add #[serde(default)] to Node (and, perhaps other similar structs)

[CupOfTeaJay]

Objective

Many structs that implement Serialize and Deserialize (when Bevy's serialize feature is enabled) also implement Default, but do not apply the #[serde(default)] attribute. This requires those who want to deserialize said structs to explicitly specify all struct fields in their data-interchange format, even if they all end up being their default values.

For example, I am trying to implement a plugin that allows me to specify a UI tree in JSON files. I noticed that when I tried to deserialize the Node component (which has 39 fields!) with serde, I was met with an error that in essence would have required me to specify all fields of Node in the JSON file, of which I was only using non-default values for:

• height
• width
• border
• flex_direction

Solution

1. Apply #[serde(default)] to the Node struct
2. If '1' is reasonable, perhaps start a conversation about applying this attribute to all structs that implement Serialize, Deserialize, and Default ?

After applying this attribute on a local fork of Bevy, I was able to deserialize my UI tree exactly as intended with minimal markup in my JSON files.

There might be a reason not to do this that I am not aware of, I tried looking for similar past issues but could not find much conversation about #[serde(default)]. Documentation on this attribute can be found here.

#[serde(default = "path")]

When deserializing, any missing fields should be filled in from the object returned by the given function or method. The function must be callable as fn() -> T. For example default = "my_default" would invoke my_default() and default = "SomeTrait::some_default" would invoke SomeTrait::some_default(). Only allowed on structs.

Testing

I did not write any formal tests for this. Informally, I simply added #[serde(default)] to Node to a local fork of Bevy and successfully deserialized an instance of Node with default values not specified in my JSON file.

Sample JSON

Note how I only needed to specify 4/39 fields for Node. Upon deserialization, serde invoked Node::default() to populate the missing fields.

{
    "UiRoot": {
        "Node": {
            "height": {"Percent": 100.0},
            "width": {"Percent": 100.0},
            "border": {"left": {"Px": 5.0}, "right": {"Px": 5.0}, "top": {"Px": 5.0}, "bottom": {"Px": 5.0}},
            "flex_direction": "Row"
        },
        "BorderColor": {"Srgba": {"red": 1.0, "green": 0.0, "blue": 0.0, "alpha": 1.0}},
        "Children": ["CreateGameButton"]
    },
    "CreateGameButton": {
        "Node": {
            "height": {"Percent": 25.0},
            "width": {"Percent": 25.0},
            "border": {"left": {"Px": 5.0}, "right": {"Px": 5.0}, "top": {"Px": 5.0}, "bottom": {"Px": 5.0}},
            "flex_direction": "Row"
        },
        "BorderColor": {"Srgba": {"red": 0.0, "green": 1.0, "blue": 0.0, "alpha": 1.0}},
        "Interaction": "None"
    }
}

[Henauxg]

EDIT possible alternatives:

• (Never mind for this one, adding #[serde(default)] to your own wrapped fields is not a valid alternative for this behavior)
• However, there are working alternatives in the form of reflection being worked on for the new Bevy Scene/UI system. While the official update is not there yet, contributors at the bevy_editor_prototypes made a working prototype: https://github.com/bevyengine/bevy_editor_prototypes/blob/3e1dad501ed762fddc3503748464800a4d94ab23/crates/bevy_bsn/examples/bsn_asset.rs