UIScrollView极限优化:两个UIImageView循环利用

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

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

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[self.visibleImageViews addObject:imageView];
[self.scrollView addSubview:imageView];

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UIImageView *imageView = [self.reusedImageViews anyObject];

if (imageView) {
    [self.reusedImageViews removeObject:imageView];
} else {
    imageView = [[UIImageView alloc] init];
    imageView.contentMode = UIViewContentModeScaleAspectFit;
}

// imageView的配置
// ...

[self.visibleImageViews addObject:imageView];
[self.scrollView addSubview:imageView];

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// 回收不再显示的ImageView
NSInteger imageViewIndex = 0;
for (UIImageView *imageView in self.visibleImageViews) {
    imageViewIndex = imageView.tag;
    // 不在显示范围内
    if (imageViewIndex < firstIndex || imageViewIndex > lastIndex) {
        [self.reusedImageViews addObject:imageView];
        [imageView removeFromSuperview];
    }
}

[self.visibleImageViews minusSet:self.reusedImageViews];