Block的初步用法

block说明

block是苹果推荐使用的数据类型。这种数据类型保存了某一段代码，可以在恰当的时机(调用它的时候)再取出来执行.既然是数据类型，那么block也有定义，赋值，使用的过程，跟一般的NSString，NSInterger类似：

//定义一个变量名为myblock的block,类比NSString *str;
void(^myblock)();

//给myblock赋值,类比str = @"wuliao"
myblock = ^{
    NSLog(@"hahahahshfAAAA");//{}大括号之间就保存了一个代码片段，在我们调用这个block的时候，就会执行这个代码片段。
};

//调用这个block。这里的用法又有点类似函数。
 myblock();

简单的应用场景

我们要传递一个要消失的页面的数据的时候，我们会想到用delegate，通知中心，键值观察模式，甚至是单例。每种方式都会有它的局限性和优势。这里我们尝试用block来实现。
　以下代码就模拟一个登陆的场景。通常我们的app中有些功能需要登陆后才能查看。大体流程是点击按钮，判断是否是登录状态，否，则跳到登录页面，在登录页面录入信息。登录成功，返回之前页面并把数据加载到之前的页面。
　代码：我们只有两个viewcontroller，一个是用户未登陆下的界面，一个是登陆界面。

登录.gif

Talk is cheap show me the code

未登录的界面.m文件
#import "ViewController.h"
#import "LoginViewController.h"

@interface ViewController ()
{
    UILabel *myName;
}

@end

@implementation ViewController

- (void)viewDidLoad {
    [super viewDidLoad];
   
    self.view.backgroundColor = [UIColor whiteColor];
    UIButton *bun = [UIButton buttonWithType:UIButtonTypeRoundedRect];
    bun.backgroundColor = [UIColor orangeColor];
    bun.frame  = CGRectMake(10, 200, [UIScreen mainScreen].bounds.size.width-20, 30);
    bun.layer.cornerRadius = 8;
    bun.clipsToBounds = YES;
    
    [bun setTitle:@"个人信息" forState:UIControlStateNormal];
    [bun setTitleColor:[UIColor whiteColor] forState:UIControlStateNormal];
    [bun addTarget:self action:@selector(loginsuccess:) forControlEvents:UIControlEventTouchUpInside];
    [self.view addSubview:bun];
    
    myName = [[UILabel alloc]initWithFrame:CGRectMake(100, 250, 100, 40)];
    myName.frame  = CGRectMake(10, CGRectGetMaxY(bun.frame)+10, [UIScreen mainScreen].bounds.size.width-20, 40);
    [self.view addSubview:myName];
}

- (void)loginsuccess:(UIButton *)sender {
    
    LoginViewController *login = [[LoginViewController alloc]init];
    [login loginResult:^(NSDictionary *successDict) {
        myName.text = successDict[@"myName"];
    }];
    [self presentViewController:login animated:YES completion:nil];
}

@end

登录界面 .h
#import <UIKit/UIKit.h>

typedef void(^LoginSuccess) (NSDictionary *successDict);

@interface LoginViewController : UIViewController

- (void)loginResult:(LoginSuccess)loginResult;

@end

另一种写法,直接在 登录界面 .h 定义一个block类型的成员变量
我们就不用typedef来定义Block类型，而是直接定义一个block类型的成员变量LoginSuccess

@property (nonatomic,copy) void (^LoginSuccess)(NSDictionary *successDict);

如果是按照这种写法，那么在 未登录.m 中，点击登录按钮的时候代码应该这样子写

login.LoginSuccess = ^(NSDictionary *successDict){
  myName.text = successDict[@"myName"];
  };

而且也不用在登录的控制器里，实现下面这句代码了

  - (void)loginResult:(LoginSuccess)loginResult;

大家可以体会两种写法的好处

登录界面 .m

#import "LoginViewController.h"

@interface LoginViewController ()
{
    UITextField  *userName;
    LoginSuccess _loginSuccess;
}

@end

@implementation LoginViewController

- (void)viewDidLoad {
    [super viewDidLoad];
    
    self.view.backgroundColor = [UIColor whiteColor];
    
    userName = [[UITextField alloc]initWithFrame:CGRectMake(20, 250, [UIScreen mainScreen].bounds.size.width-40, 45)];
    userName.placeholder = @"请输入用户名";
    userName.borderStyle = UITextBorderStyleRoundedRect;
    [self.view addSubview:userName];
    
    UITextField *password = [[UITextField alloc]initWithFrame:CGRectMake(20, 320, [UIScreen mainScreen].bounds.size.width-40, 45)];
    password.placeholder = @"请输入密码";
    password.secureTextEntry = YES;
    password.borderStyle = UITextBorderStyleRoundedRect;
    [self.view addSubview:password];
    
    //登录按钮
    UIButton *bun = [UIButton buttonWithType:UIButtonTypeRoundedRect];
    bun.backgroundColor = [UIColor orangeColor];
    bun.tag = 1212;
    bun.frame  = CGRectMake(20, [UIScreen mainScreen].bounds.size.height-350, [UIScreen mainScreen].bounds.size.width-40, 45);
    bun.layer.cornerRadius = 8;
    bun.clipsToBounds = YES;
    
    [bun setTitle:@"登陆" forState:UIControlStateNormal];
    [bun setTitleColor:[UIColor whiteColor] forState:UIControlStateNormal];
    [bun setTitleColor:[UIColor grayColor] forState:UIControlStateDisabled];
    [bun addTarget:self action:@selector(loginsuccess) forControlEvents:UIControlEventTouchUpInside];
    [self.view addSubview:bun];
    
    //加载数据菊花
    UIActivityIndicatorView *activityview = [[UIActivityIndicatorView alloc]init];
    activityview.center = self.view.center;
    activityview.tag = 1131;
    activityview.color = [UIColor grayColor];
    [activityview hidesWhenStopped];
    activityview.accessibilityNavigationStyle = UIActivityIndicatorViewStyleGray;
    [self.view addSubview:activityview];
    
    
}
-(void)loginResult:(LoginSuccess)loginResult
{
    _loginSuccess = loginResult;
}

-(void)loginsuccess
{
    [self.view endEditing:YES];
    UIActivityIndicatorView *activityview = (UIActivityIndicatorView*)[self.view viewWithTag:1131];
    [activityview startAnimating];
    UIButton *bun = (UIButton *)[self.view viewWithTag:1212];
    bun.enabled = NO;
    [self performSelector:@selector(askData) withObject:self afterDelay:0.5];
    
}

-(void)askData
{
    UIActivityIndicatorView *activityview = (UIActivityIndicatorView*)[self.view viewWithTag:1131];
    [activityview stopAnimating];
    
    NSDictionary *dic= @{@"state":@"登陆成功！",
                         @"myName":userName.text};
    
    //调用空的block时 ，报错exc_bad_access，所有每次调用block的时候必须要，先检查
    if (_loginSuccess) {
        //登陆成功，把数据返回上个界面
        _loginSuccess(dic);
    }
    [self dismissViewControllerAnimated:YES completion:^{
        
    }];

}
@end

有说的不对的地方，也请大家不吝赐教哈_谢谢