GATT library directly reading characteristic values without iterating through services - linux

Question

I'm trying to use go on a raspberry pi to query bluetooth low energy devices. It's functional, I can connect to the device I want and iterate through the services and characteristics of the connected device. Now I'm just trying to streamline things and just read/write the values I'm interested in. It isn't working.
Code:
func onPeriphConnected(p gatt.Peripheral, err error) {
fmt.Println("Connected")
defer p.Device().CancelConnection(p)
if err := p.SetMTU(500); err != nil {
fmt.Printf("Failed to set MTU, err: %s\n", err)
}
batteryServiceId := gatt.MustParseUUID("180f")
// Direct read attempt (not working)
batterySerivce := gatt.NewService(batteryServiceId)
batteryLevelUUID := gatt.MustParseUUID("2a19")
batteryChar := gatt.NewCharacteristic(batteryLevelUUID,batterySerivce,gatt.Property(0x12),0,0)
e, err := p.ReadCharacteristic(batteryChar)
if err != nil {
fmt.Printf("Failed to read battery level, err: %s\n", err)
} else {
fmt.Println(e)
}
// iterate services read (working)
ss, err := p.DiscoverServices(nil)
if err != nil {
fmt.Printf("Failed to discover services, err: %s\n", err)
return
}
for _, s := range ss {
if(s.UUID().Equal(batteryServiceId)) {
fmt.Println("Found the battery service")
// Discovery characteristics
cs, err := p.DiscoverCharacteristics(nil, s)
if err != nil {
fmt.Printf("Failed to discover characteristics, err: %s\n", err)
continue
}
for _, c := range cs {
msg := " Characteristic " + c.UUID().String()
if len(c.Name()) > 0 {
msg += " (" + c.Name() + ")"
}
msg += "\n properties " + c.Properties().String()
fmt.Println(msg)
if (c.Properties() & gatt.CharRead) != 0 {
b, err := p.ReadCharacteristic(c)
if err != nil {
fmt.Printf("Failed to read characteristic, err: %s\n", err)
continue
}
fmt.Printf(" value %x\n", b)
}
}
}
}
}
Results:
Connected
[10 0 0 1]
Found the battery service
Characteristic 2a19 (Battery Level)
properties read notify
value 53
You can see where I expect to get a hex value of 53 I'm instead getting an array of [10 0 0 1]. I'm pretty new to go so I'm probably missing something here or just assembling my read incorrectly. Any pointers are much appreciated. Thanks!

Answer 1

A link to the appropriate documentation would be advisable. I don't know if this link is correct as there seem to be multiple different versions of package gatt.
Edit: see also https://godoc.org/github.com/paypal/gatt/examples/service#NewBatteryService at https://github.com/paypal/gatt/blob/master/examples/service/battery.go, which appears to show the right way of creating a battery service directly. Original answer below.
Having had a quick scan through said documentation, two things leap out at me:
The battery level ID is 2a19. The battery service ID is 180f. You use:
batteryServiceId := gatt.MustParseUUID("180f")
batterySerivce := gatt.NewService(batteryServiceId)
batteryChar := gatt.NewCharacteristic(batteryLevelUUID, batterySerivce,
gatt.Property(0x12), 0, 0)
e, err := p.ReadCharacteristic(batteryChar)
(I kept variable name spellings, but added a bit of white space to fit better on the StackOverflow display.) You never call NewDescriptor nor either of AddDescriptor or SetDescriptors on batteryChar. Are such calls required? I don't know; the documentation doesn't say. But the call that works uses DiscoverServices followed by DiscoverCharacteristics, which perhaps does create these (documented but undescribed) Descriptors. They look like they interpose themselves in value-oriented operations, so they might be critical.
Looking further at the code, after or instead of creating a characteristic directly, I think you do have to at least link the characteristic back into the service. The right call might be AddCharacteristic or SetCharacteristics. See this chunk of code in DiscoverCharacteristics.
(Minor.) gatt.Property(0x12) is definitely the wrong way to construct the constant. You probably should use:
gatt.Property.CharRead | gatt.Property.CharNotify