---
title: Lua ↔ MD event exchange
summary: A complete request route from the UI to Mission Director and a reply back to Lua.
section: ui
kind: example
status: example
tags: ["X4","modding","md","lua","bridge"]
sources: ["https://wiki.egosoft.com/X4%20Foundations%20Wiki/Modding%20Support/"]
---
## Exchange directions


| Direction | Function or condition |
| --- | --- |
| Lua → MD | `AddUITriggeredEvent(screen, control, payload)` |
| MD receiver | `event_ui_triggered`, with `event.param3` in this contract |
| MD → Lua | `raise_lua_event` |
| Lua receiver | `RegisterEvent(name, callback)` |


Names must match exactly. Register the receiver before sending if a reply can arrive immediately.



## Lua subscription and request


The example assumes the UI file is already registered through the manifest. It illustrates the call form, not universal world readiness. In a real mod, send only at an established point in both UI and MD lifecycles.




```lua
local received = false

RegisterEvent("Wiki.Pong", function(_, payload)
    received = payload == "pong"
end)

AddUITriggeredEvent("WikiBridge", "ping", "hello")
```

## MD handling and reply


Insert this fragment in your script's `cues`. It matches screen/control, validates the simple hello payload and sends the named pong reply.




```xml
<cue name="Ping" instantiate="true" namespace="this">
  <conditions>
    <event_ui_triggered screen="'WikiBridge'" control="'ping'" />
  </conditions>
  <actions>
    <do_if value="event.param3 == 'hello'">
      <raise_lua_event name="'Wiki.Pong'" param="'pong'" />
    </do_if>
  </actions>
</cue>
```

## When an exchange becomes an operation


Validate input types and object existence before a game effect. Handle repeated clicks separately for actions that create objects or spend money. Show success after actual execution. Assemble complete multipart replies before replacing a displayed list. See [custom windows](wiki:create-ui) and [AddUITriggeredEvent](wiki:add-ui-triggered-event).