Building Multi level tree ALV with class CL_GUI_LIST_TREE

DATA :    lr_cust TYPE REF TO cl_gui_custom_container,
              lr_list_tree TYPE REF TO cl_gui_list_tree,
              lt_flight TYPE TABLE OF scarr,
              ls_flight TYPE scarr,
              lt_spfli TYPE TABLE OF spfli,
              ls_spfli TYPE spfli,
              lt_node TYPE  treev_ntab,
              ls_node TYPE LINE OF treev_ntab,
              lt_item TYPE TABLE OF mtreeitm,
              ls_item TYPE mtreeitm .

START-OF-SELECTION.
  CALL SCREEN 0001.
*&---------------------------------------------------------------------*
*&      Module  STATUS_0001  OUTPUT
*&---------------------------------------------------------------------*
MODULE status_0001 OUTPUT.
  SET PF-STATUS 'LIST_STATUS'.
ENDMODULE.                 " STATUS_0001  OUTPUT
*&---------------------------------------------------------------------*
*&      Module  GET_DATA  OUTPUT
*&---------------------------------------------------------------------*
MODULE get_data OUTPUT.
  IF lt_flight IS INITIAL.
    SELECT * FROM scarr INTO TABLE lt_flight.
    IF lt_flight IS NOT INITIAL.
      SELECT * FROM spfli INTO TABLE lt_spfli FOR ALL ENTRIES IN lt_flight
                                           WHERE carrid = lt_flight-carrid.
    ENDIF.
  ENDIF.
ENDMODULE.                 " GET_DATA  OUTPUT
*&---------------------------------------------------------------------*
*&      Module  BUILD_LIST_TREE  OUTPUT
*&---------------------------------------------------------------------*
MODULE build_list_tree OUTPUT.
  IF lr_cust IS NOT BOUND.
    CREATE OBJECT lr_cust
      EXPORTING
        container_name = 'LIST_TREE'
        repid          = sy-repid
        dynnr          = sy-dynnr.
  ENDIF.

  IF lr_list_tree IS NOT BOUND.
    CREATE OBJECT lr_list_tree
      EXPORTING
        parent                      = lr_cust
        node_selection_mode         = cl_gui_list_tree=>node_sel_mode_single
*       hide_selection              =     " Visibility of Selection
        item_selection              = 'X'  " Can Individual Items be Selected?
        with_headers                = ' '    " 'X': With Headers
*       hierarchy_header            =     " Hierarchy Header
*       list_header                 =     " List Header
*       name                        =     " Name
      EXCEPTIONS
        lifetime_error              = 1
        cntl_system_error           = 2
        create_error                = 3
        illegal_node_selection_mode = 4
        failed                      = 5
        OTHERS                      = 6.
  ENDIF.

  IF lr_list_tree IS BOUND.
* Building Root Node
    ls_node-node_key = 'Flight'.
    ls_node-hidden = ' '.
    ls_node-hidden = ' '.
    ls_node-isfolder = 'X'.
    APPEND ls_node TO lt_node.
* Building sub node and relationship with root node
    LOOP AT  lt_flight INTO ls_flight.
      ls_node-node_key = ls_flight-carrid.
      ls_node-relatkey = 'Flight'.
      ls_node-relatship = cl_gui_list_tree=>relat_last_child.
      ls_node-isfolder = 'X'.
      APPEND ls_node TO lt_node.
      LOOP AT lt_spfli INTO ls_spfli WHERE carrid = ls_flight-carrid.
        ls_node-node_key = ls_spfli-connid.
        ls_node-relatkey = ls_flight-carrid.
        ls_node-relatship = cl_gui_list_tree=>relat_last_child.
        ls_node-isfolder = 'X'.
        APPEND ls_node TO lt_node.
      ENDLOOP.
    ENDLOOP.
* Building root Node Text
    ls_item-node_key = 'Flight'.
    ls_item-item_name = '1'.
    ls_item-class = cl_gui_list_tree=>item_class_text.
    ls_item-alignment = cl_gui_list_tree=>align_auto.
    ls_item-font = cl_gui_list_tree=>item_font_prop.
    ls_item-text = 'Flight_Info'.
    APPEND ls_item TO lt_item.
* Bulding all Item node text
    LOOP AT  lt_flight INTO ls_flight.
      ls_item-node_key = ls_flight-carrid.
      ls_item-item_name = '1'.
      ls_item-class = cl_gui_list_tree=>item_class_text.
      ls_item-alignment = cl_gui_list_tree=>align_auto.
      ls_item-font = cl_gui_list_tree=>item_font_prop.
      ls_item-text = ls_flight-carrname.
      APPEND ls_item TO lt_item.
      LOOP AT lt_spfli INTO ls_spfli WHERE carrid = ls_flight-carrid.
        ls_item-node_key = ls_spfli-connid.
        ls_item-item_name = '1'.
        ls_item-class = cl_gui_list_tree=>item_class_text.
        ls_item-alignment = cl_gui_list_tree=>align_auto.
        ls_item-font = cl_gui_list_tree=>item_font_prop.
        CONCATENATE ls_spfli-countryfr '-' ls_spfli-cityfrom '-'
                   ls_spfli-countryto '-' ls_spfli-cityto INTO ls_item-text.

        APPEND ls_item TO lt_item.

      ENDLOOP.
    ENDLOOP.

    CALL METHOD lr_list_tree->add_nodes_and_items
      EXPORTING
        node_table                     = lt_node
        item_table                     = lt_item
        item_table_structure_name      = 'MTREEITM'
      EXCEPTIONS
        failed                         = 1
        cntl_system_error              = 2
        error_in_tables                = 3
        dp_error                       = 4
        table_structure_name_not_found = 5
        OTHERS                         = 6.

  ENDIF.
ENDMODULE.                 " BUILD_LIST_TREE  OUTPUT
*&---------------------------------------------------------------------*
*&      Module  USER_COMMAND_0001  INPUT
*&---------------------------------------------------------------------*
*       text
*----------------------------------------------------------------------*
MODULE user_command_0001 INPUT.
  CASE sy-ucomm.
    WHEN 'BACK' OR 'EXIT' OR 'CANCEL'.
      LEAVE TO SCREEN 0.
  ENDCASE.
ENDMODULE.                 " USER_COMMAND_0001  INPUT 

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Execute.