Question

Just learning to add actions with code in my .m file and my first attempt throws an exception and crashes the app.

This is an authentication script that I hope will communicate with my web server. Here is the relevant code (this is all in the .m file, very simple .h file):

.h (header) file:

#import <UIKit/UIKit.h>

@interface Login_ViewController : UIViewController <UITextFieldDelegate>

@end

.m (implementation) file:

- (void)viewDidLoad {
    UILabel *emailLabel = [[UILabel alloc] initWithFrame:CGRectMake(31.0f, 75.0f, 256.0f, 20.0f)];
    emailLabel.text = @"Email Address";
    emailLabel.backgroundColor = [UIColor grayColor];
    emailLabel.textColor = [UIColor whiteColor];
    UITextField *emailField = [[UITextField alloc] initWithFrame:CGRectMake(31.0f, 100.0f, 256.0f, 32.0f)];
    emailField.delegate = self;
    emailField.placeholder = @"Enter email address";
    emailField.borderStyle = UITextBorderStyleRoundedRect;
    [self.view addSubview:emailLabel];
    [self.view addSubview:emailField];
    UILabel *passLabel = [[UILabel alloc] initWithFrame:CGRectMake(31.0f, 175.0f, 256.0f, 20.0f)];
    passLabel.text = @"Password";
    passLabel.backgroundColor = [UIColor grayColor];
    passLabel.textColor = [UIColor whiteColor];
    UITextField *passField = [[UITextField alloc] initWithFrame:CGRectMake(31.0f, 200.0f, 256.0f, 32.0f)];
    passField.delegate = self;
    passField.placeholder = @"Enter Password";
    passField.borderStyle = UITextBorderStyleRoundedRect;
    passField.secureTextEntry = YES;
    UIButton *loginButton = [[UIButton alloc] initWithFrame:CGRectMake(31.0f, 275.0f, 256.0f, 32.0f)];
    [loginButton setTitle:@"Login" forState:UIControlStateNormal];
    [loginButton addTarget:self action:@selector(authenticate) forControlEvents:UIControlEventTouchUpInside];
    [self.view addSubview:passLabel];
    [self.view addSubview:passField];
    [self.view addSubview:loginButton];
    [self.view setBackgroundColor: [UIColor grayColor]];
    UIAlertView *noAccount = [[UIAlertView alloc] initWithTitle:@"Welcome to T-of-U" message:@"Please enter your TofU credentials." delegate:self cancelButtonTitle:@"continue" otherButtonTitles: nil];
    [noAccount show];
    [super viewDidLoad];
}

also in the .m file (IBAction for the button):

-(IBAction)authenticate:(id)sender{
    // This will be the validation code
    // if we validate we will set the default Setting with accountID here.
    UIAlertView *auth1 = [[UIAlertView alloc] initWithTitle:@"You clicked a button" message:@"Oh, do you want to login do you?" delegate:self cancelButtonTitle:@"Sure.. Why not!" otherButtonTitles: nil];
    [auth1 show];
}

I am using a storyboard with a View that has a custom class of login_ViewController and it loads successfully based on a check for a local setting for account ID. This window is only pulled up if they have not logged in before, so I can authenticate and get that account ID to store locally. I am still experimenting with programmatically creating all these items as you can see in the viewDidLoad section.

Is this failing because I don’t have the button/IBAction reference in the .h file , or is it ok to use the addTarget method where its all in the .m file?

Crash info from the log:

2012-01-08 04:54:32.868 TofU-5[10452:10103] *** Terminating app due to uncaught exception 'NSInvalidArgumentException', reason: '-[Login_ViewController authenticate]: unrecognized selector sent to instance 0x6c94780'

Answer 1

Your fix may be as easy as:

[loginButton addTarget:self action:@selector(authenticate:) forControlEvents:UIControlEventTouchUpInside];

(add a COLON after the authenticate name in your @selector). If a method takes any parameter, there will be a colon after it and for the run time environment to be able to send a message to it, that colon needs to be included in the selector of the action target.

I hope this makes sense and helps you out!

p.s. it wouldn’t hurt to add

-(IBAction)authenticate:(id)sender

to the .h file.

IBAction here is an abbreviation for InterfaceBuilder-Action so this actually helps a lot more if you’re building the interface in a XIB file. Because you’re setting the target programatically, you could just use -(void) authenticate (no colon, because your method uses or needs no parameters), but don’t do this yet until you get more overall experience with Objective C coding and actions.