block 反向传值详细解释

1. 两个控制器，viewcontroller---push---secondviewcontroller。。。viewcontroler中有一个button（用来push到第二个控制器，还有一个lable（用来展示第二个页面blcok传过来的值
2. 下面贴过来两个控制器中详细代码

viewcontroller.h

typedef NSString *(^mssblock)(NSString *);
@interface ViewController : UIViewController
@property (nonatomic ,copy) mssblock mss;
@property (weak, nonatomic) IBOutlet UILabel *showtexlable;
@end

viewcontroller.m

#import "ViewController.h"
#import "SecondViewController.h"
@interface ViewController ()
@end
@implementation ViewController
- (IBAction)pushpressed:(id)sender {
    SecondViewController *vc =[SecondViewController new];
    [self.navigationController pushViewController:vc animated:YES];
    [vc returnmessage:^(NSString *str) {
        self.showtexlable.text =str;
    }];
}
@end

secondviewcontroller.h

#import 

typedef void(^mssblcokdemo)(NSString *str);
@interface SecondViewController : UIViewController
@property (weak, nonatomic) IBOutlet UITextField *textfild;
@property (nonatomic ,strong) mssblcokdemo block;
-(void)returnmessage:(mssblcokdemo)block;
@end

secondviewcontroller.m

#import "SecondViewController.h"
@interface SecondViewController ()
@end
@implementation SecondViewController
- (IBAction)backpressed:(id)sender {
    self.block(_textfild.text);
    [self.navigationController popViewControllerAnimated:YES];
}
-(void)returnmessage:(mssblcokdemo)block{
    self.block=block;
}
@end

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