UIDocument saveToURL:导致严重延迟

Question

UIDocument的saveToURL:forSaveOperation:completionHandler:应该是异步的(在文档中，它说它在后台队列上执行此操作)，但我的代码在调用它时有7-10秒的延迟。

代码如下：

NSLog(@"saving image...");
[picDoc saveToURL:[picDoc fileURL] 
 forSaveOperation:UIDocumentSaveForCreating 
completionHandler:^(BOOL success) {


    dispatch_async(dispatch_get_main_queue(), ^{
        if (success) {
            NSLog(@"image save successful!");
            UIImage *thumbnail = [image imageByScalingAndCroppingForSize:CGSizeMake(146, 110)];
            Picture *picture = [[Picture alloc] initWithThumbnail:thumbnail filename:fileName];
            [[set pictures] addObject:picture];




            [self setupPictures];

            [scrollView scrollRectToVisible:((UIView *)[[scrollView subviews] lastObject]).frame animated:YES];
            currentIndex = [[set pictures] count] - 1;
            [self showPicture];

        } else {
            NSLog(@"failed saving image");
        }
        [SVProgressHUD dismiss];
    });

    [picDoc closeWithCompletionHandler:nil];
}];
NSLog(@"exit");

和控制台：

2012-05-15 07:07:27.417 Picventory[5939:707] saving image...
2012-05-15 07:07:34.120 Picventory[5939:707] exit
2012-05-15 07:07:34.740 Picventory[5939:707] image save successful!

当调用是异步的时，为什么会有如此巨大的延迟？谢谢

Answer 1

将文件写入磁盘是异步的，但获取文档数据的快照则不是。您应该使用Instruments中的Time Profiler来找出实际花费这么长时间的原因。我猜您可能需要优化您的contentsForType:error: (或等效的)方法，但首先要测量！