类之间的NSString [英] NSString between classes

问题描述

我需要在我的应用程序的两个类之间传递一个NSString（从ViewController Class_A到ViewController Class_B），我在StackOverflow上搜索，但这是我的情况的简化代码：

Class_A.h

  #importClass_B.h
 
 @interface ViewController： UIViewController {
 NSString * stringToPass; 
} 
 
 @property（retain，nonatomic）NSString * stringToPass; 
  - （NSString *）stringPassage：（id）sender; 
 
 @end 
  

Class_A.m

   - （NSString *）stringPassage：（id）sender {
 NSString * string = @A FANTASTIC STRING！ //这里我定义string INTO  - （NSString *）方法
 NSLog（@String to pass：％@，string）; 
 return string; 
} 
  

Class_B.h

  #importClass_A.h
  

Class_B.m

   - （void）viewDidLoad 
 {
 [super viewDidLoad]; 
 
 Class_A * Class_A_Instance = [[Class_A alloc] init]; 
 NSString * importedString = [Class_A_Instance stringPassage：self]; 
 [Class_A_Instance release]; 
 
 NSLog（@Received String：％@，importedString）; 
 
 //从其nib加载视图后执行任何其他设置。 
} 
  

好吧，它的作用就像一个魅力！当Class_B加载时，我可以在日志输出中读取String to pass：A FANTASTIC STRING！和Received String：A FANTASTIC STRING！;但如果我只改变这部分代码（我需要它！）：

Class_A.m

   - （void）viewDidLoad 
 {
 [super viewDidLoad]; 
 
 stringToPass = @ANOTHER FANTASTIC STRING！; 
 
 //在加载视图之后执行任何其他设置，通常来自nib。 
} 
 
  - （NSString *）stringPassage：（id）sender {
 NSString * string = stringToPass; //这里我必须调用字符串FROM另一个Class_A方法（在这个例子中是从 - （void）viewDidLoad 
 NSLog（@String to pass：％@，string）; 
 return string; 
} 
  

我可以在日志输出中读取String to pass： obvReceived String：（null）;它令人难以置信，因为我可以将全局变量* stringToPass完全传递给Class_A的每个方法，除了这个 - （NSString *）方法。

解决方案

让我们有两个viewControllers，即FirstVC和SecondVC。
在SecondVC中创建一个字符串，

@interface SecondVC：UIViewController {NSString * string2pass;}

@property（retain，nonatomic）NSString * string2pass;


在FirstVC中要显示SecondVC时，传递字符串为

   - （void）presentSecondView 
 {
 SecondVC * secondVC = [[SecondVC alloc] initWithNibName：@SecondVCbundle：nil]; 
 
 secondVC.string2pass = @这是一个传递字符串; 
 
 [self presentModalViewController：resView animated：YES]; 
 
} 
  - （void）viewDidLoad 
 {
 NSString * string4mFirstVC = string2Pass 
} 
  pre> 
I need to pass a NSString between two Classes (from ViewController Class_A to ViewController Class_B) of my app, I have searched on StackOverflow but this is the simplified code of my situation:

Class_A.h
#import "Class_B.h"

@interface ViewController : UIViewController {
    NSString *stringToPass;
}

@property (retain, nonatomic) NSString *stringToPass;
-(NSString *)stringPassage:(id)sender;

@end
Class_A.m
- (NSString *)stringPassage:(id)sender{
    NSString *string = @"A FANTASTIC STRING!"; //here I define string INTO the - (NSString *) method
    NSLog(@"String to pass: %@",string);
    return string;
}
Class_B.h
#import "Class_A.h"
Class_B.m
- (void)viewDidLoad
{
    [super viewDidLoad];

    Class_A *Class_A_Instance = [[Class_A alloc] init];
    NSString *importedString = [Class_A_Instance stringPassage:self];
    [Class_A_Instance release];

    NSLog(@"Received String: %@",importedString);

    // Do any additional setup after loading the view from its nib.
}
Well, it works like a charm! When Class_B is loaded I can read in the log output "String to pass: A FANTASTIC STRING!" and "Received String: A FANTASTIC STRING!"; but if I change only this parts of code (I need it!):

Class_A.m
 - (void)viewDidLoad
{
    [super viewDidLoad];

    stringToPass = @"ANOTHER FANTASTIC STRING!";

        // Do any additional setup after loading the view, typically from a nib.
   }

- (NSString *)stringPassage:(id)sender{
    NSString *string = stringToPass; // here I must call the string FROM another method of Class_A (in this example from - (void)viewDidLoad
    NSLog(@"String to pass: %@",string);
    return string;
}
I can read in the log output "String to pass: (null)" and obv "Received String: (null)"; Its incredible because I can pass the global variable *stringToPass to every method of Class_A perfectly, except for this -(NSString *) method. So what is the problem? Thanks!
解决方案
Let we have two viewControllers i.e FirstVC and the SecondVC. 
Make a string in SecondVC ,
@interface SecondVC : UIViewController { NSString *string2pass; }

@property (retain, nonatomic) NSString *string2pass;
In FirstVC when you are going to present SecondVC, pass the string as, 
- (void)presentSecondView 
  { 
    SecondVC *secondVC = [[SecondVC alloc]initWithNibName:@"SecondVC" bundle:nil]; 

    secondVC.string2pass = @"this is a passing string";

   [self presentModalViewController:resView animated:YES];

  }    
- (void)viewDidLoad 
  { 
      NSString *string4mFirstVC = string2Pass 
  }


                        
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