Perform push segue after an unwind segue

A bit late to the party but I found a way to do this without using state flags

Note: this only works with iOS 9+, as only custom segues support class names prior to iOS9 and you cannot declare an exit segue as a custom segue in storyboards

1. Subclass UIStoryboardSegue with UIStoryboardSegueWithCompletion

class UIStoryboardSegueWithCompletion: UIStoryboardSegue {
    var completion: (() -> Void)?

    override func perform() {
        super.perform()
        if let completion = completion {
            completion()
        }
    }
}

2. Set UIStoryBoardSegueWithCompletion as the class for your exit segue

note: the action for this segue should be unwindToMainMenu to match the original question

3. Update your unwind @IBAction to execute the code in the completion handler

@IBAction func unwindToMainMenu(segue: UIStoryboardSegue) {
    if let segue = segue as? UIStoryboardSegueWithCompletion {
        segue.completion = { 
            self.performSegueWithIdentifier("Categories", sender: self) 
        }
    }
}

Your code will now execute after the exit segue completes its transition

Taking forward this answer (I only had Objective-C code)

Subclass UIStoryBoardSegue

#import <UIKit/UIKit.h>

@interface MyStoryboardSegue : UIStoryboardSegue

/**
 This block is called after completion of animations scheduled by @p self.
 */
@property (nonatomic, copy) void(^completion)();

@end

And call this completion block after completion of animations.

@implementation MyStoryboardSegue

- (void)perform {
  [super perform];
  if (self.completion != nil) {
    [self.destinationViewController.transitionCoordinator
     animateAlongsideTransition:nil
     completion:^(id<UIViewControllerTransitionCoordinatorContext> context) {
       if (![context isCancelled]) {
         self.completion();
       }
     }];
  }
}

@end

I want to provide my own solution to this problem for now. Any further answers are always welcome.

I put a boolean variable and viewDidAppear function to MainPageViewController.

var fromCamera = false

override func viewDidAppear(animated: Bool) {
    if fromCamera {
        self.performSegueWithIdentifier("categorySelection", sender: self)
        self.fromCamera = false
    }
}

I set fromCamera to true before I perform unwind segue from CropViewController. By that way, I perform segue to category screen only if an unwind segue from crop view is performed.