Xcode 4 - Tab Bar application - Problem in executing third tab

问题

I have Xcode 4 and I created an application using the Tab Bar template (and not View based app). There is a UISwitch in each of these tabs and when I change it, a UILabel switches between ON and OFF. Very simple app and no confusion. Xcode 4 creates two tabs for me by default. I need a third tab as well, so I drag TabBarItem from the Objects Library and drop it on the existing TabBarController. I create a new file, subclass of UIViewController and the following code goes into three tabs.

The following is the interface

#import <UIKit/UIKit.h>
@interface FirstViewController : UIViewController {
    UISwitch *switch1;
    UILabel *status1;
}

@property (nonatomic,retain) IBOutlet UISwitch *switch1;
@property (nonatomic,retain) IBOutlet UILabel *status1;
- (IBAction) switch1Change;
@end

The following is the implementation

#import "FirstViewController.h"
@implementation FirstViewController
@synthesize switch1;
@synthesize status1;
- (IBAction) switch1Change
{
    if (switch1.on) 
        status1.text = @"ON";
    else
        status1.text = @"OFF";
}

The same code repeats for SecondViewController and ThirdViewController with ivars changing to switch2,status2 and switch3,status3. The link to the project is here

When I run it on the simulator, everything works fine for the first and second tab. When I open the third tab, I get the following error "Terminating app due to uncaught exception 'NSUnknownKeyException', reason: [ setValue: forUndefinedKey:]: this class is not key value coding-complaint for the key switch3."

When I remove the UISwitch from the ThirdView.xib, I don't get this error. Only when I add the switch control, I get this error. Can somebody please explain what is happening?

回答1:

In Interface Builder, your third view controller is of class UIViewController (and doesn't have outlets for status3 or switch3). Change its class to ThirdViewController, wire up the outlets, and it should work.

来源：https://stackoverflow.com/questions/5786862/xcode-4-tab-bar-application-problem-in-executing-third-tab