How can I get the value of left top and right bottom point value of current working window(the window currently focusing on)?
This is described in chapter The Window Start and End Positions of the elisp manual. Specifically:
-- Function: window-start &optional window This function returns the display-start position of window WINDOW. If WINDOW is ‘nil’, the selected window is used.
-- Function: window-end &optional window update This function returns the position where display of its buffer ends in WINDOW. The default for WINDOW is the selected window.
Be sure to read the whole manual section for details.