Windows 8.1 Bluetooth Low Energy GATT javascript event handler hangs

Question

I have a simple function that identifies the BLE devices with the simple key service (e.g. TI CC2541 sensor tag).  I identify the device, service and notify characteristic.  I then set the event handler and write the notify characteristic descriptor all as shown in this file on github in the discoverBleTags function.  The code seems to work but after several key events from the BLE device are processed, the event handler appears to stop.  Subsequent key presses are not processed.  If I rerun the discoverBleTags function, it will work briefly again but often the entire UI just hangs and I must kill the app.

What is the best way to determine why the events are stopping?  I've tested the BLE device on another operating system (iOS) and the notifications happen reliably so I do not suspect a problem with the BLE device.

If you don't want to view the code in github, here is the flow without error checking, output, etc.:

function discoverBleTags() {
    var gatt = Windows.Devices.Bluetooth.GenericAttributeProfile;
    Windows.Devices.Enumeration.DeviceInformation.findAllAsync(gatt.GattDeviceService.getDeviceSelectorFromUuid(keyServiceUuid), null)
    .done(function (devicesWithKeyService) {
        for (var i = 0; i < devicesWithKeyService.length; i++) {
            gatt.GattDeviceService.fromIdAsync(devicesWithKeyService[i].id)
            .done(function (service) {
                var notifyCharacteristic = service.getCharacteristics(keyCharacteristicUuid)[0];
                notifyCharacteristic.addEventListener("valuechanged", simpleKeyEvent, false);
                notifyCharacteristic.writeClientCharacteristicConfigurationDescriptorAsync(gatt.GattClientCharacteristicConfigurationDescriptorValue.notify);
            });
        };
    });
};

function simpleKeyEvent(args) {
    // print a message
};

Thanks!

Tom

Answer 1

update:

I built and deployed in the Release configuration and see the app actually terminate after this issue appears.  Below is the text from the Windows event report:

Version=1
EventType=MoAppHang
EventTime=130276334380906944
ReportType=3
Consent=1
ReportIdentifier=06f2c242-4196-11e3-be7a-6c71d9974a88
IntegratorReportIdentifier=06f2c243-4196-11e3-be7a-6c71d9974a88
NsAppName=praid:App
Response.type=4
Sig[0].Name=Package Full Name
Sig[0].Value=72ac2404-e852-4fa5-a0be-9eaa1f759dd1_1.0.0.0_neutral__vs7ccs9ww9cca
Sig[1].Name=Application Name
Sig[1].Value=praid:App
Sig[2].Name=Application Version
Sig[2].Value=6.3.9600.16384
Sig[3].Name=Application Timestamp
Sig[3].Value=5215d0bb
Sig[4].Name=Hang Signature
Sig[4].Value=44ce
Sig[5].Name=Hang Type
Sig[5].Value=2097152
DynamicSig[1].Name=OS Version
DynamicSig[1].Value=6.3.9600.2.0.0.256.48
DynamicSig[2].Name=Locale ID
DynamicSig[2].Value=1033
DynamicSig[22].Name=Additional Hang Signature 1
DynamicSig[22].Value=44ceb9da512bb6d55af34592ff3eb595
DynamicSig[23].Name=Additional Hang Signature 2
DynamicSig[23].Value=5681
DynamicSig[24].Name=Additional Hang Signature 3
DynamicSig[24].Value=5681cfc5f3b44ce8ecbd131b8f1460f2
DynamicSig[25].Name=Additional Hang Signature 4
DynamicSig[25].Value=44ce
DynamicSig[26].Name=Additional Hang Signature 5
DynamicSig[26].Value=44ceb9da512bb6d55af34592ff3eb595
DynamicSig[27].Name=Additional Hang Signature 6
DynamicSig[27].Value=5681
DynamicSig[28].Name=Additional Hang Signature 7
DynamicSig[28].Value=5681cfc5f3b44ce8ecbd131b8f1460f2
UI[3]=Microsoft WWA Host is not responding
UI[4]=Windows can check online for a solution. If you close the program, you might lose information.
UI[5]=Check for a solution and close the program
UI[6]=Check for a solution and close the program
FriendlyEventName=Stopped responding and was closed
ConsentKey=AppHangXProcB1
AppName=72ac2404-e852-4fa5-a0be-9eaa1f759dd1_vs7ccs9ww9cca!App
AppPath=C:\Windows\System32\WWAHost.exe
ReportDescription=A problem caused this program to stop interacting with Windows.
NsPartner=windows
NsGroup=windows8
ApplicationIdentity=7C4F8E8B9894B52C2EF7EDB4EED76951

Answer 2

In each iteration, you are adding an event listener without removing the old one. Can you try unregistering the event once you are done with it?

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Answer 3

Thanks for looking at this, Prashant.

I have tested this where I only registered an event listener / handler for the first device instance and see the same behavior.

To your point though, is there a problem registering an event listener for each device instance?  Once I get this problem resolved, I do want to respond to notifications from multiple BLE devices.

I should also note that I did not see this problem on the Win 8.1 developer preview.

Answer 4

I don't think it should be a problem, but I am checking further on this issue internally. I will keep you updated.

Btw, right before the app hangs, can you step through and tell what line of code is it actually hanging on? When the UI becomes unresponsive and the app hangs, can you go to Task manager, go to the details tab, locate your process, right click and "Create a dump file"? Once you create the dump file, you can zip it and share via Skydrive.

I don't see any bug reports around this area, but let's get a dump file to understand what is going on here.

Thanks,

Prashant.

---

Windows Store Developer Solutions, follow us on Twitter: @WSDevSol|| Want more solutions? See our blog

Answer 5

Hello,

The issue with the code above is that the GattDeviceService instance you obtain by calling the FromIsAsync method is the gatekeeper for all interaction with the device, as a consequence of this, the "service" variable you obtain from FromIdAsync must be kept alive for as long as you use the device.

If you take a look at the Bluetooth Generic Attribute Profile samples (http://code.msdn.microsoft.com/windowsapps/Bluetooth-Generic-5a99ef95#content)  the HeartRateService object (heart-rate-service.js) is defined as a singleton which explicitly manages the lifetime of the service object (instantiates it in the initializeHeartRateServiceAsync method and cleans it up in the "oncheckpoint" event handler).

Without holding on to the GattDeviceService instance, the object will just get cleaned up along with any device resources being used or any registered event handlers.

Not sure what the source of the hang is though, it could happen if the service object gets cleaned up while the event handler is actually running, or it could even be unrelated.

After you've made sure that the service instance doesn't get released while you're using it, please let us know if the hang still happens, and we can help troubleshoot this further.

Thank you,

Florin

Answer 6

Florin,

Releasing the GattDeviceService was indeed my problem.  I'm no longer able to reproduce the hang.

Sincere thanks to you and Prashant for taking the time to look at this.  You've been a huge help.

Tom