Block--两个界面 间回调传值

使用Block的地方很多，其中传值只是其中的一小部分，下面介绍Block在两个界面之间的传值：
首先，创建两个视图控制器，在第一个视图控制器中创建一个UILabel和一个UIButton，其中UILabel是为了显示第二个视图控制器传过来的字符串，UIButton是为了push到第二个界面。
第二个视图控制器

#import <UIKit/UIKit.h>typedef void (^PassValueBlock) (NSString *ShowText);@interface DetailViewController : UIViewController@property (nonatomic,copy) PassValueBlock ShowTextBlock;-(void)ShowTextValue:(PassValueBlock)block;@end

#import "DetailViewController.h"@interface DetailViewController ()@end@implementation DetailViewController- (void)viewDidLoad {    [super viewDidLoad];    UIButton *button=[UIButton buttonWithType:UIButtonTypeCustom];    button.backgroundColor=[UIColor blueColor];    button.frame=CGRectMake(100, 100, 100, 20);    [button setTitle:@"回调传值" forState:UIControlStateNormal];    [button addTarget:self action:@selector(PassValue:) forControlEvents:UIControlEventTouchUpInside];    [self.view addSubview:button];}-(void)PassValue:(UIButton *)sender{    [self.navigationController popViewControllerAnimated:YES];    //block 传值    if (self.ShowTextBlock != nil) {        self.ShowTextBlock(@"20");    }}-(void)ShowTextValue:(PassValueBlock)block{    //保存block    self.ShowTextBlock = block;}

第一个视图控制器

#import "RootViewController.h"#import "DetailViewController.h"@interface RootViewController ()@property (nonatomic,strong) UILabel *label;@end@implementation RootViewController- (void)viewDidLoad {    [super viewDidLoad];    //创建显示数据的label    self.label = [[UILabel alloc] initWithFrame:CGRectMake(100, 300, 40, 60)];    self.label.backgroundColor = [UIColor grayColor];    [self.view addSubview:self.label];    //创建button    UIButton *button = [UIButton buttonWithType:UIButtonTypeCustom];    button.backgroundColor = [UIColor blueColor];    button.frame = CGRectMake(100, 100, 40, 20);    [button addTarget:self action:@selector(Push:) forControlEvents:UIControlEventTouchUpInside];    [self.view addSubview:button];}-(void)Push:(UIButton *)sender{    //跳转控制器    DetailViewController *detailVC = [[DetailViewController alloc] init];    [self.navigationController pushViewController:detailVC animated:YES];    //回调传值    [detailVC ShowTextValue:^(NSString *ShowText) {        self.label.text = ShowText;    }];}