iOS一个页面多个网络请求完成之后再执行其他操作
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)。