在开发中传值常常会用到代理和Block，今天先说说Block的使用：

1.Block 的定义：

格式： 返回值/Block名称/参数

void (^myBlock)(UIImageView *)

2.定义Block中要执行的方法

= ^(UIImageView *imageView){      NSLog(@"----------");};

3.执行一个Block

举例：

myBlock(self.imageView);

4.传值事例：

情况说明: 有A、B两个界面，A界面有一个展示内容的UILabel，B界面有一个可以输入文字的UITextField.当从A页面PUSH到B页面的时候，在B页面中的UITextField中输入内容后,从B页面返回到A界面的时候，在A页面中的UILabel 中可以看到在B页面中输入的内容。

A页面 对应 OneViewController 类

1.需要先在B界面中定义Block：

#import 
    
     typedef void (^BlockValue)(NSString *valueString);@interface SecondViewController : UIViewController@property (nonatomic,copy) BlockValue blockValue;- (void)getValue:(BlockValue)aBlock;@end
    

SecondViewController.m 中:

#import "SecondViewController.h"    @interface SecondViewController ()    @property (nonatomic,strong) UITextField *textField;    @end        @implementation SecondViewController - (void)viewDidLoad {    [super viewDidLoad];    self.view.backgroundColor = [UIColor whiteColor];    UITextField *textField = [[UITextField alloc] init];    textField.frame = CGRectMake(50, 200, 200, 50);    textField.placeholder = @"请输入需要传值的数据";    [self.view addSubview:textField];    self.textField = textField;} - (void)getValue:(BlockValue)aBlock {        self.blockValue = aBlock;} - (void)viewWillDisappear:(BOOL)animated {        [super viewWillDisappear:animated];        if (self.blockValue) {            self.blockValue(_textField.text);        }} @end

2.在A界面定义SecondViewController:

#import "OneViewController.h"#import "SecondViewController.h" @interface OneViewController ()@property (nonatomic,strong) UILabel *label;     @end@implementation OneViewController- (void)viewDidLoad {    [super viewDidLoad];    // Do any additional setup after loading the view, typically from a nib.    UILabel *label = [[UILabel alloc] init];    label.text = @"等待传值";    label.frame = CGRectMake(50, 200, 200, 50);    [self.view addSubview:label];    self.label = label;     UIButton *button = [UIButton         buttonWithType:UIButtonTypeCustom];     button.frame =CGRectMake(50, 500, 80, 50);     [button setTitle:@"下一页" forState:UIControlStateNormal];     [button setTitleColor:[UIColor blackColor] forState:UIControlStateNormal];     [button addTarget:self action:@selector(nextStep:) forControlEvents:UIControlEventTouchUpInside];     [self.view addSubview:button];}- (void)nextStep:(UIButton *)button {    SecondViewController *secondVc = [[SecondViewController alloc] init];    secondVc.blockValue = ^(NSString *valueString) {        self.label.text = valueString;    };    [self.navigationController pushViewController:secondVc animated:YES]; }- (void)didReceiveMemoryWarning {    [super didReceiveMemoryWarning];    // Dispose of any resources that can be recreated.}@end