Method swizzling in Swift

There is a little problem with UINavigationBar which I'm trying to overcome. When you hide the status bar using prefersStatusBarHidden() method in a view controller (I don't want to disable the status bar for the entire app), the navigation bar loses 20pt of its height that belongs to the status bar. Basically the navigation bar shrinks.

I was trying out different workarounds but I found that each one of them had drawbacks. Then I came across this category which using method swizzling, adds a property called fixedHeightWhenStatusBarHidden to the UINavigationBar class which solves this issue. I tested it in Objective-C and it works. Here are the header and the implementation of the original Objective-C code.

Now since I'm doing my app in Swift, I tried translating it to Swift.

The first problem I faced was Swift extension can't have stored properties. So I had to settle for computed property to declare the fixedHeightWhenStatusBarHidden property that enables me to set the value. But this sparks another problem. Apparently you can't assign values to computed properties. Like so.

self.navigationController?.navigationBar.fixedHeightWhenStatusBarHidden = true

I get the error Cannot assign to the result of this expression.

Anyway below is my code. It compiles without any errors but it doesn't work.

import Foundation
import UIKit

extension UINavigationBar {

    var fixedHeightWhenStatusBarHidden: Bool {
        return objc_getAssociatedObject(self, "FixedNavigationBarSize").boolValue
    }

    func sizeThatFits_FixedHeightWhenStatusBarHidden(size: CGSize) -> CGSize {

        if UIApplication.sharedApplication().statusBarHidden && fixedHeightWhenStatusBarHidden {
            let newSize = CGSizeMake(self.frame.size.width, 64)
            return newSize
        } else {
            return sizeThatFits_FixedHeightWhenStatusBarHidden(size)
        }

    }
    /*
    func fixedHeightWhenStatusBarHidden() -> Bool {
        return objc_getAssociatedObject(self, "FixedNavigationBarSize").boolValue
    }
    */

    func setFixedHeightWhenStatusBarHidden(fixedHeightWhenStatusBarHidden: Bool) {
        objc_setAssociatedObject(self, "FixedNavigationBarSize", NSNumber(bool: fixedHeightWhenStatusBarHidden), UInt(OBJC_ASSOCIATION_RETAIN))
    }

    override public class func load() {
        method_exchangeImplementations(class_getInstanceMethod(self, "sizeThatFits:"), class_getInstanceMethod(self, "sizeThatFits_FixedHeightWhenStatusBarHidden:"))
    }

}

fixedHeightWhenStatusBarHidden() method in the middle is commented out because leaving it gives me a method redeclaration error.

I haven't done method swizzling in Swift or Objective-C before so I'm not sure of the next step to resolve this issue or even it's possible at all.

Can someone please shed some light on this?

Thank you.

UPDATE 1: Thanks to newacct, my first issue about properties was resolved. But the code doesn't work still. I found that the execution doesn't reach the load() method in the extension. From comments in this answer, I learned that either your class needs to be descendant of NSObject, which in my case, UINavigationBar isn't directly descended from NSObject but it does implement NSObjectProtocol. So I'm not sure why this still isn't working. The other option is adding @objc but Swift doesn't allow you to add it to extensions. Below is the updated code.

The issue is still open.

import Foundation
import UIKit

let FixedNavigationBarSize = "FixedNavigationBarSize";

extension UINavigationBar {

    var fixedHeightWhenStatusBarHidden: Bool {
        get {
            return objc_getAssociatedObject(self, FixedNavigationBarSize).boolValue
        }
        set(newValue) {
            objc_setAssociatedObject(self, FixedNavigationBarSize, NSNumber(bool: newValue), UInt(OBJC_ASSOCIATION_RETAIN))
        }
    }

    func sizeThatFits_FixedHeightWhenStatusBarHidden(size: CGSize) -> CGSize {

        if UIApplication.sharedApplication().statusBarHidden && fixedHeightWhenStatusBarHidden {
            let newSize = CGSizeMake(self.frame.size.width, 64)
            return newSize
        } else {
            return sizeThatFits_FixedHeightWhenStatusBarHidden(size)
        }

    }

    override public class func load() {
        method_exchangeImplementations(class_getInstanceMethod(self, "sizeThatFits:"), class_getInstanceMethod(self, "sizeThatFits_FixedHeightWhenStatusBarHidden:"))
    }

}

UPDATE 2: Jasper helped me to achieve the desired functionality but apparently it comes with a couple of major drawbacks.

Since the load() method in the extension wasn't firing, as Jasper suggested, I moved the following code block to app delegates' didFinishLaunchingWithOptions method.

method_exchangeImplementations(class_getInstanceMethod(UINavigationBar.classForCoder(), "sizeThatFits:"), class_getInstanceMethod(UINavigationBar.classForCoder(), "sizeThatFits_FixedHeightWhenStatusBarHidden:"))

And I had to hardcode the return value to 'true' in the getter of fixedHeightWhenStatusBarHidden property because now that the swizzling code executes in the app delegate, you can't set a value from a view controller. This makes the extension redundant when it comes to reusability.

So the question is still open more or less. If anyone has an idea to improve it, please do answer.