Question

我有一个NSObject的共享单例类，我运行了一些操作队列。我遇到了崩溃：

[super observeValueForKeyPath:keyPath ofObject:object change:change context:context];

似乎我需要使用'removeObserver：'来防止这种情况发生，但我如何在共享对象上正确地执行此操作？

CODE：

-(void)synchronizeToDevice{
    queue = [NSOperationQueue new];
    queue.name = @"SynchronizeToDeviceQueue";
    //Sync Active User
    NSInvocationOperation *operationUser = [[NSInvocationOperation alloc] initWithTarget:self
                                                                                selector:@selector(downloadUserData:)
                                                                              object:[self activeUserID]];

    [queue addOperation:operationUser];

    //Sync Video Data
    NSInvocationOperation *operationVideos = [[NSInvocationOperation alloc] initWithTarget:self
                                                                            selector:@selector(downloadVideoData)
                                                                              object:nil];
    [queue addOperation:operationVideos];


    [queue addObserver:self forKeyPath:@"operations" options:0 context:NULL];
}
- (void)observeValueForKeyPath:(NSString *)keyPath ofObject:(id)object change:(NSDictionary *)change context:(void *)context
{
    if (object == queue && [keyPath isEqualToString:@"operations"]) {
        //Synchronization Queue
        if ([queue.name isEqualToString:@"SynchronizeToDeviceQueue"] && [queue.operations count] == 0) {
            //Queue Completed
            //Notify View Synchronization Completed
            [self performSelectorOnMainThread:@selector(postNotificationDidFinishSynchronizationToDevice) withObject:nil waitUntilDone:NO];
        }
        //Video Download Queue
        if ([queue.name isEqualToString:@"VideoFileDownloadQueue"] && [queue.operations count] == 0) {
            //Notify View Video File Download Completed
            [self performSelectorOnMainThread:@selector(postNotificationDidFinishDownloadingVideo) withObject:nil waitUntilDone:NO];
        }
        //Active User Sync Queue
        if ([queue.name isEqualToString:@"SynchronizeActiveUserToDeviceQueue"] && [queue.operations count] == 0) {
            //Queue Completed
            //Notify View Synchronization Completed
            [self performSelectorOnMainThread:@selector(postNotificationDidFinishActiveUserSynchronizationToDevice) withObject:nil waitUntilDone:NO];
        }
    }
    else {
        [super observeValueForKeyPath:keyPath ofObject:object change:change context:context];
    }
}

CRASH LOG：

2013-03-14 21:48:42.167 COMPANY[1946:1103] *** Terminating app due to uncaught exception 'NSInternalInconsistencyException', reason: '<DataManager: 0x1c54a420>: An -observeValueForKeyPath:ofObject:change:context: message was received but not handled.
Key path: operations
Observed object: <NSOperationQueue: 0x1c5d3360>{name = 'SynchronizeActiveUserToDeviceQueue'}
Change: {
    kind = 1;
}
Context: 0x0'
*** First throw call stack:
(0x336262a3 0x3b4b197f 0x336261c5 0x33f1a56d 0x21bd1 0x33eb46b9 0x33eb4313 0x33eb3a25 0x33eb3817 0x33f2b689 0x3b8ccb97 0x3b8cf139 0x3b8cd91d 0x3b8cdac1 0x3b8fda11 0x3b8fd8a4)
libc++abi.dylib: terminate called throwing an exception

Answer 1

在“Key-Value Observing Programming Guide”的Receiving Notification of a Change中，给出了observeValueForKeyPath的示例实现，其中包含注释：

确保调用超类的实现，如果它实现它。 NSObject没有实现该方法。

您说您的班级是NSObject的子类，因此您不应该致电[super observeValueForKeyPath:...]。

如果在同一个共享实例上多次调用synchronizeToDevice，则会出现另一个问题。在这种情况下，您创建一个新的queue并注册一个观察者。但是旧队列的观察者没有被删除。

因此，可能会为“旧队列”和检查调用observeValueForKeyPath if (object == queue)失败，导致不必要的超级电话。

因此，如果可以多次调用synchronizeToDevice，则应首先删除旧的观察者。

Answer 2

我怀疑您拨打synchronizeToDevice的电话不止一次。如果是这样，您将继续观察旧队列以及一些新队列。当observeValueForKeyPath:...触发时，它可能会传递给你的旧队列，然后你忽略它，调用super，因为你没有处理你要求的观察而抛出异常。

这里你真正的问题是你没有使用访问器。那会更加清晰。例如，这是您实现setQueue:

的方式

-(void)setQueue:(NSOperationQueue *)queue {
  if (_queue) {
    [_queue removeObserver:self forKeyPath:@"operations"];
  }

  _queue = queue;

  if (_queue) {
    [_queue addObserver:self forKeyPath:@"operations" options:0 context:NULL];
  }
}

现在，当您致电self.queue = [NSOperationQueue new];时，一切都会自动生效。您停止观察旧队列并开始观察新队列。如果您致电self.queue = nil，它会自动取消注册。

您仍然需要确保在dealloc中取消注册：

- (void)dealloc {
  if (_queue) {
    [_queue removeObserver:self forKeyPath:@"operations"];
  }
}

iOS - 如何从单身NSObject中删除KVO的观察者？

2 个答案: