GCD总结1

dispatch_sync、dispatch_async用来控制是否开启新的线程。
串行队列、并发队列决定任务的执行方式。

dispatch_sync：立马在当前线程同步执行任务。

#pragma mark interview1 相互等待，产生死锁1
- (void)question6 {
    dispatch_queue_t queue = dispatch_get_main_queue();
    NSLog(@"执行任务1");
    dispatch_sync(queue, ^{
        NSLog(@"主队列执行任务2 -- %@", [NSThread currentThread]);
    });
    NSLog(@"执行任务3");
}
#pragma mark interview2 相互等待，产生死锁2
- (void)question7 {
    NSLog(@"执行任务1");
    dispatch_queue_t queue = dispatch_queue_create("SERIALQUEUE", DISPATCH_QUEUE_SERIAL);
    dispatch_async(queue, ^{
        NSLog(@"执行任务2 -- %@", [NSThread currentThread]);
        dispatch_sync(queue, ^{
            NSLog(@"执行任务3");
        });
        NSLog(@"执行任务4");
    });
    NSLog(@"执行任务5");
}

 (void)question13 {
    dispatch_queue_t queue = dispatch_get_global_queue(0, 0);
    dispatch_async(queue, ^{
        NSLog(@"1");
        [self performSelector:@selector(test) withObject:nil];
        NSLog(@"3");
    });
    //打印结果：1 2 3
}
- (void)question14 {
    dispatch_queue_t queue = dispatch_get_global_queue(0, 0);
    dispatch_async(queue, ^{
        NSLog(@"1");
        //performSelector withObject afterDelay的本质是：往runloop里面添加NSTimer定时器，子线程默认没有启动runloop。
        [self performSelector:@selector(test) withObject:nil afterDelay:.0];
        NSLog(@"3");
        
    });
    //打印结果：1 3
}
- (void)question15 {
    dispatch_queue_t queue = dispatch_get_global_queue(0, 0);
    dispatch_async(queue, ^{
        NSLog(@"1");
        //performSelector withObject afterDelay的本质是：往runloop里面添加NSTimer定时器，子线程默认没有启动runloop。
        [self performSelector:@selector(test) withObject:nil afterDelay:.0];
        NSLog(@"3");
        //启动runloop
        [[NSRunLoop currentRunLoop] addPort:[[NSPort alloc] init] forMode:NSDefaultRunLoopMode];
        [[NSRunLoop currentRunLoop] runMode:NSDefaultRunLoopMode beforeDate:[NSDate distantFuture]];
    });
    //打印结果：1 3 2
}
- (void)question16 {
    NSLog(@"1");
    //主线程的runloop默认是开启的
    [self performSelector:@selector(test) withObject:nil afterDelay:.0];
    NSLog(@"3");
    //打印结果：1 3 2
}
- (void)test {
    NSLog(@"2");
}

- (void)question17 {
    NSThread *thread = [[NSThread alloc] initWithBlock:^{
        NSLog(@"1");
        //为子线程添加一个runloop，不让线程销毁
        [[NSRunLoop currentRunLoop] addPort:[[NSPort alloc] init] forMode:NSDefaultRunLoopMode];
        [[NSRunLoop currentRunLoop] runMode:NSDefaultRunLoopMode beforeDate:[NSDate distantFuture]];
    }];
    [thread start];
    [self performSelector:@selector(test) onThread:thread withObject:nil waitUntilDone:YES];
    //打印结果：1 2
}
- (void)question18 {
    NSThread *thread = [[NSThread alloc] initWithBlock:^{
        NSLog(@"1");
    }];
    [thread start];
    [self performSelector:@selector(test) onThread:thread withObject:nil waitUntilDone:YES];
    //打印结果：1，程序崩溃
}

GCD-1.png GCD-2.png

产生死锁原因：
使用sync函数往当前串行队列里面添加任务，会卡住当前串行队列（产生死锁）。