iOS 網(wǎng)絡(luò)請(qǐng)求順序處理（信號(hào)量，group）

一、日常開(kāi)發(fā)中，會(huì)經(jīng)常遇到這樣的需求：
  • 網(wǎng)絡(luò)請(qǐng)求A返回成功后，調(diào)用網(wǎng)絡(luò)請(qǐng)求B，網(wǎng)絡(luò)請(qǐng)求B結(jié)果返回成功后調(diào)用網(wǎng)絡(luò)請(qǐng)求C（即：A—>B—>C）。
  • 循環(huán)調(diào)用網(wǎng)絡(luò)請(qǐng)求A，確保每次請(qǐng)求返回成功后繼續(xù)調(diào)用網(wǎng)絡(luò)請(qǐng)求A（即：A—>A—>A）。
  • 網(wǎng)絡(luò)請(qǐng)求A和B返回成功后，調(diào)用網(wǎng)路請(qǐng)求C（即：A&B—>C）。
二、以上三種需求，分別在此列舉下示例：
  • 在GCD中跟信號(hào)量有關(guān)的三個(gè)函數(shù)如下：：
//創(chuàng)建時(shí)，值為<0返回null；值為0時(shí)，當(dāng)前線程無(wú)法直接執(zhí)行wait，需等待signal信號(hào)+1后執(zhí)行wait
dispatch_semaphore_t sema = dispatch_semaphore_create(0); semaphore
//發(fā)送一個(gè)信號(hào) +1
dispatch_semaphore_signal(sema);
//等待信號(hào)（signal）>0執(zhí)行
dispatch_semaphore_wait(sema, DISPATCH_TIME_FOREVER);

A—>B—>C：信號(hào)量處理

- (void)viewDidLoad {
  NSLog(@"開(kāi)始====%@",@"A");
  [self loadListMenuWithFinishMark:@"A"];
  NSLog(@"開(kāi)始====%@",@"B");
  [self loadListMenuWithFinishMark:@"B"];
  NSLog(@"開(kāi)始====%@",@"C");
  [self loadListMenuWithFinishMark:@"C"];
}

- (void)loadListMenuWithFinishMark:(NSString *)mark{
    // 創(chuàng)建信號(hào)量
    dispatch_semaphore_t sema = dispatch_semaphore_create(0);
    NSString *urlString = @"http://www.qubaobei.com/ios/cf/dish_list.php?stage_id=1&limit=20&page=1";
    [[AFHTTPSessionManager manager] GET:urlString parameters:nil headers:nil progress:^(NSProgress * _Nonnull downloadProgress) {
    } success:^(NSURLSessionDataTask * _Nonnull task, id  _Nullable responseObject) {
        // 發(fā)送信號(hào) （+ 1）
        dispatch_semaphore_signal(sema);
        NSLog(@"結(jié)束======%@",mark);
    } failure:^(NSURLSessionDataTask * _Nullable task, NSError * _Nonnull error) {
        dispatch_semaphore_signal(sema);
        NSLog(@"結(jié)束======%@", mark);
    }];
    // 等待signal執(zhí)行+1后（>0），才執(zhí)行wait后的業(yè)務(wù)
    dispatch_semaphore_wait(sema, DISPATCH_TIME_FOREVER);
}

打印結(jié)果（異常）:

2022-05-12 10:35:50.871154+0800 SemaphoreDemo[12896:3290528] 開(kāi)始====A

原因是：
如果在主線程運(yùn)行以上代碼，會(huì)發(fā)現(xiàn)沒(méi)有結(jié)果輸出；這是因?yàn)?code>AFNetworking回調(diào)默認(rèn)是主線程，這樣dispatch_semaphore_wait和dispatch_semaphore_signal在同一個(gè)線程，這樣就死鎖了。
所以以上代碼需要放在異步線程執(zhí)行如下：

- (void)viewDidLoad {
  dispatch_async(dispatch_get_global_queue(0, 0), ^{
        NSLog(@"開(kāi)始====%@",@"A");
        [self loadListMenuWithFinishMark:@"A"];
        NSLog(@"開(kāi)始====%@",@"B");
        [self loadListMenuWithFinishMark:@"B"];
        NSLog(@"開(kāi)始====%@",@"C");
        [self loadListMenuWithFinishMark:@"C"];
    });
}

- (void)loadListMenuWithFinishMark:(NSString *)mark{
    // 創(chuàng)建信號(hào)量
    dispatch_semaphore_t sema = dispatch_semaphore_create(0);
    NSString *urlString = @"http://www.qubaobei.com/ios/cf/dish_list.php?stage_id=1&limit=20&page=1";
    [[AFHTTPSessionManager manager] GET:urlString parameters:nil headers:nil progress:^(NSProgress * _Nonnull downloadProgress) {
    } success:^(NSURLSessionDataTask * _Nonnull task, id  _Nullable responseObject) {
        // 發(fā)送信號(hào) （+ 1）
        dispatch_semaphore_signal(sema);
        NSLog(@"結(jié)束======%@",mark);
    } failure:^(NSURLSessionDataTask * _Nullable task, NSError * _Nonnull error) {
        dispatch_semaphore_signal(sema);
        NSLog(@"結(jié)束======%@", mark);
    }];
    // 等待signal執(zhí)行+1后（>0），才執(zhí)行wait后的業(yè)務(wù)
    dispatch_semaphore_wait(sema, DISPATCH_TIME_FOREVER);
}

打印結(jié)果:

