Add observer pattern

Add observer design pattern to the code.

The issue does not specify where or how to add the observer pattern. First, examine the existing codebase to understand the calculator's architecture and identify components that could benefit from an observer, such as UI updates or state changes. Look for existing event handling or notification mechanisms. Determine what events should be observed and what actions should be triggered, then design and implement the pattern accordingly.

Do mô hình lập chỉ mục viết ra từ nội dung của issue.