做文件共享的时候，iTunes往app导入本地视频，实时监听文件的复制进度
NSInteger lastSize = 0;
NSDictionary *fileAttrs = [[NSFileManager defaultManager] attributesOfItemAtPath:videoPath error:nil];
NSInteger fileSize = [[fileAttrs objectForKey:NSFileSize] intValue];
do {
lastSize = fileSize;
[NSThread sleepForTimeInterval:0.5];
self.isFinishedCopy = NO;
fileAttrs = [[NSFileManager defaultManager] attributesOfItemAtPath:videoPath error:nil];
fileSize = [[fileAttrs objectForKey:NSFileSize] intValue];
NSLog(@"文件正在复制");
} while (lastSize != fileSize);
self.isFinishedCopy = YES;
NSLog(@"%@文件复制完成");
