# Flow API<no value>
The functions here are designed to warn add-on packages about the different states of the simulation flow and the potential [FLT files](../../../content-configuration/flt-files/flt-file-properties/) that are loaded. The Flow API is a "global" API that is mainly used to deal with [Back On Track](../../../content-configuration/flt-files/back-on-track/) events, and it is available across the following platforms:

-   [WebAssembly](../../wasm/webassembly/)
-   [JavaScript](../javascript/)
-   [SimConnect SDK](../../simconnect/simconnect-api-reference/#flow)


You can find a sample project to use as a reference when using the Flow API here:

-   [FlowAircraft](../../../samples-tutorials/samples/simobjects-aircraft/modularaircraft/flowaircraft/)


There are also debugging tools available in DevMode which can be used to help debug Flow Events and API events, which is described on the following page:

-   [Global API Debugging](../../../devmode/menus/debug_info/debug-platform-dispatcher/)


### Using The Flow API

To receive flow event from the simulation, you must follow the steps outlined below:


1.  Register the listener in the JavaScript code:

    ``` js
    this.platformDispatcherListener = RegisterViewListener('JS_LISTENER_COMM_BUS');
    // or 
    this.platformDispatcherListener = RegisterCommBusListener(() => {
        ...
    });
    ```

     

2.   Create a callback which will be used when a flow event is received:

    ``` js
    ReceiveEvent(dataStr) {
         var data = JSON.parse(dataStr);
         // dataStr is a json which respects the format : 
         // { "event" : id_of_the_event, "flt_path" : "path_of_the_flt" }
    }
    ```

    The data string that is sent requires the following two data-points:
{{< table-wrapper >}}

    | Parameters        | Description                                                                                                 |
    |-------------------|-------------------------------------------------------------------------------------------------------------|
    | `id_of_the_event` | The ID of a flow event that follows the enum defined in [FsFlowEvent](../../wasm/flow-api/fsflowevent/). |
    | `path_of_the_flt` | The VFS path of a potential FLT file loaded with the event received.                                        |

{{< /table-wrapper >}}

     

3.  Finally you need to register the corresponding `__FLOW_API__ ` to call the callback on Event:

    ``` js
    this.platformDispatcherListener.on("__FLOW_API__", this.ReceiveEvent);
    ```