Block

1、用法1 传值

B页面向A页面反向传值

在B.h中 声明一个block属性

@property(nonatomic,copy)void (^myBlock)(NSString *t);

在b.m中

- (IBAction)button:(id)sender {
    self.myBlock(@"fdslfjdslfjsdlk");
    [self dismissViewControllerAnimated:YES completion:nil];
}

在A.m中

    UIStoryboard *sb = [UIStoryboard storyboardWithName:@"Main" bundle:nil];
    
    ViewControllerB *vc = [sb instantiateViewControllerWithIdentifier:@"vcm"];
    vc.myBlock = ^(NSString *st){
    
        [self.label setText:st];
        
    };
    [self presentViewController:vc animated:YES completion:nil];