我想到了一个回答，所以我会发布，以防它可以帮助某人：

首先，在调用 AVAssetExportSession 的方法中，您必须设置一个计时器来更新 UIProgressView 一旦你开始导出：

  //`AVAssetExportSession`代码在这里
 self.exportProgressBarTimer = [NSTimer scheduledTimerWithTimeInterval：.1 target：self selector：@selector（updateExportDisplay）userInfo：nil repeats：YES]; 
 ... 
 

然后，您需要一种方法来更新您的显示， progress属性 AVAssetExportSession 从0到1：

   - ）updateExportDisplay {
 self.exportProgressBar.progress = exportSession.progress; 
 if（self.exportProgressBar.progress> .99）{
 [self.exportProgressBarTimer invalidate]; 
} 
} 
 

I've got an app that exports an AVMutableComposition into a .mov file, and I'd like for the user to see the status of the export with a progress bar the same way that you would if you sent a text message or uploaded a file.

I know how to create a progress bar when I know the duration of a task (such as playing an audio file), but since there's no set duration for the export I'm unsure how to proceed.

I've got an activity indicator currently but it doesn't provide the best user experience.

Does anyone have any pointers?

I came up with an answer a while back so I'll post it in case it can help someone:

First, in the method in which you call AVAssetExportSession you've got to set up a timer to update your UIProgressView once you've initiated the export:

//`AVAssetExportSession` code here
    self.exportProgressBarTimer = [NSTimer scheduledTimerWithTimeInterval:.1 target:self selector:@selector(updateExportDisplay) userInfo:nil repeats:YES];
...

Then you need a method to update your display taking into account that the progress property on AVAssetExportSession goes from 0 - 1:

- (void)updateExportDisplay {
    self.exportProgressBar.progress = exportSession.progress;
    if (self.exportProgressBar.progress > .99) {
        [self.exportProgressBarTimer invalidate];
    }
}