为什么字符串值为空？

Question

为什么字符串值得到空值。有人请找出错误吗？ 当从 SecondViewController 调用该方法时，我如何获取字符串值我需要另一个字符串还是有人请提出一些更正建议。

我需要这个字符串有一些价值

@property (nonatomic,strong) NSString *str;

当下面的方法被调用时。

[查看getTotal];

标签在第一个视图控制器中。

提前致谢。

在 ViewController.m 中

#import "ViewController.h"
#import "SecondViewController.h"
@interface ViewController ()
{
    NSInteger total;

}
@property (weak, nonatomic) IBOutlet UILabel *lbl;
@property (weak, nonatomic) IBOutlet UIButton *btn;
@property (nonatomic,strong) NSString *str;

- (id)initWithInitialNumber:(NSInteger)initialNumber;

- (void)addNumber:(NSInteger)newNumber;

- (NSInteger)getTotal;

@end

@implementation ViewController


- (void)viewDidLoad {
    [super viewDidLoad];
    NSLog(@"%@",_str);
    self.lbl.text = _str;
    // Do any additional setup after loading the view, typically from a nib.
}
-(void)viewDidAppear:(BOOL)animated{
    [super viewDidAppear:animated];
    NSLog(@"viewDidAppear called");
    [self viewDidLoad];
}
-(id)initWithInitialNumber:(NSInteger)initialNumber{

    total = initialNumber;
    return self;

}

-(void)addNumber:(NSInteger)newNumber{

    total += newNumber;

}
-(NSInteger)getTotal{

    NSLog(@"Number is: %ld",total);

    self.str = [NSString stringWithFormat:@"%ld",total]; 
    NSLog(@"%@",self.str);
    return total;
}

- (IBAction)btnAct:(id)sender {

    UIStoryboard *sb = [UIStoryboard storyboardWithName:@"Main" bundle:nil];
    SecondViewController *SC = [sb instantiateViewControllerWithIdentifier:@"SC"];
    [self presentViewController:SC animated:YES completion:nil];


}

-(NSString *)str{
    return _str;
}


- (void)didReceiveMemoryWarning {
    [super didReceiveMemoryWarning];
    // Dispose of any resources that can be recreated.
}


@end

在 SecondViewController.m 中

#import "SecondViewController.h"
#import "ViewController.h"

@interface SecondViewController ()<Sendata>
{
    NSInteger *num;
}

@end

@implementation SecondViewController
- (IBAction)backBtn:(id)sender {

    ViewController *VC = [self.storyboard instantiateViewControllerWithIdentifier:@"VC"];

    [self presentViewController:VC animated:YES completion:nil];

}

- (void)viewDidLoad {
    [super viewDidLoad];
    ViewController *view = [[ViewController alloc]initWithInitialNumber:10];
    view.delegate = self;
    [self Print:@"Hello"];
    [view addNumber:2];
    [view getTotal];
    // Do any additional setup after loading the view.
}

- (void)didReceiveMemoryWarning {
    [super didReceiveMemoryWarning];
    // Dispose of any resources that can be recreated.
}
-(void)Print:(NSString *)str{

    NSLog(@"%@",str);

}
@end

Answer 1

我从您的代码中可以理解的是，您在单击按钮时从 ViewController 呈现 secondViewController。在第二个视图控制器上，您要添加两个数字并再次在 Firstview 控制器上显示它们。

what I can see from your code , you are pushing the new instance of ViewController every time. So thevalue you are intialising in your view didLoad in diffrent object then the one your pushing again on button action of back. IF you can modify you code like this , it will work : 


@interface ViewController ()
{
    NSInteger total;

}
@property (weak, nonatomic) IBOutlet UILabel *lbl;
@property (weak, nonatomic) IBOutlet UIButton *btn;
@property (nonatomic,strong) NSString *str;

- (id)initWithInitialNumber:(NSInteger)initialNumber;

- (void)addNumber:(NSInteger)newNumber;

- (NSInteger)getTotal;

@end
@implementation ViewController


- (void)viewDidLoad {
    [super viewDidLoad];
    NSLog(@"%@",_str);
    self.lbl.text = _str;
    // Do any additional setup after loading the view, typically from a nib.
}
-(void)viewDidAppear:(BOOL)animated{
    [super viewDidAppear:animated];
    NSLog(@"viewDidAppear called");
    [self viewDidLoad];
}
-(id)initWithInitialNumber:(NSInteger)initialNumber{

    total = initialNumber;
    return self;

}

-(void)addNumber:(NSInteger)newNumber{

    total += newNumber;

}
-(NSInteger)getTotal{

    NSLog(@"Number is: %ld",total);

    self.str = [NSString stringWithFormat:@"%ld",total];
    NSLog(@"%@",self.str);
    return total;
}

- (IBAction)btnAct:(id)sender {

    UIStoryboard *sb = [UIStoryboard storyboardWithName:@"Main" bundle:nil];
    SecondViewController *SC = [sb instantiateViewControllerWithIdentifier:@"SC"];
    [self.navigationController pushViewController:SC animated:YES];

}

-(NSString *)str{
    return _str;
}


- (void)didReceiveMemoryWarning {
    [super didReceiveMemoryWarning];
    // Dispose of any resources that can be recreated.
}


@end


@interface SecondViewController ()
{
    NSInteger *num;
    ViewController *viewController;
}

@end

@implementation SecondViewController
- (IBAction)backBtn:(id)sender {

    [self.navigationController popViewControllerAnimated:YES]


}

- (void)viewDidLoad {
    [super viewDidLoad];
    NSMutableArray *VCs = [NSMutableArray arrayWithArray: self.navigationController.viewControllers];
    if ([[VCs objectAtIndex:[VCs count] - 2] isKindOfClass:[ViewController class]])
    {
        ViewController *view = [[VCs objectAtIndex:[VCs count] - 2]
                                view.delegate = self;
                                [self Print:@"Hello"];
                                [view addNumber:2];
                                [view getTotal];
    }

    // Do any additional setup after loading the view.
}

- (void)didReceiveMemoryWarning {
    [super didReceiveMemoryWarning];
    // Dispose of any resources that can be recreated.
}
-(void)Print:(NSString *)str{

    NSLog(@"%@",str);

}
@end

我已经获取了导航堆栈中已经存在的 ViewController 的实例，更改了值，然后我弹出了 SecondViewController。