Casting an array of PropMaps inside a PropMap

[grimsteel]

I have a PropMap (a{sv}). One of the keys in this PropMap, foo, maps to an array of PropMaps (aa{sv}). However, when I try to cast this to a Vec<PropMap>, it returns None.

Setup code:

// This is one of the PropMaps in the inner array
let mut inner_prop_map = PropMap::new();
inner_prop_map.insert("foobar".into(), Variant(Box::new(123)));

let prop_map_array = vec![inner_prop_map];

let mut prop_map = PropMap::new();
prop_map.insert("foo".into(), Variant(Box::new(prop_map_array)));

Doing the prop cast on prop_map directly works:

assert!(cast::<Vec<PropMap>>(prop_map.get("foo").unwrap()).is_some());

But when I send it into a Message and bring it back out, it doesn't work (in reality, I'm getting this PropMap as a return value from org.freedesktop.NetworkManager for GetSettings)

let msg = Message::new_method_call("a.b.c", "/a/b/c", "a.b.c", "AaBb").unwrap().append1(prop_map);
let another_map = msg.get1::<PropMap>().unwrap();
assert!(cast::<Vec<PropMap>>(another_map.get("foo").unwrap().is_none());

The only thing that works is using as_iter or as_static_inner. I'm not sure why casting doesn't work.

(I want to be able to use cast_mut to get a mutable reference to the array so I can edit specific fields on some of the inner dicts. Let me know if this isn't the right approach.)

[mulkieran]

@grimsteel I'm afraid I can not explain the behavior you've observed. I did notice that the last line of your last example:

assert!(cast::<Vec<PropMap>>(another_map.get("foo").unwrap().is_none());

is missing a closing paren.

[grimsteel]

Yes, it seems like my example code had a couple of typos.

This:

use dbus::{Message, arg::{PropMap, Variant, cast}};

// This is one of the PropMaps in the inner array
let mut inner_prop_map = PropMap::new();
inner_prop_map.insert("foobar".into(), Variant(Box::new(123)));

let prop_map_array = vec![inner_prop_map];

let mut prop_map = PropMap::new();
prop_map.insert("foo".into(), Variant(Box::new(prop_map_array)));

assert!(cast::<Vec<PropMap>>(&prop_map.get("foo").unwrap().0).is_some());

let msg = Message::new_method_call("a.b.c", "/a/b/c", "a.b.c", "AaBb").unwrap().append1(prop_map);
let inner_map = &another_map.get("foo").unwrap().0;

// The problem clearly shows itself here - the signature is
// an array of dicts of strings to variants, but it can't be casted to a vec of propmaps
println!("{:?}", inner_map.signature()); // Signature("aa{sv}\0")
assert!(cast::<Vec<PropMap>>(inner_map).is_none());

compiles and runs with all of the assertions passing.

[diwic]

Signatures that start with aa cannot be downcasted to, as they don't have a stable RefArg type.

This is kind of implied by the argument guide 's reference section, but the docs could be clearer.

[grimsteel]

That makes more sense; I didn't really understand what "N/A" meant.

I can access the data inside with as_iter and as_static_inner, but is there any way to mutate the data inside of the array instead of copying the entire data structure?

[diwic]

@grimsteel I doubt it. I guess we have a few ways forward:

1. Just clarify the docs. Copying data is likely pretty fast, comparing to other things libdbus and dbus-rs do anyway
2. Add a as_mut_iter method, not sure how easy or hard this is.
3. Stabilize aa{sv} so there actually is a Vec<PropMap>. The implementation must be done carefully to avoid increased binary size (out of type explosion).

Any of these you feel like doing a PR for?

[diwic]

Added a sentence about "N/A" in the docs now.