Encapsulates `beginBackgroundTaskWithExpirationHandler` in a safer manner (should never crash the app with "active assertions beyond permitted time"). For a longer discussion see: http://bit.ly/ZVDo0V

Example.m

  // nb. backgroundTask is a member variable

  backgroundTask = [WDBackgroundTask beginBackgroundTaskWithSafeExpirationHandler:^{
	  backgroundTask = UIBackgroundTaskInvalid;
	}];

  // your task code here
  // ...

  backgroundTask = [WDBackgroundTask endBackgroundTaskSafe:backgroundTask];

WDBackgroundTask.h

#import <Foundation/Foundation.h>

@interface WDBackgroundTask : NSObject

+ (UIBackgroundTaskIdentifier) beginBackgroundTaskWithSafeExpirationHandler:(void (^)(void))handler;
+ (UIBackgroundTaskIdentifier) endBackgroundTaskSafe:(UIBackgroundTaskIdentifier)backgroundTaskIdentifier;

@end

WDBackgroundTask.m

#import "WDBackgroundTask.h"

@implementation WDBackgroundTask

+ (UIBackgroundTaskIdentifier) beginBackgroundTaskWithSafeExpirationHandler:(void (^)(void))handler
{
	UIBackgroundTaskIdentifier localBackgroundTaskIdentifier = UIBackgroundTaskInvalid;
	
	// block captures the local variable for safety, so it will always end that identifier
	localBackgroundTaskIdentifier = [[UIApplication sharedApplication] beginBackgroundTaskWithExpirationHandler:^{
		handler();
		[[UIApplication sharedApplication] endBackgroundTask:localBackgroundTaskIdentifier];
	}];
	
	return localBackgroundTaskIdentifier;
}


+ (UIBackgroundTaskIdentifier) endBackgroundTaskSafe:(UIBackgroundTaskIdentifier)backgroundTaskIdentifier
{
	if (backgroundTaskIdentifier != UIBackgroundTaskInvalid)
	{
		[[UIApplication sharedApplication] endBackgroundTask:backgroundTaskIdentifier];
	}
	return UIBackgroundTaskInvalid;
}

@end