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

#import <UIKit/UIKit.h>

//为要声明的Block重新定义了一个名字myBlock
typedef void (^myBlock)(NSString *str);// 一会要传的值为NSString类型

@interface SecondViewController : UIViewController
// 声明block属性
@property (nonatomic, copy) myBlock block;
// 声明block方法
- (void)getStr:(myBlock)block;

@end

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

#import "SecondViewController.h"
@interface SecondViewController ()

@end

@implementation SecondViewController

- (void)viewDidLoad {
    [super viewDidLoad];
    
}

- (void)getStr:(myBlock)block {
    
    //把传进来的Block块保存实例变量block中（SecondViewController.h中定义的属性）
    self.block = block;
    
    //使用block要要确定其不为nil
    if (self.block != nil) {
        self.block(@"要传给FirstViewController的字符串");
    }
}

@end

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

#import "FirstViewController.h"
#import "SecondViewController.h"

@interface FirstViewController ()
@property (nonatomic, strong) SecondViewController *secondVc;
@end

@implementation FirstViewController

- (void)viewDidLoad {
    [super viewDidLoad];
    
    //先拿到第二个控制器
    SecondViewController *secondVc = [[SecondViewController alloc]init];
    _secondVc = secondVc;
    
    //添加获取字符串按钮
    [self addGetStrBtn];
    
}

- (void)addGetStrBtn {
    UIButton *btn = [[UIButton alloc]initWithFrame:CGRectMake(150, 250, 50, 40)];
    [self.view addSubview:btn];
    [btn setTitle:@"获取" forState:UIControlStateNormal];
    btn.backgroundColor = [UIColor redColor];
    [btn addTarget:self action:@selector(getStr) forControlEvents:UIControlEventTouchUpInside];
}


- (void)getStr {
    [_secondVc getStr:^(NSString *str) {
        NSString *string = str;
        NSLog(@"从SecondViewController获取的字符串为:%@",string);
    }];
}

@end