[Vala] Setting property doesn't emit notify signal

[Yu Feng <rainwoodman gmail com>]

vala will translate setting property:
---
a.prop = "value"
----
to
--- 
class_a_set_prop (a, "value");
---
and this won't emit a notify signal, because the signal is only emitted
by g_object_set.

However, if I want to catch the signal, what should I do?

Yu