Step 3: Events ​

The framework calls main on every roundtrip — on the initial start and again after each user interaction. As soon as an app reacts to input, main has to tell those calls apart. That job falls to the lifecycle checks, and main becomes a dispatcher:

CLASS zcl_app_walkthrough DEFINITION PUBLIC.
  PUBLIC SECTION.
    INTERFACES z2ui5_if_app.

  PROTECTED SECTION.
  PRIVATE SECTION.
ENDCLASS.

CLASS zcl_app_walkthrough IMPLEMENTATION.
  METHOD z2ui5_if_app~main.

    IF client->check_on_navigated( ).

      DATA(view) = z2ui5_cl_ui5_view_builder=>factory(
          )->ele( n = `View` ns = `mvc`
              )->a( n = `xmlns`     v = `sap.m`
              )->a( n = `xmlns:mvc` v = `sap.ui.core.mvc`

              )->ele( `Shell`
                  )->ele( `Page`
                      )->a( n = `title` v = `Walkthrough - Step 3`

                      )->tag( `Text`
                          )->a( n = `text` v = `Hello World`
                      )->tag( `Button`
                          )->a( n = `text`  v = `Say Hello`
                          )->a( n = `press` v = client->_event( `SAY_HELLO` ) ).

      client->view_display( view->stringify( ) ).

    ELSEIF client->check_on_event( `SAY_HELLO` ).

      client->message_toast_display( `Hello from abap2UI5!` ).

    ENDIF.

  ENDMETHOD.
ENDCLASS.

The Lifecycle ​

┌──────────┐       ┌──────────┐       ┌──────────┐
│ Browser  │──────>│  main()  │──────>│ Browser  │
│ (Start)  │  HTTP │  view    │  HTTP │ (View)   │
└──────────┘       └──────────┘       └────┬─────┘
                                           │ user clicks
┌──────────┐       ┌──────────┐       ┌────┴─────┐
│ Browser  │<──────│  main()  │<──────│ Browser  │
│ (Toast)  │  HTTP │  event   │  HTTP │ (Event)  │
└──────────┘       └──────────┘       └──────────┘

• check_on_navigated( ) is true when the app has to draw its screen — on the first start, and again whenever the user navigates back to it later. This branch displays the view.
• check_on_event( ) is true when the user triggered an event. The argument names which one.
• client->_event( `SAY_HELLO` ) wires the button: it returns the press handler that sends the event — with the name you chose — back to main.

Each check_* method is true only for its own phase, so the IF/ELSEIF chain cleanly dispatches every roundtrip. The full picture is on the Life Cycle page.

Next: data leaves the browser and reaches your class — without a single line of transfer code.

Working Samples ​

Complete apps from the sample catalog that use what this page describes. Each is a single class in abap2UI5/samples unless its row names another of the three sample repositories — pull that repository with abapGit and start the class with ?app_start=<class>.