block是個好東西，是蘋果官方推薦的一種方法，效率高，程式碼簡單，今天用block主要是做一個簡單傳，暫時不對block詳細介紹，關於__block的用法，後期會一一介紹；

接下來，通過一個block，將 SecondViewController 的數值傳給  FirstViewController ;

FirstViewController.m

#import "FirstViewController.h"
#import "SecondViewController.h"
@interface FirstViewController ()<UITextFieldDelegate>
{
    //切換到下一個介面
    UIButton *_btn;
    
    //現實從 SecondViewController 傳過來的值
    UILabel *_label;
}

@end

@implementation FirstViewController

- (void)viewDidLoad {
    [super viewDidLoad];
    [self.view setBackgroundColor: [UIColor yellowColor]];
    
    [self layouUI];
    
}

-(void)layouUI
{
//    建立lable
    _label=[[UILabel alloc] initWithFrame:CGRectMake(100, 60, 120, 40)];
    _label.backgroundColor=[UIColor whiteColor];
    [self.view addSubview:_label];
    
    //切換到 SecondViewController
    _btn=[[UIButton alloc]initWithFrame:CGRectMake(100, 220, 120, 40)];
    [_btn setBackgroundColor:[UIColor cyanColor]];
    [_btn addTarget:self action:@selector(dobtn) forControlEvents:UIControlEventTouchDown];
    [self.view addSubview:_btn];
}

-(void)dobtn
{
    SecondViewController *second=[[SecondViewController alloc]init];
    second.mb=^(NSString *str){
        _label.text=str;
    };
    [self.navigationController pushViewController:second animated:YES];
    
}
@end
 

建立第二個控制器：

SecondViewController.h

#import <UIKit/UIKit.h>

//定義block
typedef void(^Myblock)(NSString *str);

@interface SecondViewController : UIViewController

@property(strong,nonatomic)Myblock mb;


@end

SecondViewController.m

#import "SecondViewController.h"

@interface SecondViewController ()
{
    UIButton *_btn;
    UILabel *_label;
    UITextField *_textField;
}

@end

@implementation SecondViewController

- (void)viewDidLoad {
    [super viewDidLoad];
    [self.view setBackgroundColor:[UIColor purpleColor]];
    [self layouUI];
}
-(void)layouUI
{
    //    建立一個textfield
    _textField=[[UITextField alloc]initWithFrame:CGRectMake(100, 140, 120, 40)];
    _textField.borderStyle=UITextBorderStyleLine;
    [self.view addSubview:_textField];

    
    _btn=[[UIButton alloc]initWithFrame:CGRectMake(100, 220, 120, 40)];
    [_btn setBackgroundColor:[UIColor cyanColor]];
    [_btn addTarget:self action:@selector(dobtn) forControlEvents:UIControlEventTouchDown];
    [self.view addSubview:_btn];
}
-(void)dobtn
{
//    在這裡呼叫block
    self.mb(_textField.text);
    [self.navigationController popViewControllerAnimated:YES];
}

@end
 

然後在 _textField 寫入文字，例如寫 welcome to air

如下圖：

在第一個控制器上的label就會顯示出來第二控制器上寫的字串；

如下圖：