绑定值不会更改由Apple的设计。要在更改后自己将值传播给模型，请修改此代码：

The bound value doesn't change by Apple's design. To propagate the value to the model yourself after a change, adapt this code:

NSDictionary *bindingInfo = [self infoForBinding:NSValueBinding];
[[bindingInfo valueForKey:NSObservedObjectKey] setValue:self.doubleValue
                                             forKeyPath:[bindingInfo valueForKey:NSObservedKeyPathKey]];

（感谢@DrummerB的苹果链接！）

(Thanks @DrummerB for that Apple link!)