Design pattern for Photoshop analog

I am designing the Photoshop analog using ImGui. I update state of application every frame, even if there is no changes. But there are some computationally expensive operations and I want to update state of application only when something is changed. However when you have several control panels and trying to check their state you get ugly code which is difficult to support. I am aware of signal-slot paradigm in Qt, but I want to implement it myself. What design patterns or approaches should I use in this situation? May be there are some examples which I can not find.