serviceChanged() recommended workflow?

[chmorgan]

I'm working on a BLE device that has changed characteristics over time. Ran into an odd offset issue with iOS where one characteristic was showing up when another was read and in running it down found out about ble_svc_gatt_changed(), characteristic 0x2A05 etc.

My understanding is that if there is a change to the service that I should call ble_svc_gatt_changed() to inform the stack that all bonded peers need to rescan when they connect in the future. It looks like this functionality is exposed via serviceChanged().

Is there a flow you'd recommend in this use case? Let me throw out my straw man so we have something to discuss against. Perhaps this could become a PR for a doc addition as I don't find any mention of serviceChanged() or ble_svc_gatt_changed() anywhere except in the code.

NimBLEDevice::init();
auto pServer = NimBLEDevice::createServer();
auto pService = pServer->createService(serviceUUID);
// add characteristics
pService->start();

// determine if service has changed
auto getHash = pService->getHash(); // this doesn't appear to exist
if(getHash != nvs_get_hash())
{
    pService->serviceChanged();
   nvs_set_hash();
}

[h2zero]

@chmorgan that flow is pretty much correct, you could use all kinds of ways to determine if you should signal the services changed. The simplest however is just to call that on each boot then you don't have to think about it. Not a big deal for the connecting peers to discover services again.

[chmorgan]

@h2zero when would I make the pService->serviceChanged() call in that case, after starting the service as I've noted above?

[chmorgan]

oh wait, how do I call it? serviceChanged() is private?

[h2zero]

You would want to call it after all services at started.

You'll have to to use the underlying stack call for now, I'll change the member function to public later.

[chmorgan]

@h2zero alright, so I've got this mismatch case happening right now and the pServer->serviceChanged() isn't resolving the issue. I have a UUID like 0B01 that is being set to a float value every second. I've walked through the device code to confirm but on the BLE side with light blue it's reading a large json payload, just like you'd expect with a characteristic mismatch.

Perhaps I didn't put the call to serviceChanged() in the correct location? This is how the code looks right now:

    characteristics added here

    ...

    pResetRequest->setCallbacks(new ResetRequestCallbacks(this));

    constexpr auto US_PER_MS = 1000;

    auto end = esp_timer_get_time();
    auto duration_us = end - start;
    auto duration_ms = duration_us / US_PER_MS;

    ESP_LOGI(TAG, "%s(): %d characteristics and callbacks configured in %lld ms",
        __FUNCTION__, __LINE__, duration_ms);

    // restore the whitelist
    ret = WhiteListManager::Restore();
    if(ret != 0)
    {
        ESP_LOGE(TAG, "WhiteListManager::Restore()");
    } else
    {
        ESP_LOGI(TAG, "WhiteListManager::Restore() complete");
    }

    pService->start();

    pServer->serviceChanged();

    NimBLEAdvertising *pAdvertising = NimBLEDevice::getAdvertising(); // create advertising instance
    ret = pAdvertising->setName(deviceName);
    if(ret != true)
    ....

[h2zero]

Hmm this seems tricky, if you are still connected to the peer when you change the services then they will not take effect, they cannot be changed while there is active connections or advertising.

[chmorgan]

@h2zero I can confirm that the call occurred before the central connected, just rebooted and reconnected to the device and the characteristic is still wrong.

[h2zero]

@chmorgan Are you bonded with this device?
It's possible the peer device isn't listening to the indication and if not bonded then the stack won't send the indication after reset.

After the server starts (advertising->start) call ble_gatts_show_local(); and see if it looks correct to you, if so then it's the other side.

[chmorgan]

@h2zero alright, I'll check the output of ble_gatts_show_local(), maybe its on the iOS side? That seems highly unlikely though doesn't it?

[chmorgan]

@h2zero looking at ble_gatts_show_local() it looks like the correct set of characteristics. Do I need to expose the gatt service changed characteristics in my app before it will work?

[h2zero]

You may need to call this ble_svc_gatt_changed(0x0001, 0xffff); It should be called automatically when serviceChanged is set but maybe the timing is off.

[chmorgan]

@h2zero alright, calling ble_svc_gatt_changed() in place of the call to pServer->serviceChanged(); worked, lightblue is reading the correct value now.

I see that the serviceChanged() sets a flag but this is only checked in a few spots, maybe there is a gap where I should have called it before some other call? I also noticed that I'm seeing a compile warning as I'm calling pService->start(), the comment says:

bool NimBLEService::start()' is deprecated: NimBLEService::start() has no effect. Services are started when the server is started.

but I never call anything server->start(), I do however start advertising and maybe that gets back through to a call to start the server?

[h2zero]

You're right, pServer->serviceChanged() just sets a flag that is used when the GATT server is started, which if you don't call pServer->start() yourself the advertising start will do it for you. What's interesting is that with the service changed flag set the server should call ble_svc_gatt_changed when it's started, but it either didn't do that or something doesn't work at the time it's called.

pService->start() is no longer needed, it's an automatic now, you can remove those calls from your code.

[chmorgan]

I'll drop the call to service::start(). If there is anything else I can do, let me know.

[h2zero]

I see why this happens now, since you called serviceChanged before the server was started it didn't set the flag.
That function was intended to work with situations where the service has changed after the server was already running, not before starting, like adding a service when a peer writes a command etc..

If you want to keep working with the serviceChanged call, just call pServer->start() before serviceChanged and then start advertising. Otherwise just stick with the ble_svc_gatt_changed call.