优化图片加载方法

图片加载方式一：imageWithContentsOfFile利用这种方式加载图片是很耗内存的。因为它是实时的加载图片，当加载的图片较多时，且是在主线程中，每一帧加载相同大小的图片，会造成卡顿的现象

- (void)viewDidLoad {
    [super viewDidLoad];
    // Do any additional setup after loading the view, typically from a nib.
    self.view.backgroundColor = [UIColor lightGrayColor];
//    self.layerView = [[UIView alloc] initWithFrame:[UIScreen mainScreen].bounds];
//    [self.view addSubview:self.layerView];
   self.imagePaths= [[NSBundle mainBundle] pathsForResourcesOfType:@".png" inDirectory:@""];
    [self.collectionView registerClass:[UICollectionViewCell class] forCellWithReuseIdentifier:@"cell"];
}

-(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];
    }
    //set image
    NSString *imagePath = self.imagePaths[indexPath.row];
    imageView.image = [UIImage imageWithContentsOfFile:imagePath];
    return cell;
}

第二种方式：开启一个线程，将耗性能的代码写在一个线程里，在主线程中加载图片。解决图片卡顿，造成图片延迟的加载。但对于图片本身的加载来说，并没有得到解决

 //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];
    }
    //tag cell with index and clear current image
    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;

第三种方式：ImageIO框架，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

- (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];
    }
    //tag cell with index and clear current image
    cell.tag = indexPath.row;
    imageView.image = nil;
    //switch to background thread
    dispatch_async(dispatch_get_global_queue(DISPATCH_QUEUE_PRIORITY_LOW, 0), ^{
        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);
        //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;
}

第四种方式：实现图片解压，则把整张图片绘制到CGContext可能是最佳的方式了。尽管你可能认为多余的绘制相较别的解压技术而言性能不是很高，但是新创建的图片（在特定的设备上做过优化）可能比原始图片绘制的更快。

同样，如果想显示图片到比原始尺寸小的容器中，那么一次性在后台线程重新绘制到正确的尺寸会比每次显示的时候都做缩放会更有效（尽管在这个例子中我们加载的图片呈现正确的尺寸，所以不需要多余的优化）

 //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;
            }

第五种方式：CATiledLayer可以用来异步加载和显示大型图片，而不阻塞用户输入

- (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];
    }
    //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();
}