@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;
}