Re: [gdbus-codegen] handle property set on the service-side

[Simon McVittie <smcv collabora com>]

On Mon, 05 Feb 2018 at 15:22:00 +0100, Gabriel LUCAS wrote:
> I think I might have reached some limitation with this method while trying
> to handle a property set from the service side. The generated code provides
> a vtable for the interface with the property_get and property_set handlers
> set. When a property set is received, the GObject property is automatically
> changed without having to be handled by the developer.
>
> My opinion is that the service should be able to refuse a property set in
> some cases.

One option is to set properties via method calls: this makes it easier to
document which errors they can raise, makes it possible to have several
properties change atomically as a batch, and makes it (I suspect)
more likely that all D-Bus clients will be able to make those changes
asynchronously and cope with failure (or cope with the property being
set to a differing value, e.g. undergoing some sort of normalization).

> For example, a property GNSSEnabled can be set true to activate
> the GNSS output only if the modem is powered on. Then, I'll need to return
> an error and prevent the property set if the modem is off.

For example, you could have a read-only GNSSEnabled property paired with
an EnableGNSS() method; or maybe a read-only GNSSEnabled property
paired with an EnableFeatures() method that takes an array of strings
or integers representing features as its argument.

The underlying GDBusInterfaceVTable makes it possible to handle setting
properties asynchronously (and the documentation doesn't explicitly say
so, but this is also failable) but I don't think gdbus-codegen currently
has a way to generate code that makes use of that feature.

    smcv