Add default value for properties

[mdtusz]

Context

When creating a component, there are often cases where the component can and should function without having an explicit value provided as a prop. While currently, props structs will provide a default value for a field if Default is implemented for it's type, this is not always the value that is desired or required by the component. For example, a bool will default to false and numerics will default to 0. In many sitiuations, a component will require alternative defaults than the type defaults.

Possibilities

With serde, default values can be provided using a syntax like

#[derive(Deserialize, Debug)]
struct Request {
    // Use the result of a function as the default if "resource" is
    // not included in the input.
    #[serde(default = "default_resource")]
    resource: String,
}

fn default_resource() -> String {
    "/".to_string()
}

A similar approach could be taken for property structs:

#[derive(Properties)]
struct MyProps {
    #[props(default = "default_value")]
    value: u32,
    #[props(default = "always_true")]
    enabled: bool
}

fn default_value() -> u32 {
  100
}

fn always_true() -> bool {
  true
}