为GCD并发队列设置最大并发数

// 创建一个信号量，初始值为3，表示同时最多允许3个任务并发执行
dispatch_semaphore_t semaphore = dispatch_semaphore_create(3);

// 创建一个dispatch group
dispatch_group_t group = dispatch_group_create();

// 创建一个全局并发队列
dispatch_queue_t queue = dispatch_get_global_queue(DISPATCH_QUEUE_PRIORITY_DEFAULT, 0);

// 添加第一个任务到group
dispatch_group_async(group, queue, ^{
    dispatch_group_enter(group);
    // 等待信号量，如果信号量为0，则此任务将被阻塞
    dispatch_semaphore_wait(semaphore, DISPATCH_TIME_FOREVER);
    
    // 模拟任务执行时间
    for (int i = 0; i < 20; ++i) {
        [NSThread sleepForTimeInterval:2]; // 模拟耗时操作
        NSLog(@"Task 1 --- %@", [NSThread currentThread]); // 打印当前线程
    }
    
    // 任务完成后，释放信号量
    dispatch_semaphore_signal(semaphore);
    
    // 离开group
    dispatch_group_leave(group);
});

// 添加第二个任务到group
dispatch_group_async(group, queue, ^{
    dispatch_group_enter(group);
    dispatch_semaphore_wait(semaphore, DISPATCH_TIME_FOREVER);
    
    for (int i = 0; i < 20; ++i) {
        [NSThread sleepForTimeInterval:2]; // 模拟耗时操作
        NSLog(@"Task 2 --- %@", [NSThread currentThread]); // 打印当前线程
    }
    
    dispatch_semaphore_signal(semaphore);
    
    dispatch_group_leave(group);
});

// 添加第三个任务到group
dispatch_group_async(group, queue, ^{
    dispatch_group_enter(group);
    dispatch_semaphore_wait(semaphore, DISPATCH_TIME_FOREVER);
    
    for (int i = 0; i < 20; ++i) {
        [NSThread sleepForTimeInterval:2]; // 模拟耗时操作
        NSLog(@"Task 3 --- %@", [NSThread currentThread]); // 打印当前线程
    }
    
    dispatch_semaphore_signal(semaphore);
    
    dispatch_group_leave(group);
});

// 等待所有任务完成
dispatch_group_wait(group, DISPATCH_TIME_FOREVER);

dispatch_group_notify(group, dispatch_get_main_queue(), ^{
    // 所有文件都下载完成，执行这里的代码
    NSLog(@"Task 所有文件都下载完成 --- %@", [NSThread currentThread]); // 打印当前线程

});