效果图

  背景:使用CollectionView加载11张图片，每张图片大小是800*600,一屏展示。
  分析:在iPhone 5c上，进页面明显有1s以上的延迟;在iPhone 8上，能感觉卡一下再进入。一张图片710KB,11张图片是7.81MB,性能强悍如iPhone8依然感觉到卡。
  一直保持着测卡顿就要兼容性能最差的机器，我使用iPhone5c。

  先贴一份原始代码:

@interface ImageIOViewController ()
@property (weak, nonatomic) IBOutlet UICollectionView *collectionView;
@property (nonatomic, copy) NSArray *imagePaths;
@end

@implementation ImageIOViewController

- (void)viewDidLoad {
    [super viewDidLoad];
    // Do any additional setup after loading the view from its nib.
    self.imagePaths = [[NSBundle mainBundle] pathsForResourcesOfType:@"png" inDirectory:@"Vacation Photos"];
    //register cell class
    [self.collectionView registerClass:[UICollectionViewCell class] forCellWithReuseIdentifier:@"Cell"];
}

- (CGSize)collectionView:(UICollectionView *)collectionView layout:(UICollectionViewLayout *)collectionViewLayout sizeForItemAtIndexPath:(NSIndexPath *)indexPath
{
    return CGSizeMake(CGRectGetWidth(collectionView.frame)/7, CGRectGetWidth(collectionView.frame)/7*4.0/3);
}

- (NSInteger)collectionView:(UICollectionView *)collectionView numberOfItemsInSection:(NSInteger)section
{
    return [self.imagePaths count];
}
- (UICollectionViewCell *)collectionView:(UICollectionView *)collectionView
                  cellForItemAtIndexPath:(NSIndexPath *)indexPath
{
    //dequeue cell
    UICollectionViewCell *cell = [collectionView dequeueReusableCellWithReuseIdentifier:@"Cell" forIndexPath:indexPath];
    //add image view
    const NSInteger imageTag = 99;
    UIImageView *imageView = (UIImageView *)[cell viewWithTag:imageTag];
    if (!imageView) {
        imageView = [[UIImageView alloc] initWithFrame: cell.contentView.bounds];
        imageView.tag = imageTag;
        [cell.contentView addSubview:imageView];
    }
    cell.tag = indexPath.row;
    imageView.image = nil;
    NSString *imagePath = self.imagePaths[indexPath.row];
    imageView.image = [UIImage imageWithContentsOfFile:imagePath];
    return cell;
}
@end

方法1:后台线程加载

- (UICollectionViewCell *)collectionView:(UICollectionView *)collectionView
                  cellForItemAtIndexPath:(NSIndexPath *)indexPath
{
    //dequeue cell
    UICollectionViewCell *cell = [collectionView dequeueReusableCellWithReuseIdentifier:@"Cell" forIndexPath:indexPath];
    //add image view
    const NSInteger imageTag = 99;
    UIImageView *imageView = (UIImageView *)[cell viewWithTag:imageTag];
    if (!imageView) {
        imageView = [[UIImageView alloc] initWithFrame: cell.contentView.bounds];
        imageView.tag = imageTag;
        [cell.contentView addSubview:imageView];
    }
    cell.tag = indexPath.row;
    imageView.image = nil;
    
    //switch to background thread
    dispatch_async(dispatch_get_global_queue(DISPATCH_QUEUE_PRIORITY_LOW, 0), ^{
        //load image
        NSInteger index = indexPath.row;
        NSString *imagePath = self.imagePaths[index];
        UIImage *image = [UIImage imageWithContentsOfFile:imagePath];
        //set image on main thread, but only if index still matches up
        dispatch_async(dispatch_get_main_queue(), ^{
            if (index == cell.tag) {
                imageView.image = image;
            }
        });
    });
    
    return cell;
}

  这里我们需要处理一下显示的图片和想展示的位置要对应，要不然会乱掉。发现效果还不错，卡顿没有了，进页面的时候图片无序的加载。这也是大家都能想到的思路。但有没有其他方案呢？

