iOS异步下载图片实例代码

@interface SQPhotoCell ()
@property (strong, nonatomic) UIImageView *photoView;
//Tag指向当前可见图片的url，可过滤掉已经滑出屏幕的图片的url
@property (strong, nonatomic) NSString *imageViewTag;
@end
-(id)initWithFrame:(CGRect)frame
{
self = [super initWithFrame:frame];
if (self)
{
_photoView = [[UIImageView alloc] initWithFrame:CGRectZero];
_photoView.userInteractionEnabled = YES;
[self.contentView addSubview:_photoView];
_imageViewTag = @"";
//注册下载完成通知
[[NSNotificationCenter defaultCenter] addObserver:self
selector:@selector(downloadCallback:)
name:NOTIFICATION_DOWNLOAD_CALLBACK
object:nil];
}
return self;
}

//通知处理事件
- (void)downloadCallback:(NSNotification *)noti
{
NSDictionary *notiDic = noti.userInfo;
NSString *urlStr = [notiDic objectForKey:@"urlStr"];
UIImage *image = [notiDic objectForKey:@"image"];
if ([self.imageViewTag isEqualToString:urlStr])
{
self.photoView.image = image;
}
}

- (void)setImageWithURL:(NSString *)urlStr placeholder:(UIImage *)placeholder
{
self.imageViewTag = urlStr;
//预设图片，用于清除复用以前可能存在的图片
self.photoView.image = placeholder;
if (urlStr)
{
SQWebImageManager *manager = [SQWebImageManager sharedManager];
[manager downloadImageWithURLString:urlStr];
}
[self setNeedsDisplay];
}

- (void)downloadImageWithURLString:(NSString *)urlStr
{
// 1.判断内存缓存 {urlStr: image}
UIImage *cacheImage = [self.imageCache objectForKey:urlStr];
if (cacheImage != nil)
{
//发出下载完成的通知，并传回urlStr和图片
[self postDownloadCompleteNotification:urlStr withImage:cacheImage];
return;
}
// 2.判断沙盒缓存
NSString *cacheImagePath = [self cacheImagePathWithURLString:urlStr];
cacheImage = [UIImage imageWithContentsOfFile:cacheImagePath];
if (cacheImage != nil)
{
// 从沙盒中读取到了图片,设置到内存缓存中,方便下次可以直接从内存中读取
[self.imageCache setObject:cacheImage forKey:urlStr];
// 返回图片
[self postDownloadCompleteNotification:urlStr withImage:cacheImage];
return;
}
// 3.判断操作缓存,防止图片多次下载 {urlStr: operation}
if (self.operationCache[urlStr] != nil)
{
// 有操作正在下载这张图片
NSLog(@"有操作正在下载这张图片");
return;
}
// 1.定义下载图片操作
SQDownloadOperation *downloadOperation = [SQDownloadOperation downloadOperationWithURLString:urlStr cacheImagePath:cacheImagePath];
// 设置操作下载完成的回调,当 downloadOperation 的 main 方法执行完成的时候回调用
__weak typeof(downloadOperation) weakDownloadOperation = downloadOperation;
downloadOperation.completionBlock = ^() {
// 1. 获取下载完成的图像
UIImage *image = [weakDownloadOperation getDownloadImage];
// 2. 从操作缓冲池中删除操作
[self.operationCache removeObjectForKey:urlStr];
// 3. 判断图像是否为空（缩略图）
if (image != nil)
{
// 设置下载的图片到图片内存缓存中
[self.imageCache setObject:image forKey:urlStr];
// 4. 主线程回调
[[NSOperationQueue mainQueue] addOperationWithBlock:^{
//发出下载完成通告
[self postDownloadCompleteNotification:urlStr withImage:image];
}];
}
else
{
//如果图片为空，返回下载失败时的默认图片
image = [UIImage imageNamed:@"default.jpg"];
// 4. 主线程回调
[[NSOperationQueue mainQueue] addOperationWithBlock:^{
//发出下载完成通告
[self postDownloadCompleteNotification:urlStr withImage:image];
}];
}
};
// 2.将下载图片操作添加到队列中
[self.downloadQueue addOperation:downloadOperation];
// 3.将下载图片操作添加到下载操作缓存中
[self.operationCache setObject:downloadOperation forKey:urlStr];
}
- (void)postDownloadCompleteNotification:(NSString *)urlStr withImage:(UIImage *)image
{
NSDictionary *dic = [NSDictionary dictionaryWithObjectsAndKeys:urlStr, @"urlStr", image, @"image",nil];
[[NSNotificationCenter defaultCenter]postNotificationName:NOTIFICATION_DOWNLOAD_CALLBACK
object:nil
userInfo:dic];
}

- (UICollectionViewCell *)collectionView:(UICollectionView *)collectionView cellForItemAtIndexPath:(NSIndexPath *)indexPath
{
SQPhotoCell *cell = (SQPhotoCell *)[collectionView dequeueReusableCellWithReuseIdentifier:reuseIdentifier
forIndexPath:indexPath];
UIImage *placeholder = [UIImage imageNamed:@"gray.jpg"];
NSString *imageUrl = @"http://www.taopic.com/uploads/allimg/110925/9117-11092509545328.jpg";
[cell setImageWithURL:imageUrl placeholder:placeholder];
return cell;
}