使用NSCondition (Objective)解锁线程

Question

我正在使用Objective，并试图使用需要相互交谈的线程。

守则如下：

-(id) init
{
    self = [super init];
    if (self) {
        _event = [[MyTestClass alloc]init]; //MyTestClass has a property of type NSCondition
    }
    return self;
}


-(void) func1
{
NSLog(@"The Function 1 is being called");
NSLog(@"Locking The First function");
[self.event.myCondition lock];
[self.event.myCondition wait];
NSLog(@"Resuming Function 1");
    NSLog(@"This is a test 1 ");
    NSLog(@"This is a test 2");
    NSLog(@"Terminating func 1");
}
-(void) func2
{
    NSLog(@"2");
    NSLog(@"Hey Hey Hey How are you 0 ");
    NSLog(@"Hey Hey Hey How are you 1 ");
    NSLog(@"Hey Hey Hey How are you 2");
    NSLog(@"Signaling function 1");
    [self.event.myCondition signal];
    NSLog(@"Hey Hey Hey How are you 3");
    NSLog(@"Terminating func 2");
}

我在两个独立的线程上运行func1和func2，如下所示

- (void)viewDidLoad {
    [super viewDidLoad];
    SuperTestClass * n = [[SuperTestClass alloc]init];
    // Do any additional setup after loading the view, typically from a nib.
    MyTestClass * m = [[MyTestClass alloc]init];
    dispatch_queue_t newQueue =     dispatch_get_global_queue(DISPATCH_QUEUE_PRIORITY_DEFAULT,0);
    dispatch_group_t newGroup = dispatch_group_create();
    dispatch_group_async(newGroup, newQueue, ^{
        [n func1];
    });
    dispatch_group_async(newGroup, newQueue, ^{
        [n func2];
    });
    dispatch_group_wait(newGroup, DISPATCH_TIME_FOREVER);
    NSLog(@"All process have terminated");
}

运行以下代码时会出现以下错误

2015-07-07 19:05:54.528 signalingInObjectiveC[31617:319892] 2
2015-07-07 19:05:54.528 signalingInObjectiveC[31617:319894] The Function 1 is being called
2015-07-07 19:05:54.529 signalingInObjectiveC[31617:319892] Hey Hey Hey How are you 0 
2015-07-07 19:05:54.529 signalingInObjectiveC[31617:319894] Locking The First function
2015-07-07 19:05:54.529 signalingInObjectiveC[31617:319892] Hey Hey Hey How are you 1 
2015-07-07 19:05:54.529 signalingInObjectiveC[31617:319892] Hey Hey Hey How are you 2 
2015-07-07 19:05:54.530 signalingInObjectiveC[31617:319892] Signaling function 1
2015-07-07 19:05:54.530 signalingInObjectiveC[31617:319892] Hey Hey Hey How are you 3 
2015-07-07 19:05:54.530 signalingInObjectiveC[31617:319894] Resuming Function 1
2015-07-07 19:05:54.530 signalingInObjectiveC[31617:319892] Terminating func 2
2015-07-07 19:05:54.530 signalingInObjectiveC[31617:319894] This is a test 1
2015-07-07 19:05:54.530 signalingInObjectiveC[31617:319894] This is a test 2
2015-07-07 19:05:54.530 signalingInObjectiveC[31617:319894] Terminating func 1
2015-07-07 19:05:54.530 signalingInObjectiveC[31617:319870] All process have terminated
2015-07-07 19:05:54.535 signalingInObjectiveC[31617:319870] *** -[NSCondition dealloc]: condition (<NSCondition: 0x7fa2aa517180> '(null)') deallocated while still in use
2015-07-07 19:05:54.535 signalingInObjectiveC[31617:319870] *** Break on _NSLockError() to debug.

我想知道我解锁的方式出了什么问题。

Answer 1

您滥用了NSCondition类。检查一下苹果文档。本质上，POSIX条件使用互斥对象来保护条件测试的共享数据。您应该有类似的内容(其中P是共享状态上的布尔谓词)。

线程1：

[cond lock];
while (!P(state)) {
   [cond wait];
}
// invariant: if you get here, P(state) is true
... Use the state... // it is protected by the lock
[cond unlock];

线程2：

[cond lock];
... change the state ... // this potentially changes the value of P
[cond signal];  // or [cond broadcast]
[cond unlock];

线程#2修改共享状态(例如，在共享buf中保存一条消息)并通知线程#1。在线程#2放弃锁之前，线程#1不能唤醒。注意调用cond在线程#1中是如何原子地解锁互斥锁并进入休眠状态的。循环是必要的，有两个原因：

1. 你可以得到虚假的唤醒(没有理由，因此不能保证P(状态)为真。
2. 您可以让多个线程执行线程#1所描述的操作。如果线程#2只保存了一段数据，那么执行消耗操作的线程中只有一个能够实际捡起一个数据项，而“使用它”将使P(state)失效，因此其他线程将回到睡眠状态。

总之，您的代码是不正确的，因为您未能解锁该条件，并在锁定时销毁它。您没有遵循使用POSIX条件的编码模式。