问题
UIButton has a state property, which appears to be KVO compliant by all accounts and there is no documentation to indicate otherwise. However, when I added an observer to a UIButton's state property, the observer callback was never invoked. How come?
回答1:
If you look at the documentation of UIControl, the state property is marked: synthesized from other flags.
I guess this is why changes to this property are not KVO compliant.
However, you can simply register and observer for the values you need - highlighted, selected, enabled. These properties are KVO compliant and you will get the observer callback when they change.
来源：https://stackoverflow.com/questions/7544037/key-value-observing-on-uibuttons-state