- (MJDownloadState)state
{
 // 如果是下载完毕
 if (self.totalBytesExpectedToWrite && self.totalBytesWritten == self.totalBytesExpectedToWrite) {
 return MJDownloadStateCompleted;
 }
 
 // 如果下载失败
 if (self.task.error) return MJDownloadStateNone;
 
 return _state;
}
#pragma mark - 状态控制
- (void)setState:(MJDownloadState)state
{
 // MJDownloadState oldState = self.state; 
 // 修改后:
 MJDownloadState oldState = _state; 
 if (state == oldState) return;
 
 _state = state;
 
 // 发通知
 [self notifyStateChange];
}