Clean Objective C helper methods to perform background tasks and perform completion on main thread.

GCDManager.h

#import <Foundation/Foundation.h>

@interface GCDManager : NSObject

+ (void)performBackgroundTask:(void (^)())block withCompletion:(void (^)())completion;
+ (void)performHighPriorityBackgroundTask:(void (^)())block withCompletion:(void (^)())completion;
+ (void)performLowPriorityBackgroundTask:(void (^)())block withCompletion:(void (^)())completion;

@end

GCDManager.m

#import "GCDManager.h"

@implementation GCDManager

+ (void)performBackgroundTask:(void (^)())block withCompletion:(void (^)())completion
{
    dispatch_async(dispatch_get_global_queue(DISPATCH_QUEUE_PRIORITY_DEFAULT, 0), ^{
        block();
        dispatch_async(dispatch_get_main_queue(), ^{
            completion();
        });
    });
}

+ (void)performHighPriorityBackgroundTask:(void (^)())block withCompletion:(void (^)())completion
{
    dispatch_async(dispatch_get_global_queue(DISPATCH_QUEUE_PRIORITY_HIGH, 0), ^{
        block();
        dispatch_async(dispatch_get_main_queue(), ^{
            completion();
        });
    });
}

+ (void)performLowPriorityBackgroundTask:(void (^)())block withCompletion:(void (^)())completion
{
    dispatch_async(dispatch_get_global_queue(DISPATCH_QUEUE_PRIORITY_LOW, 0), ^{
        block();
        dispatch_async(dispatch_get_main_queue(), ^{
            completion();
        });
    });
}

@end