How does one read/write to a GATT Characteristic using libgio/GTK?

[jaklug (John Klug)]

I need to write a multi-threading application using D-BUS/BlueZ/GATT Client and want to use libgio/GTK.

I took the scan code from author Parthiban:

https://gist.github.com/parthitce/408244ae90a13906a38f5756b0824cb7

His example scans and does not include GATT, but works quite well.

I am able to connect to the remote device easily enough:

result = g_dbus_connection_call_sync((GDBusConnection *)connection,
                        "org.bluez",  // bus name
                        object_path,  // Location of the remote device in DBUS
                        "org.bluez.Device1",  // interface name
                        "Connect",  // method name
                        NULL, // No parameters
                        NULL, // Expected reply type
                        G_DBUS_CALL_FLAGS_NONE,  // Flags
                        10000, // 10 second timeout
                        NULL, // Cancellable
                        &error  // error
);

Tracing HCI verifies that the open works.

This, as expected, created a bunch of characteristics in D-BUS that were made visible through :
prop_changed = g_dbus_connection_signal_subscribe(con,
“org.bluez”,
“org.freedesktop.DBus.Properties”,
“PropertiesChanged”,
NULL,
“org.bluez.Adapter1”,
G_DBUS_SIGNAL_FLAGS_NONE,
bluez_signal_adapter_changed,
NULL,
NULL);

GATT Characteristics that are discovered have a path that looks like this:
/org/bluez/hci0/dev_CC_F9_57_89_8B_D8/service000b/char000c

I then need to write and read the GATT characteristics. I tried to read a characteristic using this in a loop with repeat trials:

            gatt_objects = g_dbus_connection_call_sync(con,
                "org.bluez",
                "/org/bluez/hci0/dev_CC_F9_57_89_8B_D8/service000b/char000c",
                "org.freedesktop.DBus.Properties",
                "Get",
                NULL, // No parameters
                NULL,
                G_DBUS_CALL_FLAGS_NONE,  // Flags
                10000, // 10 second timeout
                NULL, // Cancellable
                &errorO  // error
            );  // User data for callback

But then I repeatedly get this error every time I attempt the Get:

0x7ff244008360: bluez_device_new_connection: org.bluez,/org/bluez/hci0/dev_CC_F9_57_89_8B_D8/service000b/char000c,org.freedesktop.DBus.Properties,Get  FAILED!
0x7ff244008360: print_error: Error message: GDBus.Error:org.freedesktop.DBus.Error.UnknownMethod: Method "Get" with signature "" on interface "org.freedesktop.DBus.Properties" doesn't exist
0x7ff244008360: print_error: Remote error
0x7ff244008360: print_error: Raw message: org.freedesktop.DBus.Error.UnknownMethod
0x7ff244008360: print_error: Stripped message: org.freedesktop.DBus.Error.UnknownMethod

I have been unable to find any GATT client examples using libgio.

[jaklug (John Klug)]

I found a solution. I was using the wrong D-BUS API.

The correct API to use is org.bluez.GattCharacteristic1

This has a ReadValue and a WriteValue interface.

           gatt_objects = g_dbus_connection_call_sync(con,
                "org.bluez",
                charp,
                "org.bluez.GattCharacteristic1",
                "ReadValue",
                g_variant_new_tuple(&v1, 1),  // Empty variant.
                NULL,
                G_DBUS_CALL_FLAGS_NONE,  // Flags
                10000, // 10 second timeout
                NULL, // Cancellable
                &errorO  // error
            );  // User data for callback