How to expose UnityEvent in editor?

I have found the way.

first you have to define your own event type (w$$anonymous$$ch is just a basic public class). however the custom event type must inherit from UnityEvent, like so.

 [System.Serializable]
 public class myFloatEvent : UnityEvent<float>{}

then you can make variables from it, like so.

 [SerializeField]
 public myFloatEvent onChangeEvent;

then you can set it in the editor

and the last step is to actually Invoke it in your custom code, w$$anonymous$$ch was the entire purpose of it.

 onChangeEvent.Invoke(newValue);