如文档所示，您可以使用键值观察来观察 runningApplications 属性：

As the document suggests, you use Key-Value Observing to observe the runningApplications property of the shared workspace object:

static const void *kMyKVOContext = (void*)&kMyKVOContext;


[[NSWorkspace sharedWorkspace] addObserver:self
                                forKeyPath:@"runningApplications"
                                   options:NSKeyValueObservingOptionNew // maybe | NSKeyValueObservingOptionInitial
                                   context:kMyKVOContext];

然后，您将实现观察方法（使用Xcode的现成代码段）：

Then, you would implement the observation method (using Xcode's ready-made snippet):

- (void) observeValueForKeyPath:(NSString*)keyPath ofObject:(id)object change:(NSDictionary*)change context:(void*)context
{
    if (context != kMyKVOContext)
    {
        [super observeValueForKeyPath:keyPath ofObject:object change:change context:context];
        return;
    }

    if ([keyPath isEqualToString:@"runningApplications"])
    {
        <#code to be executed when runningApplications has changed#>
    }
}