1、以下代码结果会如何？
NSLog(@"1===========%@",[NSThread currentThread]);
dispatch_sync(dispatch_get_main_queue(), ^{
NSLog(@"2===========%@",[NSThread currentThread]);
});
NSLog(@"3===========%@",[NSThread currentThread]);
结果如下：
1==========={number = 1, name = main}
会造成死锁，主线程中【同步执行+主队列】，造成的互相等待。
2、写一个线程安全的购票代码
- (void)initTicketStatusSave {
NSLog(@"start_%@",[NSThread currentThread]);
semaphoreLock = dispatch_semaphore_create(1);
self.ticketSurplusCount = 6;
//北京售票口
dispatch_queue_t queueBJ = dispatch_queue_create("beijin", DISPATCH_QUEUE_SERIAL);
//上海售票口
dispatch_queue_t queueSH = dispatch_queue_create("shanghai",DISPATCH_QUEUE_SERIAL);
__weak typeof(self) weakSelf = self;
dispatch_async(queueBJ, ^{
[weakSelf saleTicketSafe];
});
dispatch_async(queueSH, ^{
[weakSelf saleTicketSafe];
});
}
- (void)saleTicketSafe {
while (1) {
dispatch_semaphore_wait(semaphoreLock, DISPATCH_TIME_FOREVER);
if (self.ticketSurplusCount > 0) {
self.ticketSurplusCount--;
NSLog(@"%@",[NSString stringWithFormat:@"剩余票数：%ld 窗口：%@",self.ticketSurplusCount,[NSThread currentThread]]);
[NSThread sleepForTimeInterval:0.2];
} else {
NSLog(@"所有火车票均已售完");
dispatch_semaphore_signal(semaphoreLock);
break;;
}
dispatch_semaphore_signal(semaphoreLock);
}
}
start_{number = 1, name = main}
剩余票数：5 窗口：{number = 5, name = (null)}
剩余票数：4 窗口：{number = 6, name = (null)}
剩余票数：3 窗口：{number = 5, name = (null)}
剩余票数：2 窗口：{number = 6, name = (null)}
剩余票数：1 窗口：{number = 5, name = (null)}
剩余票数：0 窗口：{number = 6, name = (null)}
所有火车票均已售完
所有火车票均已售完
3、请说出下列的打印顺序
- (void)GCDForTest {
dispatch_queue_t testQueue = dispatch_queue_create("testQueue", DISPATCH_QUEUE_SERIAL);
NSLog(@"1 ---- %@",[NSThread currentThread]);
dispatch_async(testQueue, ^{
[NSThread sleepForTimeInterval:1];
NSLog(@"2 ---- %@",[NSThread currentThread]);
});
NSLog(@"3 ---- %@",[NSThread currentThread]);
dispatch_sync(testQueue, ^{
[NSThread sleepForTimeInterval:1];
NSLog(@"4 ---- %@",[NSThread currentThread]);
});
NSLog(@"5 ---- %@",[NSThread currentThread]);
}
打印顺序为：1、3、2、4、5，你说对了吗？
1 ---- {number = 1, name = main}
3 ---- {number = 1, name = main}
2 ---- {number = 5, name = (null)}
4 ---- {number = 1, name = main}
5 ---- {number = 1, name = main}
在任务1执行完后，将任务2添加至串行队列中，由于是异步的，所以不会堵塞线程。继续往下执行任务3，然后将任务4添加至串行队列，任务2和任务4此时在同一个串行队列，根据队列的先进先出原则，同步执行的任务4必须得等任务2执行完后才能执行，不然会堵塞线程。
4、下面的代码会如何打印
- (void)demoForSelector {
NSLog(@"1 ---- %@",[NSThread currentThread]);
dispatch_async(dispatch_get_global_queue(0, 0), ^{
NSLog(@"2 ---- %@",[NSThread currentThread]);
[self performSelector:@selector(testForSelector:) withObject:@"performSelector" afterDelay:1];
});
NSLog(@"3 ---- %@",[NSThread currentThread]);
}
- (void)testForSelector:(NSString *)msg {
NSLog(@"%@ ---- %@",msg,[NSThread currentThread]);
}
1 ---- {number = 1, name = main}
