Method onChanged

src/fl/ui/button.h:78–82

    }

    int onChanged(function<void(UIButton &)> callback) FL_NOEXCEPT {
        int id = mCallbacks.add(callback);
        mListener.addToEngineEventsOnce();
        return id;
    }

    int onClicked(function<void()> callback) FL_NOEXCEPT {
        int id = mCallbacks.add([callback](UIButton &btn) {