= startIndex; i --) {
        if ((self.cellSize.width + self.itemSpace) * i < endPoint.x) {
            endIndex = i;
            break;
        }
    }
    
    // 可见页分别向前向后扩展一个，提高效率
    startIndex = MAX(startIndex, 0);
    endIndex = MIN(endIndex, self.visibleCells.count - 1);
    self.visibleRange = NSMakeRange(startIndex, endIndex - startIndex + 1);
    
    for (NSInteger i = startIndex; i <= endIndex; i++) {
        [self addCellAtIndex:i];
    }
    
    for (NSInteger i = 0; i < startIndex; i ++) {
        [self removeCellAtIndex:i];
    }
    
    for (NSInteger i = endIndex + 1; i < self.visibleCells.count; i ++) {
        [self removeCellAtIndex:i];
    }
}

- (void)updateVisibleCellAppearance {
    if (self.showCount == 0) return;
    if (self.cellSize.width <= 0 || self.cellSize.height <= 0) return;
    
    CGFloat offsetX = self.scrollView.contentOffset.x;
    
    for (NSInteger i = self.visibleRange.location; i < NSMaxRange(self.visibleRange); i++) {
        LBPointLeftBannerViewBaseCell *cell = self.visibleCells[i];
        CGFloat originX = cell.frame.origin.x;
        CGFloat delta = fabs(originX - offsetX);
        CGRect originCellFrame = (CGRect){
   
   {self.cellSize.width * i, 0}, self.cellSize};
        
        CGFloat leftRightInset = 0;
        CGFloat topBottomInset = 0;
        CGFloat alpha = 0;
        
        if (delta < self.cellSize.width + self.itemSpace) {
            alpha = (delta / self.cellSize.width) * self.minimumCellAlpha;
            CGFloat adjustLeftRightMargin = self.leftMargin == 0 ? 0 : self.leftMargin + 1;
            CGFloat adjustTopBottomMargin = self.topBottomMargin == 0 ? 0 : self.topBottomMargin;
            
            leftRightInset = adjustLeftRightMargin * delta / self.cellSize.width;
            topBottomInset = adjustTopBottomMargin * delta / self.cellSize.width;
            
            NSInteger index = self.realCount == 0 ? 0 : i % self.realCount;
            if (index == self.currentSelectIndex) {
                [self.scrollView bringSubviewToFront:cell];
            }
        } else {
            alpha = self.minimumCellAlpha;
            
            leftRightInset = self.leftMargin;
            topBottomInset = self.topBottomMargin;
            
            [self.scrollView sendSubviewToBack:cell];
        }
        
        if (self.leftMargin == 0 && self.topBottomMargin == 0) {
            cell.frame = originCellFrame;
        }else {
            CGFloat scaleX = (self.cellSize.width - leftRightInset * 2) / self.cellSize.width;
            CGFloat scaleY = (self.cellSize.height - topBottomInset * 2) / self.cellSize.height;
            UIEdgeInsets insets = UIEdgeInsetsMake(topBottomInset, leftRightInset - 0.1, topBottomInset, leftRightInset);
            
//            cell.layer.transform = CATransform3DMakeScale(scaleX, scaleY, 1.0);
//            cell.frame = UIEdgeInsetsInsetRect(originCellFrame, insets);
        }
        
        // 获取当前cell
        if (cell.tag == self.currentSelectIndex) {
            self.currentCell = cell;
        }
    }
}

- (void)addCellAtIndex:(NSInteger)index {
    NSParameterAssert(index >= 0 && index < self.visibleCells.count);
    
    LBPointLeftBannerViewBaseCell *cell = self.visibleCells[index];
    if ((NSObject *)cell == [NSNull null]) {
        cell = [self.dataSource cycleScrollView:self cellForViewAtIndex:index % self.realCount];
        if (cell) {
            [self.visibleCells replaceObjectAtIndex:index withObject:cell];
            
            cell.tag = index % self.realCount;
            
            __weak __typeof(self) weakSelf = self;
            cell.didCellClick = ^(NSInteger index) {
                [weakSelf handleCellSelectWithIndex:index];
            };
            cell.frame = CGRectMake((self.cellSize.width + self.itemSpace) * index, 0, self.cellSize.width, self.cellSize.height);
            if (!cell.superview) {
                [self.scrollView addSubview:cell];
            }
        }
    }
}

