how to fix the "unrecognized selector sent to instance"
===

A few days ago I created a app based on the scroll-view application I saw on the book Beginning iOS 4 Application Development .The scroll-view will displays smartly when I touch the textfield.
but it always crash when I touch the another textfield.

the debugger display:

    2012-07-31 21:32:35.721 View-based Application[1515:c07] -[ViewController keyboardDidShow:]: unrecognized selector sent to instance 0x6a3e400
    2012-07-31 21:32:35.723 View-based Application[1515:c07] *** Terminating app due to uncaught exception 'NSInvalidArgumentException', reason: '-[ViewController keyboardDidShow:]: unrecognized selector sent to instance 0x6a3e400'
    *** First throw call stack:
    (0x14b2022 0xeb2cd6 0x14b3cbd 0x1418ed0 0x1418cb2 0x9d7a29 0x147d855 0x147d778 0x91c19a 0x3ab845 0xbfe1bc4 0x3a6e01 0x4f757 0x45e49 0x45f34 0xbfe5aac 0x1d8ab54 0x3d93509 0x13e9803 0x13e8d84 0x13e8c9b 0x139b7d8 0x139b88a 0x15626 0x2182 0x20f5)
    terminate called throwing an exception(lldb) 

I am new to iOS develop.I really don't know whats wrong.  Here is the viewController.m code. Perhaps it is bit long.
Please help me. If someone need the whole code,I can send it to him. My e-mail address:kururuhuang@gmail.com

    
      
   

     #import "ViewController.h"
    
    @interface ViewController ()
    
    @end
    
    @implementation ViewController
    
    @synthesize scrollView;
    
    
    -(void) viewWillAppear:(BOOL)animated{
        //---registers the notifications for keyboard---
        [[NSNotificationCenter defaultCenter]
         addObserver:self 
         selector:@selector(keyboardDidShow:) 
         name:UIKeyboardDidShowNotification 
         object:self.view.window];
        
        [[NSNotificationCenter defaultCenter]
         addObserver:self 
         selector:@selector(keyBoardDidHide) 
         name:UIKeyboardDidHideNotification 
         object:nil];
    }
    
    
    //---when a TextField view begins editing---
    -(void) textFieldDidBeginEditing:(UITextField *)textFieldView{
        currentTextField = textFieldView;
    }
    
    
    //---when the user taps on the return key on the keyboard---
    -(BOOL) textFieldShouldReturn:(UITextField *) textFieldView {
        [textFieldView resignFirstResponder];
        return NO;
    }
    
    //---when a TextField view is done editing---
    -(void) textFieldDidEndEditing:(UITextField *) textFieldView {
        currentTextField = nil;
    }
    
    //---when the keyboard appears---
    -(void) keyboardDidshow:(NSNotification *) notification {
        if (keyboardIsShown) return;
        
        NSDictionary* info = [notification userInfo];
        
        //---obtain the size of the keyboard---
        NSValue *aValue = 
        [info objectForKey:UIKeyboardFrameEndUserInfoKey];
        CGRect keyboardRect = [self.view convertRect:[aValue CGRectValue] fromView:nil];
        
        NSLog(@"%f", [aValue CGRectValue].size.height);
        
        NSLog(@"%f",keyboardRect.size.height);
        
        //---resize the scroll view (with keyboard)---
        CGRect viewFrame = [scrollView frame];
        viewFrame.size.height -= keyboardRect.size.height;
        scrollView.frame = viewFrame;
        
        //---scroll to the current text field---
        CGRect textFieldRect = [currentTextField frame];
        [scrollView scrollRectToVisible:textFieldRect animated:YES];
        
        keyboardIsShown = YES;
    }
    
    //---when the keyboard disappears---
    -(void) keyboardDidHide:(NSNotification *) notification {
        NSDictionary* info = [notification userInfo];
        
        //---obtain the size of the keyboard---
        NSValue* aValue = [info objectForKey:UIKeyboardFrameEndUserInfoKey];
        CGRect keyboardRect = [self.view convertRect:[aValue CGRectValue] fromView:nil];
        
        //---resize the scroll view back to the orginal size (without keyboard)---
        CGRect viewFrame = [scrollView frame];
        viewFrame.size.height += keyboardRect.size.height;
        scrollView.frame = viewFrame;
        
        keyboardIsShown = NO;
    }
    
    //---before the View window disapppear--
    -(void) viewWillDisappear:(BOOL)animated{
        //---removes the notifications for keyboard---
        [[NSNotificationCenter defaultCenter] removeObserver:self
                                                     name:UIKeyboardDidShowNotification
                                                      object:nil];
        
        [[NSNotificationCenter defaultCenter] removeObserver:self
                                                         name:UIKeyboardWillHideNotification
                                                      object:nil];
    }
    - (void)viewDidLoad
    {
        scrollView.frame = CGRectMake(0,0,320,460);
        [scrollView setContentSize:CGSizeMake(320, 701)];
        [super viewDidLoad];
    	// Do any additional setup after loading the view, typically from a nib.
    }
    
    - (void)viewDidUnload
    {
        [super viewDidUnload];
        // Release any retained subviews of the main view.
    }
    
    
    - (BOOL)shouldAutorotateToInterfaceOrientation:(UIInterfaceOrientation)interfaceOrientation
    {
        return (interfaceOrientation != UIInterfaceOrientationPortraitUpsideDown);
    }
    
    @end




