1、执行以下代码打印结果是什么？

- (void)interview01 {
    dispatch_queue_t queue = dispatch_get_global_queue(0, 0);
    dispatch_async(queue, ^{
        NSLog(@"1");
        [self performSelector:@selector(print) withObject:nil afterDelay:3];
        NSLog(@"3");
    });
}
- (void)print {
    NSLog(@"2");
}

打印结果：

~: 1
~: 3

原因分析：
performSelector:withObject:afterDelay:方法底层是由定时器实现，而定时器又依赖于RunLoop，子线程的RunLoop默认没有开启，所以这段代码不会打印2。
可以参考 GUNStep 来查看performSelector:withObject:afterDelay:实现逻辑，Foundation-RunLoop.m

如何能够让2打印：
方法一：开启当前线程的RunLoop

- (void)test1 {
    dispatch_queue_t queue = dispatch_get_global_queue(0, 0);
    dispatch_async(queue, ^{
        NSLog(@"1");
        [self performSelector:@selector(test2) withObject:nil afterDelay:3];
        NSLog(@"3");
        // 开启RunLoop
        NSRunLoop *runLoop = [NSRunLoop currentRunLoop];
        [runLoop runMode:NSDefaultRunLoopMode beforeDate:[NSDate distantFuture]];
    });
}

方法二：使用performSelector:方法调用print

- (void)test4 {
    dispatch_queue_t queue = dispatch_get_global_queue(0, 0);
    dispatch_async(queue, ^{
        NSLog(@"1");
        [self performSelector:@selector(test2)];
        NSLog(@"3");
    });
}

原因是performSelector底层原理是消息发送，不受RunLoop的影响。

// obcj源码 - NSObject.mm
+ (id)performSelector:(SEL)sel {
    if (!sel) [self doesNotRecognizeSelector:sel];
    return ((id(*)(id, SEL))objc_msgSend)((id)self, sel);
}

2、执行以下代码打印结果是什么？

- (void)interview02 {
    NSThread *thread = [[NSThread alloc] initWithBlock:^{
        NSLog(@"1");
    }];
    [thread start];
    [self performSelector:@selector(print) onThread:thread withObject:nil waitUntilDone:YES];
}
- (void)print {
    NSLog(@"2");
}

打印结果：

~: 1

报错： ! target thread exited while waiting for the perform
分析：
thread在执行start方法时，会调用block内部代码，打印1。
thread没有开启RunLoop所以在start方法后会退出，所以无法在线程里继续执行任务。