如何在iOS sdk中使用继承

Question

I just started to learn iOS programming and I have a problem with inheritance. There are 2 files.

First file

Header

#import <UIKit/UIKit.h>
@interface ViewController : UIViewController {
    UILabel *myLabel;
}
@property (nonatomic, retain) IBOutlet UILabel *myLabel;
@end

Implementation:

#import "ViewController.h"
#import "NewClass.h"
@implementation ViewController
@synthesize myLabel;
#pragma mark - View lifecycle
- (void)viewDidLoad {
    [super viewDidLoad];
    myLabel.text = @"ViewController text!";    
    NewClass *myClass = [[[NewClass alloc] init] autorelease];
    [myClass setLabelText];
}
@end

Second file

Header:

#import "ViewController.h"
@interface NewClass : ViewController 
-(void) setLabelText ;
@end

Implementation:

#import "NewClass.h"
@implementation NewClass
- (void) setLabelText {
    myLabel.text = @"NewClass text!";
}
-(id)init { 
    self = [super init];                                       
    if (self != nil) {                                         
    }
    return self;                           
}
@end

And i set myLabel outlet in IB.

Why when i call [myClass setLabelText]; nothing happens? There also remained "ViewController text!" on label view. Where is my problem? How can i change ViewController::myLabel.text in NewClass

Answer 1

This is not inheritance that you are doing. In your ViewController implementation, you have the following code

 NewClass *myClass = [[[NewClass alloc] init] autorelease];
[myClass setLabelText];

This is just creating an instance of the class NewClass and setting the labelText on that instance. You are never displaying it or adding it to a view.

Answer 2

This example may help u where u can change label text by clicking on button.implement the below code in ur example

//class1.h

IBOutlet UILabel *text1;

@property(nonatomic,retain)UILabel *text1;

-(IBAction)press:(id)sender;

//class1.m

-(IBAction)press:(id)sender

{

Class2 *method = [[Class2 alloc]init];
NSString *string = [method sayhello];

[method release];
text1.text = string;

}

//class2.h

{

}

-(NSString *)sayhello;

@end

//class2.m

-(NSString *)sayhello

{

return @"hello";

}

connect button to file owner..

Answer 3

NewClass *myClass = [[[NewClass alloc] init] autorelease];     
[myClass setLabelText]; 

Here you are creating the new view controller via alloc / init (not usual if you have a backing xib file for the view controller, you would usually use initWithNibName:) and then setting a property on an IBOutlet. At this point, the outlet will be nil since you haven't loaded a nib.

Also, as Nick Bull points out, you are just creating an instance, not displaying it anywhere. So even if you did use initWithNibName, you wouldn't see your changes unless you presented the view controller somehow.

Answer 4

Wait... from the looks of it you are going to have some serious issues with stack overflows: when you load a ViewController, it in turn creates myClass. Because myClass is a subclass of ViewController, this in turn will create it's own variable 'myClass' when loaded... etc. This will continue forever, or until you get a stack overflow error.

In fact, I'm pretty confused by your code. Perhaps it might be time to rethink what you are trying to achieve and what your code should be doing?