AVAssetwriter failed when enter

使用AVAssetWriter导出视频时，退到后台，进入时，失败,status = AVAssetExportSessionStatusFailed
注意：退到后台等几秒钟，进入会失败

You use an AVAssetWriter
object to write media data to a new file of a specified audiovisual container type, such as a QuickTime movie file or an MPEG-4 file, with support for automatic interleaving of media data for multiple concurrent tracks.
You can only use a given instance of AVAssetWriter
once to write to a single file. If you want to write to files multiple times, you must use a new instance of AVAssetWriter
each time.

解决方案
1.进入前台判断导入视频的文件是否成功（如是否存在），尝试重新导出
2.使用后台线程，

- (void)applicationDidEnterBackground:(NSNotification *)note
{
    UIApplication *app = [UIApplication sharedApplication];
    _backgroundRenderingID = [app beginBackgroundTaskWithExpirationHandler:^{
        [app endBackgroundTask:_backgroundRenderingID];
        _backgroundRenderingID = UIBackgroundTaskInvalid;
    }];
 NSTimeInterval time1 = app.backgroundTimeRemaining; //180s
}
//执行完任务，记得告诉应用，防止被os杀掉，You can find out how much time is available using the [backgroundTimeRemaining]property.
[[UIApplication sharedApplication] endBackgroundTask:_backgroundRenderingID];

注意：如果执行任务超过10分钟，最好就是暂停任务。（我测试返回后台保持时间是180s)
参考