多任务下载（支持后台、断点下载）

项目介绍：

基于NSURLSessionDownloadTask实现多任务后台下载，支持断点下载

1. #import "FYDownloadManager+Task.h"
2. 使用FYDownloadManager的分类FYDownloadManager+Task来创建和管理下载任务
3. eg: 获取所有下载任务 [FYDownloadManager shareManager].downloadItemArray
4. 开启一个新的下载任务
5. FYDownloadItem *item = [[FYDownloadItem alloc] init:[NSURL URLWithString:@"http://ultravideo.cs.tut.fi/video/ShakeNDry_3840x2160_30fps_420_8bit_AVC_MP4.mp4"]]; [[FYDownloadManager shareManager] addDownload:item]; [[FYDownloadManager shareManager] start:item.requestUrl];
6. 监听下载进度
7. 1.添加进度监听
8. [[NSNotificationCenter defaultCenter] addObserver:self selector:@selector(downloadStateChange:) name:@"FYDownload_Status" object:nil];
9. 2.进度监听回调
10. (void)downloadStateChange:(NSNotification *)no {
11. NSURL *requestUrl = no.userInfo[@"requestUrl"]; FYDownloadItem *currentItem = no.userInfo[@"item"];          NSLog(@"%@___%@",currentItem.countOfBytesReceived,currentItem.countOfBytesExpectedToReceive);
12. }

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git地址FYDownload  仓促编码会一直改进，欢迎批评指正，交流学习…