Understanding Unrecognized Selectors in Swift

As a developer, we have all encountered the dreaded “unrecognized selector sent to instance” error at some point. In this article, we will delve into the world of Objective-C selectors and explore why they are being sent to our Swift code.

What is an Objective-C Selector?

In Objective-C, when you want to call a method on an object, you must specify the method name. This process is called sending a message to the object. The method name is equivalent to a selector in Objective-C terminology.

Selectors are essentially methods or properties, but they can also be blocks (closure). They serve as a way for the compiler to identify the specific method that should be invoked on an object at runtime.

When you call performSegueWithIdentifier from your Swift code, it sends a message to the destination view controller. However, in this case, we are trying to call a method (setShowsCancelButton) that is not available on the navigation controller or its top view controller.

Recognizing Unrecognized Selectors

The error message [Chooze.LogInOrRegisterViewController setShowsCancelButton:animated:]: unrecognized selector sent to instance 0x7f928b93aed0 indicates that our Swift code is trying to call a method (setShowsCancelButton) on the LogInOrRegisterViewController object, but this method does not exist.

To understand why, we need to look at the properties of the navigation controller. In our case, the segue’s destination view controller is embedded in a navigation controller (UINavigationController). The topViewController property returns the top-most view controller that can be pushed or popped from the current navigation stack.

When we call performSegueWithIdentifier, it sets the new top view controller to the one identified by the segue. However, our Swift code does not know this. Instead, it tries to invoke a method (setShowsCancelButton) on the default view controller of the navigation controller.

Understanding the Navigation Controller’s View Hierarchy

To better understand what is happening here, let’s dive into the world of the navigation controller’s view hierarchy.

The UINavigationController has two key properties:

• RootViewController: This property returns the top-most view controller in the current navigation stack.
• **ViewControllers`: This property returns an array of all view controllers that are currently part of the navigation stack, excluding the root view controller.

In our case, when we call performSegueWithIdentifier, it sets the new top view controller to the one identified by the segue. However, the default view controller is not set; instead, it’s the object that we are trying to invoke the setShowsCancelButton method on.

The Solution: Accessing TopViewController

To fix this issue, we need to access the navigation controller’s topViewController property and call setShowsCancelButton on the top view controller. This is because our Swift code needs to know which view controller should be used to invoke this method.

Here is how you can do it:

// In your prepareForSegue function

override func prepareForSegue(segue: UIStoryboardSegue, sender: AnyObject?) {
    if segue.identifier == "Login" {
        let navController = segue.destinationViewController as UINavigationController
        let topController = navController.topViewController
        if let controller = topController as? LoginOrRegisterViewController {
            controller.setShowsCancelButton(false, animated: false)
        }
    }
}

In this revised solution:

• We first cast the destinationViewController to a UINavigationController.
• Then we access the navigation controller’s topViewController property.
• Finally, we check if the top view controller is of type LoginOrRegisterViewController and invoke the setShowsCancelButton method on it.

By following these steps, you can ensure that your Swift code correctly accesses the correct view controller to invoke methods like setShowsCancelButton.

Last modified on 2024-10-30