手动管理GCD调度组

本文通过模拟当前流行的网络框架AFNetworking的使用，来演示手动管理GCD调度组的过程。

代码：

AFHTTPRequestOperationManager.h：

#import <Foundation/Foundation.h>

@interface AFHTTPRequestOperationManager : NSObject

+ (instancetype)manager;

- (void)GET:(NSString *)URLString parameters:(id)parameters success:(void (^)())success failure:(void (^)())failure;

@end

AFHTTPRequestOperationManager.m：

#import "AFHTTPRequestOperationManager.h"

@implementation AFHTTPRequestOperationManager

+ (instancetype)manager {
    return [[self alloc] init];
}

- (void)GET:(NSString *)URLString parameters:(id)parameters success:(void(^)())success failure:(void(^)())failure {
    NSUInteger duration = arc4random_uniform(5) + 1;
    dispatch_after(dispatch_time(DISPATCH_TIME_NOW, (int64_t)(duration * NSEC_PER_SEC)), dispatch_get_main_queue(), ^{
        success();
    });
}

@end

ViewController.m：

#import "ViewController.h"
#import "AFHTTPRequestOperationManager.h"

@interface ViewController ()

@property (strong, nonatomic) dispatch_group_t group;

@end

@implementation ViewController

- (void)viewDidLoad {
    [super viewDidLoad];

    AFHTTPRequestOperationManager *manager = [AFHTTPRequestOperationManager manager];
    
    dispatch_group_enter(self.group);
    [manager GET:nil parameters:nil success:^{
        NSLog(@"A done.");
        dispatch_group_leave(self.group);
    } failure:nil];
    
    dispatch_group_enter(self.group);
    [manager GET:nil parameters:nil success:^{
        NSLog(@"B done.");
        dispatch_group_leave(self.group);
    } failure:nil];
    
    dispatch_group_enter(self.group);
    [manager GET:nil parameters:nil success:^{
        NSLog(@"C done.");
        dispatch_group_leave(self.group);
    } failure:nil];
    
    dispatch_group_enter(self.group);
    [manager GET:nil parameters:nil success:^{
        NSLog(@"D done.");
        dispatch_group_leave(self.group);
    } failure:nil];
    
    dispatch_group_enter(self.group);
    [manager GET:nil parameters:nil success:^{
        NSLog(@"E done.");
        dispatch_group_leave(self.group);
    } failure:nil];
    
    dispatch_group_notify(self.group, dispatch_get_main_queue(), ^{
        NSLog(@"All done.");
    });
}

- (dispatch_group_t)group {
    if (!_group) {
        dispatch_group_t group = dispatch_group_create();
        _group = group;
    }
    return _group;
}

@end

可能的输出：

D done.
A done.
C done.
E done.
B done.
All done.

备注：
dispatch_group_enter()与dispatch_group_leave()调用必须一一对应。