Re: [PATCH v4 01/10] Battery: Add Battery Service Gatt Client

Hi Chen, Johan,

On Thu, Aug 16, 2012 at 10:24 PM, Johan Hedberg [off-list ref] wrote:

Hi Chen,

On Thu, Aug 16, 2012, Chen Ganir wrote:

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However, i fail to understand why the object manager prevents this
from getting accepted - do you have a time estimation when the
object manager should be active or available ?

By the time of our next release (as per the content under the BlueZ 5
section in our TODO file).

Is there anywhere some documentation or instructions how to adapt
the API for the new object manager ? Is it backward compatible to
what we have today ? Do we really hold back DBUS additions until we
change the DBUS API ?

As an unrelated thing (but since I saw it in your commit messages too):
please spell D-Bus as D-Bus, not DBUS, DBus, or anything else in natural
written language. This keeps it consistent with the rest of our
documentation as well as how the D-Bus project itself spells the name.

Regarding the Object Manager interface the implementation details you'd
get from Lucas and Luiz who are working on it whereas the API spec is
available here:

http://dbus.freedesktop.org/doc/dbus-specification.html#standard-interfaces-objectmanager

With object manager we have to keep 2 things in mind while designing
new interfaces:

1. In case of returning new objects, as in CreateDevice, always
include the interfaces and properties e.g. D-Bus signature oa{sa{sv}}
(we will probably have a gdbus helper to do that) which is the
equivalent to InterfacesAdded signal. This avoid having to call
GetManagedObjects.
2. No need to keep a list of objects or emit signals when one is
created/destroyed, gdbus will take care of this automatically.

Now if this is really relevant here Im not sure.

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I prefer this way of putting the batteries below the device, since it
is obvious which battery belongs to each device.

I fully agree with this. It's not what I had an issue with. However, if
we really want to make this clear you could additionally have a "Device"
property for each battery object pointing back to the parent object.

I can do that, if it helps. Although it is not necessary, since the
object path for a battery already contains the device objec path in
it :
[variable prefix]/{hci0,..}/dev_XX_XX_XX_XX_XX_XX/BATT-NN-DDDD
Adding a device object path as a property will be redundant.

Even though we do have such a hierarchical construction of the paths I
don't think we should encourage this kind of magic tricks in trying look
into actual values of object paths to infer their relation to each
other. However, since the battery object paths would be discovered
through the device object there might not be any need for such a
backwards reference.

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The other option i can think of is to have another interface
registered on the device object path:

Service             org.bluez
Interface   org.bluez.Batteries
Object path [variable prefix]/{hci0,..}/dev_XX_XX_XX_XX_XX_XX

Methods     dict GetProperties()

           Returns all properties for the interface. See the
           Properties section for the available properties.

Signals             ProperyChanged(string name, variant value)

           This signal indicates a changed value of the given
           property.

Properties  array{object} Batteries [readonly]

           List of device battery object paths that represents the available
batteries on the remote devices.


Service             org.bluez
Interface   org.bluez.Battery
Object path [variable prefix]/{hci0,..}/dev_XX_XX_XX_XX_XX_XX/BATT-NN-DDDD


Methods     dict GetProperties()

           Returns all properties for the interface. See the
           Properties section for the available properties.

Signals             PropertyChanged(string name, variant value)

           This signal indicates a changed value of the given
           property.

Properties  byte Level [readonly]

                   Battery level (0-100).

Any other suggestion ?

That would work, although both of these interfaces are rather
light-weight in that they only contain a single property which begs the
question whether this is a bit over-kill.

I know. I believe so as well, but if you want to separate the
interfaces, it is a must.

quoted

Another option is to do away with the per-battery object somehow and
expose all batteries through a single interface. I'm not so sure that's
a good idea though since it could prevent future extensibility of the
API and doesn't reuse the common tools we have for object-like
abstractions (which a battery certainly is).

If we do what you suggest, a battery name (namespace+descriptor)
will be the property name, and the value should be the battery
level. i'm not too comfortable with it, and it is different than
what we used to do up until now.

So where do we go from here ?

Some extra opinions/view points could help. Maybe Marcel or Luiz have
some preferences/ideas on how to do this.

Im trying to think how having multiple objects could be useful for the
UI, most likely it only gonna show a single meter aggregating all the
batteries.

Another thing that worth adding is that there are other profiles that
do support battery status such as HFP and AVRCP, so I think this
should be made generic enough so other sources could be supported.

-- 
Luiz Augusto von Dentz