如何在objective-C中处理继承(iOS sdk)

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

我刚开始学习iOS编程，我有继承问题。有两个文件。

First file

第一个文件

Header

头

#import <UIKit/UIKit.h>
@interface ViewController : UIViewController {
    int x;
}
@end

Implementation:

实现:

#import "ViewController.h"
#import "NewClass.h"
@implementation ViewController
#pragma mark - View lifecycle
- (void)viewDidLoad
{
    [super viewDidLoad];
    x = 999;
    NewClass *myClass = [[[NewClass alloc] init] autorelease];
}
@end

Second file

第二个文件

Header:

标题:

#import "ViewController.h"

@interface NewClass : ViewController 
@end

Implementation:

实现:

#import "NewClass.h"
@implementation NewClass

-(id)init { 
    self = [super init];                                       
    if (self != nil) {                                            
        NSLog(@"%i",x);
    }
    return self;                           
}
@end

In ViewController I set x to 999, and in NewClass I want to get it, but when I call NSLog(@"%i",x); it gives me 0.

在ViewController中，我将x设为999，在NewClass中，我想要得到它，但是当我调用NSLog(@ % I,x);它给我0。

Where did I make a mistake?

我哪里出错了?

3 个解决方案

#1

You have a timing problem.

你有时间问题。

The init method gets called first (at all levels of the inheritance hierarchy, so in both ViewController and NewClass). This is when you print out your value of x, when it is still zero.

init方法首先被调用(在继承层次结构的所有级别上，所以在ViewController和NewClass中)。这是当你打印出x的值时，它还是0。
The viewDidLoad method only gets called much later, generally at a point after a view controller's view has been added to a superview. It's functionality that's specific to the UIViewController class.

viewDidLoad方法只会在以后调用，通常是在视图控制器的视图添加到父视图之后。它是特定于UIViewController类的功能。

To make your example work, put an init method in your ViewController class that looks like the one in your NewClass class, and set x there.

要使示例正常工作，请在ViewController类中放置一个init方法，该方法与NewClass类中的方法类似，并在那里设置x。

Also, you don't need to create a NewClass instance within ViewController. When you create a NewClass object, it is automatically a ViewController as well. In the same way that a dog is an animal automatically, and so is a cat. When you create a dog, you don't want to create an animal as well!

同样，您不需要在ViewController中创建一个NewClass实例。当你创建一个NewClass对象时，它也是一个ViewController。就像狗是自动的动物一样，猫也是一样。当你创造一个狗，你也不想创造一个动物!

As sidyll says, you should probably do a bit more reading about how inheritance works, I'm afraid!

正如sidyll所说，恐怕你应该多读一些关于遗传是如何运作的文章。

#2

You need to review you OOP concepts. Object-Oriented Programming with Objective-C is a must.

您需要检查OOP概念。必须使用Objective-C进行面向对象编程。

Your class NewClass indeed inherits the x variable, but not it's value. When you create an instance of it, you're creating a shiny new instance whose values have nothing to do with the parent class.

类NewClass确实继承了x变量，但不是它的值。当您创建它的实例时，您正在创建一个闪亮的新实例，其值与父类无关。

Another point of view to help you is that x was set in a object of ViewController class. The NewClass inherits from ViewController class, not from an arbitrary instance (object, where you set x).

另一个帮助你的观点是x被设置在一个ViewController类的对象中。NewClass继承自ViewController类，而不是从任意实例(对象，在其中设置x)继承。

#3

That's because -viewDidLoad is not called until well after -init returns. Your superclass should do configuration like that in its -init method.

这是因为-viewDidLoad直到-init返回后才被调用。您的超类应该在其-init方法中执行类似的配置。

#1