- (void)removeCellAtIndex:(NSInteger)index{
    LBPointLeftBannerViewBaseCell *cell = [self.visibleCells objectAtIndex:index];
    if ((NSObject *)cell == [NSNull null]) return;
    
    [self.reusableCells addObject:cell];
    
    if (cell.superview) {
        [cell removeFromSuperview];
    }
    
    [self.visibleCells replaceObjectAtIndex:index withObject:[NSNull null]];
}

- (void)handleCellSelectWithIndex:(NSInteger)index {
    if ([self.delegate respondsToSelector:@selector(cycleScrollView:didSelectCellAtIndex:)]) {
        [self.delegate cycleScrollView:self didSelectCellAtIndex:index];
    }
}

- (void)handleCellScrollWithIndex:(NSInteger)index {
    self.currentSelectIndex = index;
    // 获取当前cell
    for (NSInteger i = self.visibleRange.location; i < NSMaxRange(self.visibleRange); i++) {
        LBPointLeftBannerViewBaseCell *cell = self.visibleCells[i];
        if (cell.tag == index) {
            self.currentCell = cell;
        }
    }
    
    if ([self.delegate respondsToSelector:@selector(cycleScrollView:didScrollCellToIndex:)]) {
        [self.delegate cycleScrollView:self didScrollCellToIndex:index];
    }
}

- (void)timerUpdate {
    self.timerIndex++;
    
    // bug fixed：解决反向滑动停止后，可能出现的自动滚动错乱问题
    if (self.timerIndex > self.realCount * 2) {
        self.timerIndex = self.realCount * 2;
    }
    
    if (!self.isInfiniteLoop) {
        if (self.timerIndex >= self.realCount) {
            self.timerIndex = 0;
        }
    }
    
    [self.scrollView setContentOffset:CGPointMake((self.cellSize.width + self.itemSpace) * self.timerIndex, 0) animated:YES];
}

#pragma mark UIScrollView Delegate
- (void)scrollViewDidScroll:(UIScrollView *)scrollView{
    if (self.realCount == 0) return;
    if (self.cellSize.width <= 0 || self.cellSize.height <= 0) return;
    
    NSInteger index = 0;
    index = (NSInteger)round(self.scrollView.contentOffset.x / (self.cellSize.width + self.itemSpace)) % self.realCount;
    
    if (self.isInfiniteLoop) {
        if (self.realCount > 1) {
            CGFloat horIndex = scrollView.contentOffset.x / (self.cellSize.width + self.itemSpace);
            if (horIndex >= 2 * self.realCount) {
                scrollView.contentOffset = CGPointMake((self.cellSize.width + self.itemSpace) * self.realCount, 0);
                self.timerIndex = self.realCount;
            }
            
            if (horIndex <= (self.realCount - 1)) {
                scrollView.contentOffset = CGPointMake((self.cellSize.width + self.itemSpace) * (2 * self.realCount - 1), 0);
                self.timerIndex = 2 * self.realCount - 1;
            }
        }
        }else {
            index = 0;
        }
    
    [self setupCellsWithContentOffset:scrollView.contentOffset];
    [self updateVisibleCellAppearance];
    
    if (index >= 0 && self.currentSelectIndex != index) {
        [self handleCellScrollWithIndex:index];
    }
    [self handleScrollViewDidScroll:scrollView];
}

- (void)scrollViewWillBeginDragging:(UIScrollView *)scrollView {
    [self stopTimer];
    if ([self.delegate respondsToSelector:@selector(cycleScrollView:willBeginDragging:)]) {
        [self.delegate cycleScrollView:self willBeginDragging:scrollView];
    }
}

// 结束拖拽时调用，decelerate是否有减速
- (void)scrollViewDidEndDragging:(UIScrollView *)scrollView willDecelerate:(BOOL)decelerate {
    if (!decelerate) {
        [self startTimer];
    }
    if ([self.delegate respondsToSelector:@selector(cycleScrollView:didEndDragging:willDecelerate:)]) {
        [self.delegate cycleScrollView:self didEndDragging:scrollView willDecelerate:decelerate];
    }
}

