IOS多线程实现多图片下载(二)

@interface ViewController ()//所有数据@property (nonatomic,strong)NSArray *apps;//内存缓存图片@property (nonatomic,strong)NSMutableDictionary *imgCache;/**所有操作*/@property (nonatomic,strong)NSMutableDictionary *operations;/**队列对象*/@property (nonatomic,strong) NSOperationQueue *queue;@end 

- (NSOperationQueue *)queue {if (!_queue) {_queue = [[NSOperationQueue alloc] init];//最大并发数_queue.maxConcurrentOperationCount = 3;}return _queue;} 

- (UITableViewCell *)tableView:(UITableView *)tableView cellForRowAtIndexPath:(NSIndexPath *)indexPath {static NSString *ID = @"app";UITableViewCell *cell = [tableView dequeueReusableCellWithIdentifier:ID];DDZApp *app = self.apps[indexPath.row];cell.textLabel.text = app.name;cell.detailTextLabel.text = app.download;//先从内存中取出图片UIImage *image = self.imgCache[app.icon];if (image) {cell.imageView.image = image;}else {//内存中没有图片//将图片文件数据写入到沙盒中NSString *cachesPath = [NSSearchPathForDirectoriesInDomains(NSCachesDirectory, NSUserDomainMask, YES) firstObject];//获得文件名NSString *filename = [app.icon lastPathComponent];//计算出文件的全路径NSString *file = [cachesPath stringByAppendingPathComponent:filename];//加载沙盒的文件数据NSData *data = [NSData dataWithContentsOfFile:file];//判断沙盒中是否有图片if (data) {//直接加载沙盒中图片UIImage *image = [UIImage imageWithData:data];cell.imageView.image = image;//存到字典(内存)中self.imgCache[app.icon] = image;}else {//下载图片//占位图片cell.imageView.image = [UIImage imageNamed:@"place.jpg"];//先判断是否有下载任务//加载失败后可以重复下载NSOperation *operation = self.operations[app.icon];if (operation == nil) {//这张图片没有下载任务operation = [NSBlockOperation blockOperationWithBlock:^{NSData *data = [NSData dataWithContentsOfURL:[NSURL URLWithString:app.icon]];//数据加载失败if(data == nil) {//移除操作[self.operations removeObjectForKey:app.icon];return ;}UIImage *image = [UIImage imageWithData:data];//存到内存中self.imgCache[app.icon] = image;//回到主线程显示图片[[NSOperationQueue mainQueue] addOperationWithBlock:^{//会出现重复占位的问题//cell.imageView.image = image;//只需找到图片所在的行即可[tableView reloadRowsAtIndexPaths:@[indexPath] withRowAnimation:UITableViewRowAnimationNone];}];//将图片数据写入到沙盒中[data writeToFile:file atomically:YES];//移除操作[self.operations removeObjectForKey:app.icon];}];//添加到下载队列[self.queue addOperation:operation];//添加到字典self.operations[app.icon] = operation;}}}return cell;}