下载网络视频到沙盒

//沙盒的路径
 NSArray *paths = NSSearchPathForDirectoriesInDomains(NSDocumentDirectory, NSUserDomainMask, YES);
    NSString *documentsDirectory = [paths objectAtIndex:0];
    
    
    AFHTTPSessionManager * manager = [AFHTTPSessionManager manager];
    //沙盒的全路径
    NSString  *fullPath = [NSString stringWithFormat:@"%@/%@", documentsDirectory, @"test11.mov"];
    NSURL *url = [NSURL URLWithString:@"http://tb-video.bdstatic.com/tieba-smallvideo-transcode/20985849_722f981a5ce0fc6d2a5a4f40cb0327a5_3.mp4"];
    NSURLRequest *request = [NSURLRequest requestWithURL:url];
//targetPath为目标路径
 NSURLSessionDownloadTask *task =
    [manager downloadTaskWithRequest:request
                            progress:nil destination:^NSURL *(NSURL *targetPath, NSURLResponse *response) {
//返回的是全路径
                                return [NSURL fileURLWithPath:fullPath];
                            }
                   completionHandler:^(NSURLResponse *response, NSURL *filePath, NSError *error) {   
                       //filePath就是文件路径
                   }];
    [task resume];