// 结束减速是调用
- (void)scrollViewDidEndDecelerating:(UIScrollView *)scrollView {
    [self startTimer];
    if ([self.delegate respondsToSelector:@selector(cycleScrollView:didEndDecelerating:)]) {
        [self.delegate cycleScrollView:self didEndDecelerating:scrollView];
    }
}

- (void)scrollViewWillEndDragging:(UIScrollView *)scrollView withVelocity:(CGPoint)velocity targetContentOffset:(inout CGPoint *)targetContentOffset {
    if (self.realCount > 1 && self.isAutoScroll) {
        NSInteger index = round(targetContentOffset->x / self.cellSize.width);
        self.timerIndex = index;
    }
}

- (void)scrollViewDidEndScrollingAnimation:(UIScrollView *)scrollView {
    [self updateVisibleCellAppearance];
    
    if ([self.delegate respondsToSelector:@selector(cycleScrollView:didEndScrollingAnimation:)]) {
        [self.delegate cycleScrollView:self didEndScrollingAnimation:scrollView];
    }
}

- (void)handleScrollViewDidScroll:(UIScrollView *)scrollView {
    if ([self.delegate respondsToSelector:@selector(cycleScrollView:didScroll:)]) {
        [self.delegate cycleScrollView:self didScroll:scrollView];
    }
    
    if ([self.delegate respondsToSelector:@selector(cycleScrollView:scrollingFromIndex:toIndex:ratio:)]) {
        BOOL isFirstRevirse = NO; // 是否在第一个位置反向滑动
        CGFloat ratio = 0;   // 滑动百分比
        CGFloat offsetX = scrollView.contentOffset.x;
        CGFloat maxW = self.realCount * scrollView.bounds.size.width;
        
        CGFloat changeOffsetX = self.isInfiniteLoop ? (offsetX - maxW) : offsetX;
        if (changeOffsetX < 0) {
            changeOffsetX = -changeOffsetX;
            isFirstRevirse = YES;
        }
        ratio = (changeOffsetX / scrollView.bounds.size.width);
        if (ratio > self.realCount || ratio < 0) return; // 越界，不作处理
        ratio = MAX(0, MIN(self.realCount, ratio));
        NSInteger baseIndex = floor(ratio);
        if (baseIndex + 1 > self.realCount) {
            // 右边越界了
            baseIndex = 0;
        }
        CGFloat remainderRatio = ratio - baseIndex;
        if (remainderRatio <= 0 || remainderRatio >= 1) return;
        NSInteger toIndex = 0;
        if (isFirstRevirse) {
            baseIndex = self.realCount - 1;
            toIndex = 0;
            remainderRatio = 1 - remainderRatio;
        }else if (baseIndex == self.realCount - 1) {
            toIndex = 0;
        }else {
            toIndex = baseIndex + 1;
        }
        [self.delegate cycleScrollView:self scrollingFromIndex:baseIndex toIndex:toIndex ratio:remainderRatio];
    }
}

#pragma mark - 懒加载
- (UIScrollView *)scrollView {
    if (!_scrollView) {
        _scrollView = [[UIScrollView alloc] initWithFrame:self.bounds];
        _scrollView.scrollsToTop = NO;
        _scrollView.delegate = self;
        _scrollView.pagingEnabled = YES;
        _scrollView.clipsToBounds = NO;
        _scrollView.showsHorizontalScrollIndicator = NO;
        _scrollView.showsVerticalScrollIndicator = NO;
    }
    return _scrollView;
}

- (NSMutableArray *)visibleCells {
    if (!_visibleCells) {
        _visibleCells = [NSMutableArray new];
    }
    return _visibleCells;
}

- (NSMutableDictionary *)viewClsDict
{
    if (!_viewClsDict) {
        _viewClsDict = [NSMutableDictionary dictionary];
    }
    return _viewClsDict;
}

- (NSMutableArray *)reusableCells {
    if (!_reusableCells) {
        _reusableCells = [NSMutableArray new];
    }
    return _reusableCells;
}

@end