gattc: add Read() characteristic method implementations for Linux and nRF528xx

Signed-off-by: deadprogram <ron@hybridgroup.com>

adapter_nrf528xx.go

@@ -219,6 +219,16 @@ func handleEvent() {
 					}
 				}
 			}
+		case C.BLE_GATTC_EVT_READ_RSP:
+			readEvent := gattcEvent.params.unionfield_read_rsp()
+			if debug {
+				println("evt: read response, data length", readEvent.len)
+			}
+			readingCharacteristic.handle_value.Set(readEvent.handle)
+			readingCharacteristic.offset = readEvent.offset
+			// Create a Go slice from the data.
+			readingCharacteristic.value = (*[255]byte)(unsafe.Pointer(&readEvent.data[0]))[:readEvent.len:readEvent.len]
+
 		case C.BLE_GATTC_EVT_HVX:
 			hvxEvent := gattcEvent.params.unionfield_hvx()
 			switch hvxEvent._type {

examples/discover/main.go

@@ -68,6 +68,12 @@ func main() {
 		}
 		for _, char := range chars {
 			println("-- characteristic", char.UUID().String())
+			val, err := char.Read()
+			if err != nil {
+				println("---", err.Error())
+			} else {
+				println("--- value =", string(val))
+			}
 		}
 	}
 

gattc_darwin.go

@@ -145,3 +145,8 @@ func (c *DeviceCharacteristic) EnableNotifications(callback func(buf []byte)) er
 
 	return nil
 }
+
+// Read reads the current characteristic value.
+func (c DeviceCharacteristic) Read() (data []byte, err error) {
+	return nil, nil
+}

gattc_linux.go

@@ -232,3 +232,9 @@ func (c *DeviceCharacteristic) EnableNotifications(callback func(buf []byte)) er
 	}()
 	return c.characteristic.StartNotify()
 }
+
+// Read reads the current characteristic value.
+func (c DeviceCharacteristic) Read() ([]byte, error) {
+	options := make(map[string]interface{})
+	return c.characteristic.ReadValue(options)
+}

gattc_sd.go

@@ -337,6 +337,38 @@ func (c *DeviceCharacteristic) WriteWithoutResponse(p []byte) (n int, err error)
 	return len(p), nil
 }
 
+// A global used to pass information from the event handler back to the
+// Read function below.
+var readingCharacteristic struct {
+	handle_value volatile.Register16
+	offset       uint16
+	value        []byte
+}
+
+// Read reads the current characteristic value up to MTU length.
+// A future enhancement would be to be able to retrieve a longer
+// value by making multiple calls.
+func (c DeviceCharacteristic) Read() ([]byte, error) {
+	errCode := C.sd_ble_gattc_read(c.connectionHandle, c.valueHandle, 0)
+	if errCode != 0 {
+		return nil, Error(errCode)
+	}
+
+	// wait for response with data
+	for readingCharacteristic.handle_value.Get() == 0 {
+		arm.Asm("wfe")
+	}
+
+	// copy data since value is slice to unsafe pointer.
+	data := make([]byte, len(readingCharacteristic.value))
+	copy(data, readingCharacteristic.value)
+
+	// prepare for next read
+	readingCharacteristic.handle_value.Set(0)
+
+	return data, nil
+}
+
 type gattcNotificationCallback struct {
 	connectionHandle uint16
 	valueHandle      uint16 // may be 0 if the slot is empty