# 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: ```abap 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 ```text ┌──────────┐ ┌──────────┐ ┌──────────┐ │ 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](/cookbook/event_navigation/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](https://abap2ui5.github.io/playground/samples/) that use what this page describes. Each is a single class in [abap2UI5/samples](https://github.com/abap2UI5/samples) unless its row names another of the three sample repositories — pull that repository with [abapGit](https://abapgit.org) and start the class with `?app_start=`. | Sample | Class | |---|---| | Basics III — Lifecycle: Init, Event, Navigated | [`Z2UI5_CL_SMP_APP_495`](https://github.com/abap2UI5/samples/blob/main/src/01/z2ui5_cl_smp_app_495.clas.abap) | | Basics IV — Events, Views and Roundtrips | [`Z2UI5_CL_SMP_APP_004`](https://github.com/abap2UI5/samples/blob/main/src/00/97/z2ui5_cl_smp_app_004.clas.abap) |