GCD:如何从两个线程写入和读取变量

Question

这可能听起来像一个新手问题,我是GCD的新手,

我正在创建并运行以下两个线程.第一个将数据放入ivar mMutableArray,第二个读取数据.如何锁定和取消线程以避免崩溃并保持代码线程安全？

// Thread for writing data into mutable array 
dispatch_queue_t queue = dispatch_get_global_queue(DISPATCH_QUEUE_PRIORITY_DEFAULT, 0);
dispatch_source_t timer = dispatch_source_create(DISPATCH_SOURCE_TYPE_TIMER, 0, 0, queue);
if (timer) {
    dispatch_source_set_timer(timer, dispatch_time(DISPATCH_TIME_NOW, NSEC_PER_SEC * interval), interval * NSEC_PER_SEC, leeway);
    dispatch_source_set_event_handler(timer, ^{
        ...
        // Put data into ivar
        [mMutableArray addObject:someObject];
        ...
    });
    dispatch_resume(timer);
}

// Thread for reading from mutable array
dispatch_source_t timer1 = dispatch_source_create(DISPATCH_SOURCE_TYPE_TIMER, 0, 0, queue);
if (timer1) {
    dispatch_source_set_timer(timer1, dispatch_time(DISPATCH_TIME_NOW, NSEC_PER_SEC * interval), interval * NSEC_PER_SEC, leeway);
    dispatch_source_set_event_handler(timer1, ^{
        ...
        if (mMutableArray) {
            // Read data from ivar
            SomeObject* someobject = [mMutableArray lastObject];
        }
        ...
    });
    dispatch_resume(timer1);
}

Answer 1

您使用它是错误的,通过锁定对变量的访问,您只是失去了GCD的任何好处.创建一个与要修改的变量(在本例中为可变数组)相关联的串行队列.然后使用该队列进行写入和读取,这将在保证的串行序列中发生,并且锁定开销最小.您可以在http://www.fieryrobot.com/blog/2010/09/01/synchronization-using-grand-central-dispatch/上的 "异步设置器"中阅读有关它的更多信息.只要您通过其关联的调度队列访问共享变量,就不会出现并发问题.