求一张appStore开发者后台界面截图

代码如下：
UIWindow *screenWindow = [[UIApplication sharedApplication] keyWindow];
UIGraphicsBeginImageContext(screenWindow.frame.size);
[screenWindow.layer renderInContext:UIGraphicsGetCurrentContext()];
UIImage *viewImage = UIGraphicsGetImageFromCurrentImageContext();

UIGraphicsEndImageContext();
return viewImage;

后台测试函数：
- (void)applicationDidEnterBackground:(UIApplication *)application
{
// Use this method to release shared resources, save user data, invalidate timers, and store enough application state information to restore your application to its current state in case it is terminated later.
// If your application supports background execution, this method is called instead of applicationWillTerminate: when the user quits.
if ([[UIDevice currentDevice] respondsToSelector:@selector(isMultitaskingSupported)]) { //Check if our iOS version supports multitasking I.E iOS 4
if ([[UIDevice currentDevice] isMultitaskingSupported]) { //Check if device supports mulitasking
UIApplication *application = [UIApplication sharedApplication]; //Get the shared application instance
__block UIBackgroundTaskIdentifier background_task; //Create a task object
background_task = [application beginBackgroundTaskWithExpirationHandler: ^ {
[application endBackgroundTask: background_task]; //Tell the system that we are done with the tasks
background_task = UIBackgroundTaskInvalid; //Set the task to be invalid
//System will be shutting down the app at any point in time now
}];
//Background tasks require you to use asyncrous tasks
dispatch_async(dispatch_get_global_queue(DISPATCH_QUEUE_PRIORITY_DEFAULT, 0), ^{
//Perform your tasks that your application requires
NSLog(@"\n\nRunning in the background!\n\n");

[self SendStart];

[application endBackgroundTask: background_task]; //End the task so the system knows that you are done with what you need to perform
background_task = UIBackgroundTaskInvalid; //Invalidate the background_task
});
}
}
}

但是遇到下面几个现象，求解：

1.运行在前台（程序主界面）的话，可以截得到图，当按了home键到后台后，程序进入到后台，这时截到的图依旧是程序背景图片，而非截到Ipad当前屏幕的图片。

2.截图函数运行在后台线程，日志跟踪无挂起，还在不停记录日志，有数据发送流量，数据大小正常。

3.未关闭程序情况下，再次打开程序，能截到当前程序的操作图片，即是程序运行在前台的情况下使用正常。