一，工程图。

二，代码。

RootViewController.h

#import <UIKit/UIKit.h>

@interface RootViewController : UIViewController

@end

 

RootViewController.m

#import "RootViewController.h"

//加入头文件
#import "Reachability.h"

@interface RootViewController ()

@end

@implementation RootViewController

- (id)initWithNibName:(NSString *)nibNameOrNil bundle:(NSBundle *)nibBundleOrNil
{
    self = [super initWithNibName:nibNameOrNil bundle:nibBundleOrNil];
    if (self) {
        // Custom initialization
    }
    return self;
}

- (void)viewDidLoad
{
    [super viewDidLoad];
    // Do any additional setup after loading the view.
    
    if ([self isConnectionAvailable]) {
        
        UIAlertView *alert=[[UIAlertView alloc]initWithTitle:@"提醒" message:@"网络已连接，请您一会儿重新发送" delegate:self cancelButtonTitle:@"取消" otherButtonTitles:@"确定", nil];
        [alert show];

    };
}
#pragma -mark -functions
//判断网络是否连接。
-(BOOL) isConnectionAvailable{
    
    BOOL isExistenceNetwork = YES;
    Reachability *reach = [Reachability reachabilityWithHostName:@"www.apple.com"];
    switch ([reach currentReachabilityStatus]) {
        case NotReachable:
            isExistenceNetwork = NO;
            //NSLog(@"notReachable");
            break;
        case ReachableViaWiFi:
            isExistenceNetwork = YES;
            //NSLog(@"WIFI");
            break;
        case ReachableViaWWAN:
            isExistenceNetwork = YES;
            //NSLog(@"3G");
            break;
    }
     if (!isExistenceNetwork) {
     
     UIAlertView *alert=[[UIAlertView alloc]initWithTitle:@"提醒" message:@"网络未连接，请您一会儿重新发送" delegate:self cancelButtonTitle:@"取消" otherButtonTitles:@"确定", nil];
     [alert show];
     }
     return isExistenceNetwork;
}