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ios 一道面试题

admin 1月 28, 2021 0

iOS 一道面试题

写出以下代码的打印结果

dispatch_queue_t queue = dispatch_get_global_queue(0, 0);
dispatch_async(queue, ^{
    NSLog(@"1");
    [self performSelector:@selector(test) withObject:nil afterDelay:.0];
    NSLog(@"3");
});

- (void)test
{
    NSLog(@"2");
}

打印结果：1 3

[self performSelector:@selector(test) withObject:nil afterDelay:.0];
这句代码的本质是往Runloop中添加定时器，但是子线程的Runloop默认是未启动的。

修改代码

dispatch_queue_t queue = dispatch_get_global_queue(0, 0);
dispatch_async(queue, ^{
    NSLog(@"1");
    [self performSelector:@selector(test) withObject:nil afterDelay:.0];
    NSLog(@"3");
    [[NSRunLoop currentRunLoop] runMode:NSDefaultRunLoopMode beforeDate:[NSDate distantFuture]];
});

- (void)test
{
    NSLog(@"2");
}

[[NSRunLoop currentRunLoop] runMode:NSDefaultRunLoopMode beforeDate:[NSDate distantFuture]];启动Runloop后

打印结果 1 3 2