// 創(chuàng)建一個信號量，初始值為3，表示同時最多允許3個任務(wù)并發(fā)執(zhí)行
dispatch_semaphore_t semaphore = dispatch_semaphore_create(3);

// 創(chuàng)建一個dispatch group
dispatch_group_t group = dispatch_group_create();

// 創(chuàng)建一個全局并發(fā)隊列
dispatch_queue_t queue = dispatch_get_global_queue(DISPATCH_QUEUE_PRIORITY_DEFAULT, 0);

// 添加第一個任務(wù)到group
dispatch_group_async(group, queue, ^{
    dispatch_group_enter(group);
    // 等待信號量，如果信號量為0，則此任務(wù)將被阻塞
    dispatch_semaphore_wait(semaphore, DISPATCH_TIME_FOREVER);
    
    // 模擬任務(wù)執(zhí)行時間
    for (int i = 0; i < 20; ++i) {
        [NSThread sleepForTimeInterval:2]; // 模擬耗時操作
        NSLog(@"Task 1 --- %@", [NSThread currentThread]); // 打印當前線程
    }
    
    // 任務(wù)完成后，釋放信號量
    dispatch_semaphore_signal(semaphore);
    
    // 離開group
    dispatch_group_leave(group);
});

// 添加第二個任務(wù)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]; // 模擬耗時操作
        NSLog(@"Task 2 --- %@", [NSThread currentThread]); // 打印當前線程
    }
    
    dispatch_semaphore_signal(semaphore);
    
    dispatch_group_leave(group);
});

// 添加第三個任務(wù)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]; // 模擬耗時操作
        NSLog(@"Task 3 --- %@", [NSThread currentThread]); // 打印當前線程
    }
    
    dispatch_semaphore_signal(semaphore);
    
    dispatch_group_leave(group);
});

// 等待所有任務(wù)完成
dispatch_group_wait(group, DISPATCH_TIME_FOREVER);

dispatch_group_notify(group, dispatch_get_main_queue(), ^{
    // 所有文件都下載完成，執(zhí)行這里的代碼
    NSLog(@"Task 所有文件都下載完成 --- %@", [NSThread currentThread]); // 打印當前線程

});