2022-05-12 10:29:55.555207+0800 SemaphoreDemo[12269:3284921] 開(kāi)始====A
2022-05-12 10:29:55.706748+0800 SemaphoreDemo[12269:3284782] 結(jié)束======A
2022-05-12 10:29:55.706758+0800 SemaphoreDemo[12269:3284921] 開(kāi)始====B
2022-05-12 10:29:55.712722+0800 SemaphoreDemo[12269:3284782] 結(jié)束======B
2022-05-12 10:29:55.712728+0800 SemaphoreDemo[12269:3284921] 開(kāi)始====C
2022-05-12 10:29:55.715101+0800 SemaphoreDemo[12269:3284782] 結(jié)束======C

循環(huán)請(qǐng)求：信號(hào)量處理


- (void)viewDidLoad {
    [super viewDidLoad];
    dispatch_async(dispatch_get_global_queue(0, 0), ^{
        for (NSInteger i = 0; i < 3; i ++) {
            NSLog(@"開(kāi)始====%ld",i);
            [self loadListMenuWithFinishIndex:i];
        }
    });
}

- (void)loadListMenuWithFinishIndex:(NSInteger)index{
    dispatch_semaphore_t sema = dispatch_semaphore_create(0);
    NSString *urlString = @"http://www.qubaobei.com/ios/cf/dish_list.php?stage_id=1&limit=20&page=1";
    [[AFHTTPSessionManager manager] GET:urlString parameters:nil headers:nil progress:^(NSProgress * _Nonnull downloadProgress) {
    } success:^(NSURLSessionDataTask * _Nonnull task, id  _Nullable responseObject) {
        dispatch_semaphore_signal(sema);
        NSLog(@"結(jié)束======%ld",index);
    } failure:^(NSURLSessionDataTask * _Nullable task, NSError * _Nonnull error) {
        dispatch_semaphore_signal(sema);
        NSLog(@"結(jié)束======%ld", index);
    }];
    dispatch_semaphore_wait(sema, DISPATCH_TIME_FOREVER);
}

打印結(jié)果:

2022-05-12 10:45:15.329772+0800 SemaphoreDemo[13910:3299813] 開(kāi)始====0
2022-05-12 10:45:15.439159+0800 SemaphoreDemo[13910:3299720] 結(jié)束======0
2022-05-12 10:45:15.439178+0800 SemaphoreDemo[13910:3299813] 開(kāi)始====1
2022-05-12 10:45:15.444384+0800 SemaphoreDemo[13910:3299720] 結(jié)束======1
2022-05-12 10:45:15.444405+0800 SemaphoreDemo[13910:3299813] 開(kāi)始====2
2022-05-12 10:45:15.464591+0800 SemaphoreDemo[13910:3299720] 結(jié)束======2

多個(gè)網(wǎng)絡(luò)請(qǐng)求都完成之后進(jìn)行某操作（A&B—>C）: Group處理

#import "AFNetworking.h"

@interface ViewController ()
@property (nonatomic,strong)dispatch_group_t group;
@end

@implementation ViewController
- (void)viewDidLoad {
    [super viewDidLoad];
    self.group = dispatch_group_create();
    NSLog(@"開(kāi)始====%@",@"A");
    [self loadListFinishMark:@"A"];
    NSLog(@"開(kāi)始====%@",@"B");
    [self loadListFinishMark:@"B"];
    // 完成A&B后執(zhí)行C請(qǐng)求
    dispatch_group_notify(self.group, dispatch_get_main_queue(), ^{
        NSLog(@"開(kāi)始====%@",@"C");
        [self loadListFinishCMark];
    });
}

#pragma mark - Group Request
- (void)loadListFinishMark:(NSString *)mark{
    //  enter 與 leave 一一對(duì)應(yīng)
    dispatch_group_enter(self.group);
    NSString *urlString = @"http://www.qubaobei.com/ios/cf/dish_list.php?stage_id=1&limit=20&page=1";
    [[AFHTTPSessionManager manager] GET:urlString parameters:nil headers:nil progress:^(NSProgress * _Nonnull downloadProgress) {
    } success:^(NSURLSessionDataTask * _Nonnull task, id  _Nullable responseObject) {
        NSLog(@"結(jié)束======%@",mark);
        dispatch_group_leave(self.group);
    } failure:^(NSURLSessionDataTask * _Nullable task, NSError * _Nonnull error) {
        NSLog(@"結(jié)束======%@", mark);
        dispatch_group_leave(self.group);
    }];
}

- (void)loadListFinishCMark{
    NSString *urlString = @"http://www.qubaobei.com/ios/cf/dish_list.php?stage_id=1&limit=20&page=1";
    [[AFHTTPSessionManager manager] GET:urlString parameters:nil headers:nil progress:^(NSProgress * _Nonnull downloadProgress) {
    } success:^(NSURLSessionDataTask * _Nonnull task, id  _Nullable responseObject) {
        NSLog(@"結(jié)束======C");
    } failure:^(NSURLSessionDataTask * _Nullable task, NSError * _Nonnull error) {
        NSLog(@"結(jié)束======C");
    }];
}

打印結(jié)果：

2022-05-12 10:52:32.505854+0800 SemaphoreDemo[14680:3307180] 開(kāi)始====A
2022-05-12 10:52:32.508026+0800 SemaphoreDemo[14680:3307180] 開(kāi)始====B
2022-05-12 10:52:32.626408+0800 SemaphoreDemo[14680:3307180] 結(jié)束======B
2022-05-12 10:52:32.626604+0800 SemaphoreDemo[14680:3307180] 結(jié)束======A
2022-05-12 10:52:32.627107+0800 SemaphoreDemo[14680:3307180] 開(kāi)始====C
2022-05-12 10:52:32.631006+0800 SemaphoreDemo[14680:3307180] 結(jié)束======C

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