方法2:延迟解压

  一旦图片文件被加载就必须要进行解码，解码过程是一个相当复杂的任务，需要消耗非常长的时间。解码后的图片将同样使用相当大的内存。
  有三种方法来实现延迟解压:
  1.最简单的方法就是使用 UIImage 的 +imageNamed: 方法避免延时加载。问题在于 +imageNamed: 只对从应用资源束中的图片有效，所以对用户生成的图片内容或者是下载的图片就没法使用了。
  2.另一种立刻加载图片的方法就是把它设置成图层内容，或者是 UIImageView 的 image 属性。不幸的是，这又需要在主线程执行，所以不会对性能有所提升。
  3.第三种方式就是绕过 UIKit ，像下面这样使用ImageIO框架：

NSInteger index = indexPath.row;
NSURL *imageURL = [NSURL fileURLWithPath:self.imagePaths[index]];
NSDictionary *options = @{(__bridge id)kCGImageSourceShouldCache: @YES};
CGImageSourceRef source = CGImageSourceCreateWithURL((__bridge CFURLRef)imageURL, NULL);
CGImageRef imageRef = CGImageSourceCreateImageAtIndex(source, 0,(__bridge CFDictionaryRef)options);
UIImage *image = [UIImage imageWithCGImage:imageRef];
CGImageRelease(imageRef);
CFRelease(source);

这样就可以使用 kCGImageSourceShouldCache 来创建图片，强制图片立刻解压，然后在图片的生命周期保留解压后的版本。

最后一种方式就是使用UIKit加载图片，但是立刻会知道 CGContext 中去。图片必须要在绘制之前解压，所以就强制了解压的及时性。这样的好处在于绘制图片可以再后台线程（例如加载本身）执行，而不会阻塞UI。

- (UICollectionViewCell *)collectionView:(UICollectionView *)collectionView
                  cellForItemAtIndexPath:(NSIndexPath *)indexPath
{
    //dequeue cell
    UICollectionViewCell *cell = [collectionView dequeueReusableCellWithReuseIdentifier:@"Cell" forIndexPath:indexPath];
    //add image view
    const NSInteger imageTag = 99;
    UIImageView *imageView = (UIImageView *)[cell viewWithTag:imageTag];
    if (!imageView) {
        imageView = [[UIImageView alloc] initWithFrame: cell.contentView.bounds];
        imageView.tag = imageTag;
        [cell.contentView addSubview:imageView];
    }
    cell.tag = indexPath.row;
    imageView.image = nil;
    dispatch_async(dispatch_get_global_queue(DISPATCH_QUEUE_PRIORITY_LOW, 0), ^{
        //load image
        NSInteger index = indexPath.row;
        NSString *imagePath = self.imagePaths[index];
        UIImage *image = [UIImage imageWithContentsOfFile:imagePath];
        //redraw image using device context
        UIGraphicsBeginImageContextWithOptions(imageView.bounds.size, YES, 0);
        [image drawInRect:imageView.bounds];
        image = UIGraphicsGetImageFromCurrentImageContext();
        UIGraphicsEndImageContext();
        //set image on main thread, but only if index still matches up
        dispatch_async(dispatch_get_main_queue(), ^{
            if (index == cell.tag) {
                imageView.image = image;
            }
        });
    });
}

  注意不要在子线程访问imageView的属性，否则XCode会给提示，需要绘制的大小我们是可以提前知道的。

方法3 CATiledLayer替代UIImageView

  CATiledLayer 可以用来异步加载和显示大型图片，而不阻塞用户输入。

@interface ImageTiledLayerIOViewController ()<CALayerDelegate>
@property (nonatomic,strong) NSMutableSet<CATiledLayer *> * tiledLayerSet;
@property (weak, nonatomic) IBOutlet UICollectionView *collectionView;
@property (nonatomic, copy) NSArray *imagePaths;
@end

@implementation ImageTiledLayerIOViewController

- (void)viewDidLoad {
    [super viewDidLoad];
    _tiledLayerSet = [NSMutableSet new];
    
    // Do any additional setup after loading the view from its nib.
    self.imagePaths = [[NSBundle mainBundle] pathsForResourcesOfType:@"png" inDirectory:@"Vacation Photos"];
    [self.collectionView registerClass:[UICollectionViewCell class] forCellWithReuseIdentifier:@"Cell"];
}

