iOS Objective-C块回调问题

Question

我想用纯代码在viewController之间创建UI练习块传递值。但回调块不起作用。 NSLog方法没有打印任何东西在调试区域。这是代码。给我一些提示，谢谢。

VC.h

#import <UIKit/UIKit.h> 

@interface SecondViewController : UIViewController 
    @property (copy, nonatomic) void (^callBack)(NSString *text); 
@end 

VC.m

- (UITextField *)textField { 
    if (!_textField) { 
     _textField = [[UITextField alloc] init]; 
     _textField.backgroundColor = [UIColor whiteColor]; 
    } 
    return _textField; 
} 

- (UIButton *)button { 
    if (!_button) { 
     _button = [[UIButton alloc] init]; 
     _button.backgroundColor = [UIColor blueColor]; 
     [_button addTarget:self action:@selector(buttonAction) forControlEvents:UIControlEventTouchUpInside]; 
    } 
    return _button; 
} 

- (void)setupUI { 
    [self.view addSubview:self.textField]; 
    [self.view addSubview:self.button]; 

    [self.textField mas_makeConstraints:^(MASConstraintMaker *make) { 
     make.width.mas_equalTo(200); 
     make.height.mas_equalTo(50); 
     make.centerX.mas_equalTo(self.view.mas_centerX); 
     make.centerY.mas_equalTo(self.view); 
    }]; 

    [self.button mas_makeConstraints:^(MASConstraintMaker *make) { 
     make.width.mas_equalTo(200); 
     make.height.mas_equalTo(50); 
     make.centerX.mas_equalTo(self.view); 
     make.centerY.mas_equalTo(self.view).offset(100); 
    }]; 
} 

- (void)buttonAction { 

    NSString *str = self.textField.text; 
    if (self.callBack != nil) { 
     self.callBack(str); 
     NSLog(@"This statement didnt print in log"); 
    } 
} 

- (void)viewDidLoad { 
    [super viewDidLoad]; 
    [self setupUI]; 
    // Do any additional setup after loading the view. 

    self.view.backgroundColor = [UIColor redColor]; 
} 

---

更新代码

VC2.m

- (void)viewWillAppear:(BOOL)animated{ 

      self.callBack = ^(NSString *text){ 

      }; 
    } 
- (void)buttonAction { 

    if (self.callBack) { 
     NSLog(@"It worked on debug area %@", self.textField.text); 
     self.callBack(self.textField.text); 
    } 
    self.textField.text = @""; 
} 

VC1.m

- (void)viewDidLoad { 
    [super viewDidLoad]; 
    _secondVc = [[SecondViewController alloc] init]; 
    _secondVc.callBack = ^(NSString *str){ 

    }; 
    [self setupUI]; 

    self.view.backgroundColor = [UIColor greenColor]; 
} 

- (void)viewWillAppear:(BOOL)animated { 

    if (_secondVc.callBack != nil) { 
     NSLog(@"It wrked on debug screen"); 
     _secondVc.callBack = ^(NSString *str){ 
      NSLog(@"It didn't worked on debug screen"); 
      //I want set my label.text = str; 
     }; 
}; 

}

Answer 1

唯一的办法是你财产

@property (copy, nonatomic) void (^callBack)(NSString *text); 

是空的。尝试在buttonAction方法中放置断点并查看属性。

Answer 2

由于桑德和克里希纳CA提到你的callBack是零。我建议你创建块的这样一个定义：

typedef void(^TextBlock)(NSString *text); 

那么你的属性更改为：

@property (copy, nonatomic) TextBlock callBack; 

在你的第一个视图控制器创建块的副本：

@interface FirstViewController() 

@property (copy, nonatomic) TextBlock firstViewControllerCallBack; 

@end 

初始化回调拷贝（即在viewDidLoad中）

- (void)viewDidLoad { 
    [super viewDidLoad]; 
    self.firstViewControllerCallBack = ^(NSString *text){ 
     NSLog(@"Second view controller's button tapped!"); 
    }; 
} 

正确呈现前指定回调第二视图控制器/推：

SecondViewController *secondVC = [[SecondViewController alloc] init]; 
secondVC.callBack = self.firstViewControllerCallBack; // Assign the callback 
// ... Presenting the view controller 

清理完毕块，你用它做后（即在viewWillDisappear）：

- (void)viewWillDisappear:(BOOL)animated { 
    [super viewWillDisappear:animated]; 
    self.firstViewControllerCallBack = nil; 
}