Unrecognized selector sent to instance" problem

my code broke somewhere along the way, and crashes when using the navigation bar buttons.

Error message: *** Terminating app due to uncaught exception 'NSInvalidArgumentException', reason: '-[UIView newMemoViewController:didAddMemo:]: unrecognized selector sent to instance 0x5b55a60'

When de开发者_开发知识库bugging, the program does run the cancel method, and throws an exception at the @synthesize line. However, I cannot see anything wrong with it.

The symptoms are identical, so I am including the relevant code only for the Cancel button:

NewMemoViewController.h

#import <UIKit/UIKit.h>
@protocol NewMemoDelegate;


@class AKVoiceMemo;


@interface NewMemoViewController : UIViewController {
    @private
        AKVoiceMemo *voiceMemo;
        id <NewMemoDelegate> delegate;
}

@property (nonatomic, retain) AKVoiceMemo *voiceMemo;
@property (nonatomic, assign) id <NewMemoDelegate> delegate;

@end

@protocol NewMemoDelegate <NSObject>
- (void)newMemoViewController:(NewMemoViewController *)newMemoViewController didAddMemo:(AKVoiceMemo *)voiceMemo;


@end

NewMemoViewController.m

#import "NewMemoViewController.h"

@synthesize delegate;


- (void)viewDidLoad {
    UIBarButtonItem *cancelButtonItem = [[UIBarButtonItem alloc] initWithTitle:@"Cancel" style:UIBarButtonItemStyleBordered target:self action:@selector(cancel)];
    self.navigationItem.leftBarButtonItem = cancelButtonItem;
    [cancelButtonItem release];
}


- (void)cancel {
    [self.delegate newMemoViewController:self didAddMemo:nil];
}

Your help would be appreciated.

Edit: the delegate is the RootViewController:

- (void)newMemoViewController:(NewMemoViewController *)newMemoViewController didAddMemo:(AKVoiceMemo *)voiceMemo {
    if (voiceMemo){
        // Show the note in a new view controller
        // TODO: Implement this
    }

    [self dismissModalViewControllerAnimated:YES];
}

You're probably setting the delegate of NewMemoViewController to a UIView object instead of an object that implements the NewMemoDelegate protocol.

The error message is telling you that a newMemoViewController:didAddMemo: message was sent to a UIView object and the UIView object didn't know what to do with it. Since your cancel method calls newMemoViewController:didAddMemo: on the delegate, it is the delegate which is the UIView object that doesn't recognize the newMemoViewController:didAddMemo: message. In other words, your delegate is a UIView and it doesn't implement the NewMemoDelegate protocol.

If you are correctly setting the delegate, then @jtbandes makes a great point: The delegate is probably being released and a UIView object is taking over the same memory location, thus "becoming" the delegate by accident. You're doing the right thing by using the assign attribute for your delegate; that's fairly standard Cocoa practice. However, you do need to make sure that the delegate is retained by another object, and that object needs to make sure that the delegate sticks around as long as NewMemoViewController needs it to.

I'm guessing you've over-released the delegate. I notice you have @property (assign) ... delegate;. This means that whenever you set the delegate, that object must be retained by something else as well.

The other possibility is the delegate is actually a UIView, but I'm guessing it's the other case.