UIScrollView实现重用性能优化

#import "YHTestContentView.h"

#define CARDVIEWS 5  // 所有的卡片数量

@interface YHTestContentView ()<UIScrollViewDelegate>

@property (nonatomic, strong) UIScrollView *testView;

/**  保存可见的视图*/
@property (nonatomic, strong) NSMutableSet *visibleViews;

/** 保存可重用的视图*/
@property (nonatomic, strong) NSMutableSet *reusedViews;

@end

@implementation YHTestContentView

- (instancetype)init{
    if (self = [super init]) {
        [self initView];
    }
    return self;
}
- (void)initView{
    
    UIScrollView *testView = [[UIScrollView alloc] init];
    self.testView = testView;
    testView.showsHorizontalScrollIndicator = NO;
    testView.clipsToBounds = NO;
    testView.pagingEnabled = YES;
    testView.frame = CGRectMake(0, 0, 256.0, 143.0);
    testView.delegate = self;
    [self addSubview:testView];
    // 刚开始进来时只需显示需要显示的卡片数量
    NSInteger needShowCards = [UIScreen mainScreen].bounds.size.width / 270 + 1;
    
    for (int i = 0; i<CARDVIEWS; i++) {
        
        if (i < needShowCards) {
            UIView *view = [[UIView alloc] init];
            view.tag = i;
            view.backgroundColor = [UIColor colorWithRed:arc4random_uniform(255.0)/255.0 green:arc4random_uniform(255.0)/255.0 blue:arc4random_uniform(255.0)/255.0 alpha:1.0];
            [self.testView addSubview:view];
            view.frame = CGRectMake(i*(256.0 + 14.0), 0, 256, 143.0);
            
            [self.visibleViews addObject:view];
        }
    }
    
    CGFloat offsetX = [UIScreen mainScreen].bounds.size.width - 256.0;
    self.testView.contentSize = CGSizeMake(CARDVIEWS*(256.0 + 14.0) - offsetX, 143.0);
}

- (UIView *)hitTest:(CGPoint)point withEvent:(UIEvent *)event{
    
    return self.testView;
}

- (void)scrollViewDidScroll:(UIScrollView *)scrollView{
    // 可视区域
    CGRect visibleBounds = scrollView.bounds;
    CGFloat minx = CGRectGetMinX(visibleBounds);
    CGFloat maxX = CGRectGetMaxX(visibleBounds);
//    CGFloat width = CGRectGetWidth(visibleBounds);
    
    NSInteger firstIndex = (NSInteger)floorf(minx/270.0);
    NSInteger lastIndex = (NSInteger)floorf(maxX/270.0);
    
//    firstIndex--;
    // 这里做++是考虑到这里实现分页的方式可能会让用户看到重用的过程，所以往后扩容
    lastIndex++;
    
    // 处理越界的情况
    if (firstIndex < 0) {
        firstIndex = 0;
    }
    
    if (lastIndex >= CARDVIEWS) {
        lastIndex = CARDVIEWS - 1;
    }
    
    // 回收不再显示的view
    NSInteger imageViewIndex = 0;
    for (UIImageView *imageView in self.visibleViews) {
        imageViewIndex = imageView.tag;
        // 不在显示范围内,删除不在显示范围内的view
        if (imageViewIndex < firstIndex || imageViewIndex > lastIndex) {
            [self.reusedViews addObject:imageView];
            [imageView removeFromSuperview];
        }
    }
    
    // 把集合self.visibleViews中和self.reusedViews集合中相同的元素删除
    [self.visibleViews minusSet:self.reusedViews];
    
    // 是否需要显示新的视图
    for (NSInteger index = firstIndex; index <= lastIndex; index++) {
       // isShow表示index索引对应的view是否已经在界面上显示，因为此时self.visibleViews记录的都是已经在界面上显示的view
        BOOL isShow = NO;
        for (UIView *view in self.visibleViews) {
            if (view.tag == index) {
                isShow = YES;
            }
        }
        
        if (!isShow) {
            // isShow = NO，表示当前需要在界面上显示的view却没有显示时，这时候就要去显示
            [self showViewAtIndex:index];
        }
    }
}

#pragma mark - 显示界面上显示的view ----
- (void)showViewAtIndex:(NSInteger)index {
    // 先从缓存池取
    UIView *view = [self.reusedViews anyObject];
    if (view) {
        [self.reusedViews removeObject:view];
    } else {
        // 如果不存在再创建
        view = [[UIView alloc] init];
    }
    view.backgroundColor = [UIColor colorWithRed:arc4random_uniform(255.0)/255.0 green:arc4random_uniform(255.0)/255.0 blue:arc4random_uniform(255.0)/255.0 alpha:1.0];
    CGRect viewFrame = CGRectMake(270 *index, 0, 256, 143.0);
    view.tag = index;
    view.frame = viewFrame;

    [self.visibleViews addObject:view];
    [self.testView addSubview:view];
}
#pragma mark - 界面上显示的所有视图 ---
- (NSMutableSet *)visibleViews {
    if (_visibleViews == nil) {
        _visibleViews = [[NSMutableSet alloc] init];
    }
    return _visibleViews;
}
#pragma mark - 重用池 ----
- (NSMutableSet *)reusedViews {
    if (_reusedViews == nil) {
        _reusedViews = [[NSMutableSet alloc] init];
    }
    return _reusedViews;
}

@end