3 ---- {number = 1, name = main}
2 ---- {number = 5, name = (null)}
可以看到，testForSelector:方法没走，为什么？
是因为在GCD异步全局并发队列的子线程里，默认是没有开启对应的RunLoop的，所以这个方法就会失效。
有3中方式可以让这个方法起作用：
- 将
dispatch_get_global_queue换成dispatch_get_main_queue - 将
dispatch_async换成dispatch_async - 将
testForSelector:方法添加到RunLoop里
NSRunLoop *loop = [NSRunLoop currentRunLoop];
[self performSelector:@selector(testForSelector:) withObject:@"performSelector" afterDelay:0.1];
[loop run];
5、怎么用GCD实现多读单写？
多读单写：可以多个读者同时读取数据，但是在读的时候不能去写入数据。写的时候，只能单个写，不能有其他写者同时去写。
- (void)多读单写 {
dispatch_queue_t queue = dispatch_queue_create("duodudanxie", DISPATCH_QUEUE_CONCURRENT);
//读
for (int i = 0; i < 5; i++) {
dispatch_async(queue, ^{
NSLog(@"%d ---- %@",i,[NSThread currentThread]);
});
}
//写
dispatch_barrier_sync(queue, ^{
NSLog(@"barrier ---- %@",[NSThread currentThread]);
});
//读
for (int i = 10; i < 5; i++) {
dispatch_async(queue, ^{
NSLog(@"%d ---- %@",i,[NSThread currentThread]);
});
}
}
0 ---- {number = 5, name = (null)}
1 ---- {number = 6, name = (null)}
3 ---- {number = 7, name = (null)}
2 ---- {number = 3, name = (null)}
4 ---- {number = 8, name = (null)}
barrier ---- {number = 1, name = main}
10 ---- {number = 8, name = (null)}
13 ---- {number = 4, name = (null)}
11 ---- {number = 3, name = (null)}
12 ---- {number = 7, name = (null)}
14 ---- {number = 6, name = (null)}
6、在多个网络请求完成后才去刷新UI界面
在n个耗时并发任务都完成后，再去执行接下来的任务。
dispatch_group_async
- (void)demoForGroup {
dispatch_queue_t concurrentQueue = dispatch_queue_create("groupqueue", DISPATCH_QUEUE_CONCURRENT);
dispatch_group_t group = dispatch_group_create();
for (NSInteger i = 0; i < 5; i++) {
dispatch_group_async(group, concurrentQueue, ^{
[NSThread sleepForTimeInterval:1];
NSLog(@"网络请求：%ld ---- %@",i,[NSThread currentThread]);
});
}
dispatch_group_notify(group, concurrentQueue, ^{
NSLog(@"刷新UI ---- %@",[NSThread currentThread]);
});
}
网络请求：2 ---- {number = 6, name = (null)}
网络请求：0 ---- {number = 5, name = (null)}
网络请求：3 ---- {number = 4, name = (null)}
网络请求：1 ---- {number = 7, name = (null)}
网络请求：4 ---- {number = 3, name = (null)}
刷新UI ---- {number = 3, name = (null)}
dispatch_barrier_sync
dispatch_queue_t concurrentQueue = dispatch_queue_create("groupqueue", DISPATCH_QUEUE_CONCURRENT);
for (NSInteger i = 0; i < 5; i++) {
dispatch_async(concurrentQueue, ^{
[NSThread sleepForTimeInterval:1];
NSLog(@"网络请求：%ld ---- %@",i,[NSThread currentThread]);
});
}
dispatch_barrier_sync(concurrentQueue, ^{
NSLog(@"刷新UI ---- %@",[NSThread currentThread]);
});
NSLog(@"end");
网络请求：3 ---- {number = 8, name = (null)}
网络请求：0 ---- {number = 7, name = (null)}
网络请求：4 ---- {number = 5, name = (null)}
网络请求：2 ---- {number = 6, name = (null)}
网络请求：1 ---- {number = 4, name = (null)}
刷新UI ---- {number = 1, name = main}
end
参考
2019 iOS面试题-----多线程相关
阿里、字节：一套高效的iOS面试题（八 - 多线程 GCD）