iOS开发技术

iOS一个页面多个网络请求完成之后再执行其他操作

2018-12-27  本文已影响0人  十三楼的大笨象

前言

我们在开发的过程中,大家应该都会遇到一进入某个页面,就要请求多个接口,然后我们在完成所有请求以后再进行其他操作,对于这种需求,我们如何来设计我们的代码呢?

例如下面的场景,在首页有5个模块,我们需要分别调用5个不同的接口来获取数据,然后展示界面。


Simulator Screen Shot - iPhone 6s - 2018-12-27 at 10.16.58.png Simulator Screen Shot - iPhone 6s - 2018-12-27 at 10.17.09.png Simulator Screen Shot - iPhone 6s - 2018-12-27 at 10.17.14.png

解决方案

1、首先,我们创建一个项目,然后做一般性的做法,不做任何处理去连续请求一个接口10次:

//1.无处理
-(void)viewDidLoad{
    UIButton *Btn1 = [UIButton buttonWithType:UIButtonTypeCustom];
    Btn1.frame = CGRectMake(100, 100, 100, 40);
    Btn1.backgroundColor = [UIColor grayColor];
    [Btn1 setTitle:@"noConduct" forState:UIControlStateNormal];
    [Btn1 addTarget:self action:@selector(Btn1) forControlEvents:UIControlEventTouchUpInside];
    [self.view addSubview:Btn1];
}
-(void)Btn1{
    NSString *str = @''http://www.jianshu.com";
    NSURL *url = [NSURL URLWithString:str];
    NSURLRequest *request = [NSURLRequest requestWithURL:url];
    NSURLSession *session = [NSURLSession sharedSession];
     
    for (int i=0; i<10; i++) {
        NSURLSessionDataTask *task = [session dataTaskWithRequest:request completionHandler:^(NSData * _Nullable data, NSURLResponse * _Nullable response, NSError * _Nullable error) {
             
            NSLog(@"%d---%d",i,i);
             
        }];
         
        [task resume];
    }
     
    NSLog(@"end");
}

运行,看看我们的控制台输出:

2017-12-04 17:10:10.503 DownImage[3289:261033] end
2017-12-04 17:10:10.676 DownImage[3289:261080] 0---0
2017-12-04 17:10:10.704 DownImage[3289:261080] 1---1
2017-12-04 17:10:10.754 DownImage[3289:261096] 4---4
2017-12-04 17:10:10.760 DownImage[3289:261080] 2---2
2017-12-04 17:10:10.800 DownImage[3289:261096] 5---5
2017-12-04 17:10:10.840 DownImage[3289:261080] 7---7
2017-12-04 17:10:10.844 DownImage[3289:261082] 6---6
2017-12-04 17:10:10.846 DownImage[3289:261096] 3---3
2017-12-04 17:10:10.888 DownImage[3289:261096] 8---8
2017-12-04 17:10:10.945 DownImage[3289:261080] 9---9

很明显,无任何处理情况下,end最先被打印出来,由于网络请求的异步回调,然后各个网络请求的回调顺序是无序的。

2、使用GCD的dispatch_group_t

-(void)viewDidLoad{
    UIButton *Btn2 = [UIButton buttonWithType:UIButtonTypeCustom];
    Btn2.frame = CGRectMake(100, 200, 100, 40);
    Btn2.backgroundColor = [UIColor grayColor];
    [Btn2 setTitle:@"group--" forState:UIControlStateNormal];
    [Btn2 addTarget:self action:@selector(Btn2) forControlEvents:UIControlEventTouchUpInside];
    [self.view addSubview:Btn2];
}
-(void)Btn2{
   NSString *str = @''http://www.jianshu.com";
   NSURL *url = [NSURL URLWithString:str];
    NSURLRequest *request = [NSURLRequest requestWithURL:url];
    NSURLSession *session = [NSURLSession sharedSession];
     
    dispatch_group_t downloadGroup = dispatch_group_create();
    for (int i=0; i<10; i++) {
        dispatch_group_enter(downloadGroup);
         
        NSURLSessionDataTask *task = [session dataTaskWithRequest:request completionHandler:^(NSData * _Nullable data, NSURLResponse * _Nullable response, NSError * _Nullable error) {
             
            NSLog(@"%d---%d",i,i);
            dispatch_group_leave(downloadGroup);
             
        }];
         
        [task resume];
    }
     
    dispatch_group_notify(downloadGroup, dispatch_get_main_queue(), ^{
        NSLog(@"end");
    });
}

运行看看控制台输出:

