Geofence Application: Geofence Alerts

To send an alert to that the geofence has been escaped we'll use the FlowMessaging Point to Point API. Using SendMessageToUser we can send a message to the user who owns the WiFire.

To use this API we will include flowmessaging like so

extern "C" {
    #include "flow/flowmessaging.h"
}

Because Flow APIs are C code and we are using C++ we need to import the headers inside a block of C code.

We can define alerts to be XML messages emitted when we want to alert a user or device of some event.

<alert>
    <sent>
        2015-01-29T05:08:16Z
    </gpsreadingtime>
    <type>
        ALERT TYPE
    </type>
</alert>

We'll define the string GEOFENCE ESCAPED for our alert type

#define GEOFENCE_ESCAPED ("GEOFENCE ESCAPED")

It would be possible to add structured (XML) details to alerts later, but we only need basic alerts at this time.

In order to avoid noise and continual alerts we only want to generate an alert if the escape from the fence is for a certain time. We also should only generate an alert if the device has been inside the fence for a period of time before leaving. This can be achieved by keeping track of when we were last inside and when we were last outside the fence, allowing us to work out how long we have been inside/outside the fence. If this time exceeds a defined threshold then we consider the device to have entered/exited and can change the state for monitoring for the opposite even, along with generating an alert when a "exiting" event is detected.

static long lastInsideFence = 0;
static long lastOutsideFence = 0;
#define FENCE_TIME_THRESHOLD (1*60*1000)
if (gps.location.isValid() && fence.radius > 0)
{
    // check for escaped geofence
    double distance = gps.distanceBetween(
                            fence.latitude,     fence.longitude, 
                            gps.location.lat(),   gps.location.lng()
                      );
    if (distance > fence.radius)
    {
        // escaped
        lastOutsideFence = millis();
        if (insideFence && 
            lastOutsideFence - lastInsideFence > FENCE_TIME_THRESHOLD)
        {
            insideFence = false;
            sendAlert(GEOFENCE_ESCAPED);
        }
    }
    else 
    {
        lastInsideFence = millis();
        digitalWrite(PIN_LED1, LOW);
        if (!insideFence && 
            lastInsideFence - lastOutsideFence > FENCE_TIME_THRESHOLD)
        {
            insideFence = true;
        }
    }
}

To send an alert we can construct the alert XML using

void sendAlert(const char *type)
{
    XMLNode alert("alert");

    XMLNode &sent = alert.addChild("sent");
    setDatetimeContent(sent);

    XMLNode &typeNode = alert.addChild("type");
    typeNode.setContent(type);

    StringBuilder alertStringBuilder = StringBuilder_New(512);
    alertStringBuilder = StringBuilder_Append(alertStringBuilder, "<?xml version=\"1.0\" encoding=\"UTF-8\"?>");
    alert.appendTo(alertStringBuilder);

    FlowMessaging_SendMessageToUser(
        g_OwnerID, 
        "text/plain", 
        (char *)StringBuilder_GetCString(alertStringBuilder), 
        StringBuilder_GetLength(alertStringBuilder), 
        120
    );

    StringBuilder_Free(&alertStringBuilder);
}

Note specifically FlowMessaging_SendMessageToUser from the FlowMessaging library. To identify the target we use the FlowCloud variable for the owner's ID. To send messages to other devices we could use FlowMessaging_SendMessageToDevice. In this case the device IDs could be hard-coded or fetched programmatically using the FlowDevices API.

We've now finished our geofence application on the WiFire. The final code can be found in the GitHub repository.

FlowCloud on Arduino

Geofence Application: Geofence Alerts