iOS gcd面试题

1.异步+并发

dispatch_queue_t queue = dispatch_queue_create("com.demo.conQueue", DISPATCH_QUEUE_CONCURRENT);
    NSLog(@"1");
    dispatch_async(queue, ^{
        // 还是会按顺序执行
        NSLog(@"%@ 2-1",[NSThread currentThread]);
        NSLog(@"%@ 2-2",[NSThread currentThread]);
        NSLog(@"%@ 2-3",[NSThread currentThread]);
    });
    dispatch_async(queue, ^{
        NSLog(@"%@ 3",[NSThread currentThread]);
    });
    dispatch_async(queue, ^{
        NSLog(@"%@ 4",[NSThread currentThread]);
    });
    NSLog(@"5");

    // 结果
    /*
     1
     5
     <NSThread: 0x60000174dfc0>{number = 3, name = (null)} 2-1
     <NSThread: 0x600001704540>{number = 6, name = (null)} 3
     <NSThread: 0x600001758f40>{number = 7, name = (null)} 4
     <NSThread: 0x60000174dfc0>{number = 3, name = (null)} 2-2
     <NSThread: 0x60000174dfc0>{number = 3, name = (null)} 2-3
     */

2.同步/异步 + 串行队列

    dispatch_queue_t queue = dispatch_queue_create("com.demo.serialQueue", DISPATCH_QUEUE_SERIAL);
    NSLog(@"1");
    
    // 异步+串行 --> 能够开辟一条新线程
    dispatch_async(queue, ^{
        NSLog(@"%@ 2",[NSThread currentThread]);
        // 死锁，同步 + 当前队列 --> 死锁
        dispatch_sync(queue, ^{
            NSLog(@"3");
        });
        NSLog(@"%@ 4",[NSThread currentThread]);
    });
    NSLog(@"5");
    
    // 结果
    /*
     1
     5
     <NSThread: 0x600001749100>{number = 6, name = (null)} 2
     死锁了
     */