Understanding the Issue: Unrecognized Selector Sent to Instance

As developers, we’ve all encountered the dreaded “unrecognized selector sent to instance” error. In this article, we’ll delve into the world of Objective-C and explore what causes this issue, how to identify it, and most importantly, how to fix it.

What is an Unrecognized Selector?

In Objective-C, a selector is essentially a reference to a method or function within an object. When you call a method on an object, the runtime environment checks if that object implements the specified method. If the object does implement the method, it’s said to have sent the selector to itself. Conversely, if an object doesn’t implement the specified method, the runtime environment will raise an exception, and you’ll see the dreaded “unrecognized selector sent to instance” error.

Why Does this Error Occur?

The “unrecognized selector sent to instance” error occurs when you try to call a method on an object that doesn’t know how to handle that specific request. This can happen in several ways:

• You’ve modified the class hierarchy of your objects.
• You’ve added or removed methods from a class without updating its superclass or subclasses.
• The object’s subclass hasn’t implemented the required method.

How Can I Identify this Error?

To identify the “unrecognized selector sent to instance” error, follow these steps:

1. Check your code for any modifications you’ve made to the class hierarchy of your objects.
2. Use Instruments or Xcode’s built-in debugging tools to inspect the object’s method table and verify that it contains the specified method.

Fixing the Issue

To fix the issue, you’ll need to identify where the unrecognized selector is being sent and update the class hierarchy of your objects accordingly. Here are some steps to follow:

1. Use Instruments or Xcode’s built-in debugging tools to inspect the object’s method table and verify that it contains the specified method.
2. Check the stack trace provided by the error message to identify which line of code is causing the issue.
3. Inspect the LoginViewController class definition to see if the viewController property has been added or modified in any way.

Example Solution

The original code snippet includes a call to [loginViewControllerObject viewController];, which is likely an instance method. If you’re trying to access the navigation controller, you can try using the viewControllers property instead.

- (void)viewDidLoad {
    [super viewDidLoad];
    self.pinValidated = NO;
    
    // Accessing the view controllers property
    NSArray<UIViewController *> *controllers = self.viewControllers;
    for (UIViewController *controller in controllers) {
        if ([controller isKindOfClass:[UINavigationController class]]) {
            UINavigationController *navController = controller;
            // Perform some action using navController
        }
    }
}

Best Practices

To avoid the “unrecognized selector sent to instance” error, follow these best practices:

• Always update your class hierarchy and method tables when making changes to your code.
• Use Instruments or Xcode’s built-in debugging tools to inspect the object’s method table and verify that it contains the specified method.
• Inspect the stack trace provided by the error message to identify which line of code is causing the issue.

By following these guidelines, you can avoid the “unrecognized selector sent to instance” error and write more robust, maintainable code.

Last modified on 2023-06-04