2017-12-04 17:14:46.984 DownImage[3289:265374] 2---2
2017-12-04 17:14:46.987 DownImage[3289:265370] 1---1
2017-12-04 17:14:47.052 DownImage[3289:265383] 5---5
2017-12-04 17:14:47.065 DownImage[3289:265370] 4---4
2017-12-04 17:14:47.111 DownImage[3289:265379] 3---3
2017-12-04 17:14:47.121 DownImage[3289:265383] 6---6
2017-12-04 17:14:47.169 DownImage[3289:265383] 7---7
2017-12-04 17:14:47.192 DownImage[3289:265370] 9---9
2017-12-04 17:14:47.321 DownImage[3289:265383] 8---8
2017-12-04 17:14:47.747 DownImage[3289:265374] 0---0
2017-12-04 17:14:47.747 DownImage[3289:261033] end

从上两次输出可以看出,end确实是在所有网络请求之后才输出出来,这也是符合了我们的需求。
代码中我们只添加了4行代码

dispatch_group_t downloadGroup = dispatch_group_create();
dispatch_group_enter(downloadGroup);
dispatch_group_leave(downloadGroup);
dispatch_group_notify(downloadGroup, dispatch_get_main_queue(), ^{
    });

对以上4行代码可理解为:创建一个dispatch_group_t, 每次网络请求前先dispatch_group_enter,请求回调后再dispatch_group_leave,对于enter和leave必须配合使用,有几次enter就要有几次leave,否则group会一直存在。当所有enter的block都leave后,会执行dispatch_group_notify的block。

3、使用GCD的信号量dispatch_semaphore_t

//3.semaphore
    UIButton *Btn3 = [UIButton buttonWithType:UIButtonTypeCustom];
    Btn3.frame = CGRectMake(100, 300, 100, 40);
    Btn3.backgroundColor = [UIColor grayColor];
    [Btn3 setTitle:@"semaphore" forState:UIControlStateNormal];
    [Btn3 addTarget:self action:@selector(Btn3) forControlEvents:UIControlEventTouchUpInside];
    [self.view addSubview:Btn3];
//3.semaphore--
-(void)Btn3{
    NSString *str = @"http://www.jianshu.com/p/6930f335adba";
    NSURL *url = [NSURL URLWithString:str];
    NSURLRequest *request = [NSURLRequest requestWithURL:url];
    NSURLSession *session = [NSURLSession sharedSession];
    
    dispatch_semaphore_t sem = dispatch_semaphore_create(0);
    for (int i=0; i<10; i++) {
        
        NSURLSessionDataTask *task = [session dataTaskWithRequest:request completionHandler:^(NSData * _Nullable data, NSURLResponse * _Nullable response, NSError * _Nullable error) {
            
            NSLog(@"%d---%d",i,i);
            count++;
            if (count==10) {
                dispatch_semaphore_signal(sem);
                count = 0;
            }
            
        }];
        
        [task resume];
    }
    dispatch_semaphore_wait(sem, DISPATCH_TIME_FOREVER);
    
    dispatch_async(dispatch_get_main_queue(), ^{
        NSLog(@"end");
    });
}

运行看看控制台输出:

2017-12-04 17:36:49.098 DownImage[3428:283651] 2---2
2017-12-04 17:36:49.144 DownImage[3428:284210] 0---0
2017-12-04 17:36:49.152 DownImage[3428:284213] 3---3
2017-12-04 17:36:49.158 DownImage[3428:283651] 1---1
2017-12-04 17:36:49.167 DownImage[3428:284210] 4---4
2017-12-04 17:36:49.235 DownImage[3428:284213] 8---8
2017-12-04 17:36:49.249 DownImage[3428:283651] 5---5
2017-12-04 17:36:49.252 DownImage[3428:283651] 7---7
2017-12-04 17:36:49.324 DownImage[3428:283651] 9---9
2017-12-04 17:36:49.468 DownImage[3428:284214] 6---6
2017-12-04 17:36:49.469 DownImage[3428:283554] end

从输出可以看出,这样的方法也是满足我们的需求的,在这个方法中,我们使用了

dispatch_semaphore_t sem = dispatch_semaphore_create(0);
dispatch_semaphore_signal(sem);
dispatch_semaphore_wait(sem, DISPATCH_TIME_FOREVER);

对这三句代码可以这样理解:dispatch_semaphore信号量为基于计数器的一种多线程同步机制。如果semaphore计数大于等于1,程序继续运行。如果计数为0,则等待。dispatch_semaphore_signal(semaphore)为计数+1操作,dispatch_semaphore_wait(sema, DISPATCH_TIME_FOREVER)为设置等待时间,这里设置的等待时间是一直等待。
对于以上代码通俗一点就是,开始为0,等待,等10个网络请求都完成了,dispatch_semaphore_signal(semaphore)为计数+1,然后计数-1返回,程序继续执行。 (这里也就是为什么有个count变量的原因,记录网络回调的次数,回调10次之后再发信号量,使后面程序继续运行)。

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