@interface ViewController ()//所有數據@property (nonatomic,strong)NSArray *apps;//內存緩存圖片@property (nonatomic,strong)NSMutableDictionary *imgCache;/**所有操作*/@property (nonatomic,strong)NSMutableDictionary *operations;/**隊列對象*/@property (nonatomic,strong) NSOperationQueue *queue;@end 

- (NSOperationQueue *)queue {if (!_queue) {_queue = [[NSOperationQueue alloc] init];//最大并發數_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;//先從內存中取出圖片UIImage *image = self.imgCache[app.icon];if (image) {cell.imageView.image = image;}else {//內存中沒有圖片//將圖片文件數據寫入到沙盒中NSString *cachesPath = [NSSearchPathForDirectoriesInDomains(NSCachesDirectory, NSUserDomainMask, YES) firstObject];//獲得文件名NSString *filename = [app.icon lastPathComponent];//計算出文件的全路徑NSString *file = [cachesPath stringByAppendingPathComponent:filename];//加載沙盒的文件數據NSData *data = [NSData dataWithContentsOfFile:file];//判斷沙盒中是否有圖片if (data) {//直接加載沙盒中圖片UIImage *image = [UIImage imageWithData:data];cell.imageView.image = image;//存到字典(內存)中self.imgCache[app.icon] = image;}else {//下載圖片//占位圖片cell.imageView.image = [UIImage imageNamed:@"place.jpg"];//先判斷是否有下載任務//加載失敗后可以重復下載NSOperation *operation = self.operations[app.icon];if (operation == nil) {//這張圖片沒有下載任務operation = [NSBlockOperation blockOperationWithBlock:^{NSData *data = [NSData dataWithContentsOfURL:[NSURL URLWithString:app.icon]];//數據加載失敗if(data == nil) {//移除操作[self.operations removeObjectForKey:app.icon];return ;}UIImage *image = [UIImage imageWithData:data];//存到內存中self.imgCache[app.icon] = image;//回到主線程顯示圖片[[NSOperationQueue mainQueue] addOperationWithBlock:^{//會出現重復占位的問題//cell.imageView.image = image;//只需找到圖片所在的行即可[tableView reloadRowsAtIndexPaths:@[indexPath] withRowAnimation:UITableViewRowAnimationNone];}];//將圖片數據寫入到沙盒中[data writeToFile:file atomically:YES];//移除操作[self.operations removeObjectForKey:app.icon];}];//添加到下載隊列[self.queue addOperation:operation];//添加到字典self.operations[app.icon] = operation;}}}return cell;}