# ASIO

## Properties

Properties can be dynamically registered to an interface by calling
`register_property()`. If you need to provide a custom getter or setter for a
property, it should follow the guidelines in this section.

To return a D-Bus error reply for either GetProperty or SetProperty, throw a
custom exception derived from `sdbusplus::exception_t`. For the custom
exception, you can return a well-defined org.freedesktop.DBus.Error
(https://www.freedesktop.org/software/systemd/man/sd-bus-errors.html#) from the
`name()` or a custom/arbitrary name. The former will be automatically translated
into a matching error code
(https://github.com/systemd/systemd/blob/485c9e19e7ebcd912d5fbf11f40afc62951173f8/src/libsystemd/sd-bus/bus-error.c)
that can be consumed by the caller, while the latter will always be mapped to
`EIO`, requiring a `strcmp` to determine the exact error name.

The handler may also throw any exception not derived from
`sdbusplus::exception_t`, in which case a generic
org.freedesktop.DBus.Error.InvalidArgs error will be returned to the caller.

### Get Property callback

When registering a property using ASIO methods, the get callback should meet the
following prototype:

```c++
PropertyType getHandler(const PropertyType& value);
```

The first argument is the current value cached in the D-Bus interface (i.e. the
previously "getted" value, or the initialized value), and the return value is
returned to the D-Bus caller.

### Set Property callback

When registering a writable property using ASIO methods, the set callback should
meet the following prototype:

```c++
bool setHandler(const PropertyType& newValue, PropertyType& value);
```

The first argument is the new value requested to be set by the D-Bus caller.

The second argument is the actual underlying value contained within the object
server. If the new value meets the expected constraints, the handler must set
`value = newValue` to make the set operation take effect and return true. If the
new value is invalid or cannot be applied for whatever reason, the handler must
leave `value` unmodified and return false.

If the handler returns false but doesn't throw an exception, a generic
org.freedesktop.DBus.Error.InvalidArgs error will be returned to the caller.