AVAssetWriter 输出视频文件在ios8上的结束

我的代码是:

if ([UIDevice currentDevice].systemVersion.floatValue < 6.0) {
    [assetWriter finishWriting];
} else {
    if ([assetWriter respondsToSelector:@selector(finishWritingWithCompletionHandler:)]) {
        [assetWriter finishWritingWithCompletionHandler:^(){
            NSLog(@"Over end");
        }];
        NSLog(@"Start end");
    }
    else {
#pragma clang diagnostic push
#pragma clang diagnostic ignored "-Wdeprecated-declarations"
        [assetWriter finishWriting];
#pragma clang diagnostic pop
     }
}

结果在ios6上会打印:
Start end
Over end
而在ipad mini2 和 iphone6的ios8.4上，没有打印出Over end只有Start end,谁晓得怎么回事么