Objective-C，如何从另一个类访问实例变量 [英] Objective-c, how to access an instance variable from another class

问题描述

我习惯于用Java编程并使用类变量来访问其他类中的数据.然后我发现类变量在Obj-C中的工作方式不同，并且存在问题.

I am used to programming in Java and to use class variables to access data from other classes. Then I found out that class variables does not work the same way in Obj-C, and are having problems with that.

我的问题是，用户登录后，我想在另一个类中访问用户输入的密码.在不同的论坛中都有所读过，因此我应该使用类方法(+)来访问这些数据.但是因为我需要在第二类中创建第一个类的新实例，所以这意味着输入的密码在第一类的新实例中不存在.

My problem is that i want to access the user inputted password in another class after the user is logged in. Have read in different forums and such that I should use class methods (+) to access these data. But because I need to create a new instance of the first class in class two it means that the inputted password does not exist in the new instance of class one.

我的代码如下:

class1.h

@interface class1 : UIViewController {
    UITextField *usernameField;
    UITextField *passwordField;
        UIButton *loginButton;
        NSString *password;
}

@property (nonatomic, retain) IBOutlet UITextField *usernameField;
@property (nonatomic, retain) IBOutlet UITextField *passwordField;
@property (nonatomic, retain) IBOutlet UIButton *loginButton;
@property (nonatomic, retain) NSString *password;

-(IBAction) loginButtonPushed;
+(NSString *)password;

@end

class1.m

#import "viewSwitcherViewController.h"

@implementation viewSwitcherViewController

@synthesize usernameField;
@synthesize passwordField;
@synthesize loginButton;
@synthesize password; // not needed

-(IBAction) loginButtonPushed {
        password = passwordField.text; //not needed
    // ...Code for switching view if successful login... EDITED:
    class2 *c2 = [class2 alloc] init]; //Instantiating new class2 object
    c2.password = passwordField.text; //Assigning tekst from passworField to thePassword-variable in the class2 instance.
}

class2.m

#import "class2.h"
#import "class1.h"

@implementation class2

@synthesize thePassword; //NSString variable

// this method is also not needed
-(void) someMethodUsingPassword {
         class1 *c1 = [[class1 alloc] init];
         thePassword = [c1 password];
         NSLog(@"The password is: %@", thePassword); //This call returns null
}

所以我的问题是，在class2中创建的c1实例不保存提交的密码，因此返回"null".

So my problem is that the c1 instance created in class2 does not hold the submitted password and therefore returns "null".

也许这只是我的Java方法搞砸了，但是我找不到其他方法，请帮忙:)！

Maybe this is just my Java approach messing it up, but I couldn't find any other way so please help:)!

推荐答案

Java中的公共类变量等效于C和Objective-C中的全局变量.您可以将其实现为:

A public class variable in Java is equivalent to a global variable in C and Objective-C. You would implement it as:

class1.h

extern NSString* MyGlobalPassword;

class1.m

NSString* MyGlobalPassword = nil;

然后，任何导入class1.h的人都可以读写MyGlobalPassword.

Then, anyone that imports class1.h can read and write to MyGlobalPassword.

一种更好的方法是通过类方法"进行访问.

A slightly better approach would be make it accessible via a "class method".

class1.h:

@interface Class1 : UIViewController {
    UITextField *usernameField;
    UITextField *passwordField;
    UIButton *loginButton;    
}
+ (NSString*)password;
@end

class1.m:

static NSString* myStaticPassword = nil;

@implementation Class1
+ (NSString*)password {
    return myStaticPassword;
}

- (void)didClickLoginButton:(id)sender {
    [myStaticPassword release];
    myStaticPassword = [passwordField.text retain];
}

然后其他类将通过password类方法读取密码.

Other classes would then read the password through the password class method.

Class2.m:

-(void) someMethodUsingPassword {
     thePassword = [Class1 password]; // This is how to call a Class Method in Objective C
     NSLog(@"The password is: %@", thePassword); 
}

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