- (CGSize)collectionView:(UICollectionView *)collectionView layout:(UICollectionViewLayout *)collectionViewLayout sizeForItemAtIndexPath:(NSIndexPath *)indexPath
{
    return CGSizeMake(CGRectGetWidth(collectionView.frame), CGRectGetWidth(collectionView.frame)*4.0/3);
}

- (NSInteger)collectionView:(UICollectionView *)collectionView numberOfItemsInSection:(NSInteger)section
{
    return [self.imagePaths count];
}
- (UICollectionViewCell *)collectionView:(UICollectionView *)collectionView cellForItemAtIndexPath:(NSIndexPath *)indexPath
{
    //dequeue cell
    UICollectionViewCell *cell = [collectionView dequeueReusableCellWithReuseIdentifier:@"Cell" forIndexPath:indexPath];
    //add the tiled layer
    CATiledLayer *tileLayer = [cell.contentView.layer.sublayers lastObject];
    if (!tileLayer) {
        tileLayer = [CATiledLayer layer];
        tileLayer.frame = cell.bounds;
        tileLayer.contentsScale = [UIScreen mainScreen].scale;
        tileLayer.tileSize = CGSizeMake(cell.bounds.size.width * [UIScreen mainScreen].scale, cell.bounds.size.height * [UIScreen mainScreen].scale);
        tileLayer.delegate = self;
        [tileLayer setValue:@(indexPath.row) forKey:@"index"];
        [cell.contentView.layer addSublayer:tileLayer];
        
        [_tiledLayerSet addObject:tileLayer];
    }
    //tag the layer with the correct index and reload
    tileLayer.contents = nil;
    [tileLayer setValue:@(indexPath.row) forKey:@"index"];
    [tileLayer setNeedsDisplay];
    return cell;
}

- (void)drawLayer:(CATiledLayer *)layer inContext:(CGContextRef)ctx
{
    //get image index
    NSInteger index = [[layer valueForKey:@"index"] integerValue];
    //load tile image
    NSString *imagePath = self.imagePaths[index];
    UIImage *tileImage = [UIImage imageWithContentsOfFile:imagePath];
    //calculate image rect
    CGFloat aspectRatio = tileImage.size.height / tileImage.size.width;
    CGRect imageRect = CGRectZero;
    imageRect.size.width = layer.bounds.size.width;
    imageRect.size.height = layer.bounds.size.height * aspectRatio;
    imageRect.origin.y = (layer.bounds.size.height - imageRect.size.height)/2;
    //draw tile
    UIGraphicsPushContext(ctx);
    [tileImage drawInRect:imageRect];
    UIGraphicsPopContext();
}

- (void)dealloc{
    for (CATiledLayer * tiledLayer in _tiledLayerSet.allObjects) {
        [tiledLayer removeFromSuperlayer];
    }
}

  CATiledLayer需要在dealloc中手动异常，否则会产生崩溃。其实后台线程加载、延迟解压或者使用CATiledLayer的方式已经解决的很好了，那我们还有别的方式吗?

方法4 缓存与后台线程的结合

  如果是应用程序资源下的图片用 [UIImage imageNamed:] 足以解决我们的问题，但多数情况下是网络图片。我们可以自定义缓存，当然苹果也为我们提供了一种缓存方案NSCache.
  NSCache 在系统低内存的时候自动丢弃存储的对象NSCache 用来判断何时丢弃对象的算法并没有在文档中给出，但是你可以使用 -setCountLimit: 方法设置缓存大小，以及 -setObject:forKey:cost: 来对每个存储的对象指定消耗的值来提供一些暗示。
  指定消耗数值可以用来指定相对的重建成本。如果对大图指定一个大的消耗值，那么缓存就知道这些物体的存储更加昂贵，于是当有大的性能问题的时候才会丢弃这些物体。你也可以用 -setTotalCostLimit: 方法来指定全体缓存的尺寸。

