Block传值讲解与使用

需求:在FirstViewController中点击"接收"按钮接收来自SecondViewController的block传递的值

步骤如下:

1.在SecondViewController .h文件中声明block

 1 #import <UIKit/UIKit.h>
 2 
 3 //为要声明的Block重新定义了一个名字myBlock
 4 typedef void (^myBlock)(NSString *str);// 一会要传的值为NSString类型
 5 
 6 @interface SecondViewController : UIViewController
 7 // 声明block属性
 8 @property (nonatomic, copy) myBlock block;
 9 // 声明block方法
10 - (void)getStr:(myBlock)block;
11 
12 @end

2.在SecondViewController .m文件中实现 - (void)getStr:(myBlock)block方法

 1 #import "SecondViewController.h"
 2 @interface SecondViewController ()
 3 
 4 @end
 5 
 6 @implementation SecondViewController
 7 
 8 - (void)viewDidLoad {
 9     [super viewDidLoad];
10     
11 }
12 
13 - (void)getStr:(myBlock)block {
14     
15     //把传进来的Block块保存实例变量block中（SecondViewController.h中定义的属性）
16     self.block = block;
17     
18     //使用block要要确定其不为nil
19     if (self.block != nil) {
20         self.block(@"要传给FirstViewController的字符串");
21     }
22 }
23 
24 @end

3.在FirstViewController.m文件中接收SecondViewController.m的block传来的值

 1 #import "FirstViewController.h"
 2 #import "SecondViewController.h"
 3 
 4 @interface FirstViewController ()
 5 @property (nonatomic, strong) SecondViewController *secondVc;
 6 @end
 7 
 8 @implementation FirstViewController
 9 
10 - (void)viewDidLoad {
11     [super viewDidLoad];
12     
13     //先拿到第二个控制器
14     SecondViewController *secondVc = [[SecondViewController alloc]init];
15     _secondVc = secondVc;
16     
17     //添加获取字符串按钮
18     [self addGetStrBtn];
19     
20 }
21 
22 - (void)addGetStrBtn {
23     UIButton *btn = [[UIButton alloc]initWithFrame:CGRectMake(150, 250, 50, 40)];
24     [self.view addSubview:btn];
25     [btn setTitle:@"获取" forState:UIControlStateNormal];
26     btn.backgroundColor = [UIColor redColor];
27     [btn addTarget:self action:@selector(getStr) forControlEvents:UIControlEventTouchUpInside];
28 }
29 
30 
31 - (void)getStr {
32     [_secondVc getStr:^(NSString *str) {
33         NSString *string = str;
34         NSLog(@"从SecondViewController获取的字符串为:%@",string);
35     }];
36 }
37 
38 @end