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:selfselector:@selector(downloadCallback:)name:NOTIFICATION_DOWNLOAD_CALLBACKobject: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_CALLBACKobject:niluserInfo:dic];}

- (UICollectionViewCell *)collectionView:(UICollectionView *)collectionView cellForItemAtIndexPath:(NSIndexPath *)indexPath{SQPhotoCell *cell = (SQPhotoCell *)[collectionView dequeueReusableCellWithReuseIdentifier:reuseIdentifierforIndexPath: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;}