UIImagePickerController在ios7中没有显示（花费更多时间加载）相机预览

问题描述：

我在我的应用程序中使用GCD来获取后台队列中的大量图像和数据，当我呈现uiimagepickercontroller时，它需要更多时间来显示相机预览。谷歌搜索后，我发现许多人在iOS 7中遇到过这个问题，这里还有一篇对我有用的帖子。（ iOS 7 UIImagePickerController相机无图像）。解决方案是停止后台线程然后呈现选择器控制器。但我真的很难过，不知道如何停止或暂停后台线程，呈现选择器控制器，然后恢复/启动后台线程。有人可以帮我解决这个问题吗。

I am using GCD in my app to fetch lot of images and data in the background queue and when I present uiimagepickercontroller it takes more time to show the camera preview. After googling around, I found that many have faced this issue with iOS 7 and here is one more post that made some sense to me. (iOS 7 UIImagePickerController Camera No Image). The solution was to stop the background threads and then present the picker controller. But I am really stumped and don't know how to stop or pause the background threads, present the picker controller and then resume/start the background thread. Can someone please help me with this.

以下是我在网络课上（和其他人一样）在后台获取图像和数据的方法。我的串行队列初始化如下。

Here is how I am doing my background fetching of images and data in my networking class (like everyone else). My serial queue is initialized like this.

sharedInstance.serialQueue = dispatch_queue_create("user-detail-queue", DISPATCH_QUEUE_CONCURRENT);

在后台获取内容的代码就是这样。

And the code to fetch things in the background is like this.

    dispatch_async(dispatch_get_global_queue(DISPATCH_QUEUE_PRIORITY_BACKGROUND, 0), ^{
        NSString *myUserID = [JNKeychain loadValueForKey:@"userID"];
        NSString *sessionToken = [JNKeychain loadValueForKey:@"sessionToken"];
        if(sessionToken && myUserID)
        {
            dispatch_async([BGNetworkAPI sharedInstance].serialQueue, ^{
                id object = [[BGNetworkAPI sharedInstance].userCache objectForKey:myUserID];
                if(object)
                {
                    NSDictionary *cachedResponseDictionary = (NSDictionary *)object;
                    BGUser *user = [BGUser createUserWithResponseDictionary:cachedResponseDictionary];
                    if(block)
                    {
                        block(user, nil);
                    }
                }
                else
                {
                  [[BGUserManagementClient sharedClient] fetchUserDetailsWithUserId:myUserID withSessionToken:sessionToken withSuccessBlock:^(AFHTTPRequestOperation *operation, id responseObject)
                 {
                     //NSLog(@"The response object of my user object is %@",responseObject);
                     [[BGNetworkAPI sharedInstance].userCache setObject:responseObject forKey:myUserID];
                     NSDictionary *responseDictionary = (NSDictionary *)responseObject;
                     BGUser *user = [BGUser createUserWithResponseDictionary:responseDictionary];
                     if(block)
                     {
                         block(user,nil);
                     }
                 } failure:^(AFHTTPRequestOperation *operation, NSError *error)
                 {
                     NSLog(@"The error while fetching the user details is %@", operation.responseObject);
                     if(block)
                     {
                         block(nil, error);
                     }
                 }];
                }
             }
        });

答

使用上面的代码，你没有进行提取backgroud为 dispatch_get_main_queue（）为您提供运行该接口的队列。要执行后台提取，您必须使用 dispatch_queue_create（image_queue，NULL）打开另一个队列。

With the code above you are not doing the fetching in the backgroud as dispatch_get_main_queue()gives you the queue the interface is running on. To perform a background fetch you have to open another queue with dispatch_queue_create("image_queue", NULL).

backgroud fetch已完成，您希望在主队列中使用UI执行此操作。

When the backgroud fetch is finished and you want to do something with the UI you have to do this in the main queue.

dispatch_async(dispatch_get_main_queue(),^{
  if(block) {
    block(user,nil)
  }
});

UIImagePickerController在ios7中没有显示（花费更多时间加载）相机预览

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