- (UIImage *)loadImageAtIndex:(NSUInteger)index
{
    //set up cache
    static NSCache *cache = nil;
    if (!cache) {
        cache = [[NSCache alloc] init];
    }
    //if already cached, return immediately
    UIImage *image = [cache objectForKey:@(index)];
    if (image) {
        return [image isKindOfClass:[NSNull class]]? nil: image;
    }
    //set placeholder to avoid reloading image multiple times
    [cache setObject:[NSNull null] forKey:@(index)];
    //switch to background thread
    dispatch_async( dispatch_get_global_queue(DISPATCH_QUEUE_PRIORITY_LOW, 0), ^{
        //load image
        NSString *imagePath = self.imagePaths[index];
        UIImage *image = [UIImage imageWithContentsOfFile:imagePath];
        //redraw image using device context
        UIGraphicsBeginImageContextWithOptions(image.size, YES, 0);
        [image drawAtPoint:CGPointZero];
        image = UIGraphicsGetImageFromCurrentImageContext();
        UIGraphicsEndImageContext();
        //set image for correct image view
        dispatch_async(dispatch_get_main_queue(), ^{ //cache the image
            [cache setObject:image forKey:@(index)];
            //display the image
            NSIndexPath *indexPath = [NSIndexPath indexPathForItem: index inSection:0]; UICollectionViewCell *cell = [self.collectionView cellForItemAtIndexPath:indexPath];
            UIImageView *imageView = [cell.contentView.subviews lastObject];
            imageView.image = image;
        });
    });
    //not loaded yet
    return nil;
}

- (UICollectionViewCell *)collectionView:(UICollectionView *)collectionView
                  cellForItemAtIndexPath:(NSIndexPath *)indexPath
{
//dequeue cell
UICollectionViewCell *cell = [collectionView dequeueReusableCellWithReuseIdentifier:@"Cell" forIndexPath:indexPath];
//add image view
UIImageView *imageView = [cell.contentView.subviews lastObject];
if (!imageView) {
imageView = [[UIImageView alloc] initWithFrame:cell.contentView.bounds];
imageView.contentMode = UIViewContentModeScaleAspectFit;
[cell.contentView addSubview:imageView];
}
//set or load image for this index
imageView.image = [self loadImageAtIndex:indexPath.item];
//preload image for previous and next index
if (indexPath.item < [self.imagePaths count] - 1) {
[self loadImageAtIndex:indexPath.item + 1]; }
if (indexPath.item > 0) {
[self loadImageAtIndex:indexPath.item - 1]; }
return cell;
}

方法5 分辨率交换

视网膜分辨率（根据苹果市场定义）代表了人的肉眼在正常视角距离能够分辨的最小像素尺寸。但是这只能应用于静态像素。当观察一个移动图片时，你的眼睛就会对细节不敏感，于是一个低分辨率的图片和视网膜质量的图片没什么区别了。
  为了做到图片交换，我们需要利用 UIScrollView 的一些实
现 UIScrollViewDelegate 协议的委托方法（和其他类似于 UITableView 和 UICollectionView 基于滚动视图的控件一样）：

- (void)scrollViewDidEndDragging:(UIScrollView *)scrollView willDecelerate:(BOOL)decelerate;
- (void)scrollViewDidEndDecelerating:(UIScrollView *)scrollView;

  你可以使用这几个方法来检测传送器是否停止滚动，然后加载高分辨率的图片。只要高分辨率图片和低分辨率图片尺寸颜色保持一致，你会很难察觉到替换的过程（确保在同一台机器使用相同的图像程序或者脚本生成这些图片）

小结:上图虽然是书中提出的方案，但没有具体实现。实际情况采用大小图，列表仅加载小图，点击放大再展示大图。

方法6 使用RunLoop

  我们可以使用[self performSelector:@selector(loadImage) withObject:nil afterDelay:0 inModes:@[NSDefaultRunLoopMode]],仅在RunLoop休眠时加载图片。可与其他几种方式结合使用。

总结:
1.苹果一直以流畅著称，多数情况下并不需要考虑性能问题，但涉及复杂场景，还是需要优化的。
2.本书多数内容参考<<iOS CoreAnimation>>，有兴趣同学可自行阅读。本文demo包含iOS CoreAnimation中所有案例。
3.第四种其实使用SDWebImage就能很好解决了，不需要我们再来实现。第三种可以说是很强大，如果你图片有1个G，你可以分成几等分，用CATiledLayer加载，效果才是真的好。第五种的缺点需要提供大小图了。
4.有任何问题欢迎留言评论。