Struct Topic 

pub struct Topic {
    pub name: &'static str,
    pub prefix: &'static str,
    pub gate: fn(&Context) -> bool,
    pub keys: fn(&Context) -> Vec<u32>,
    pub render: fn(&PropSet, &Context) -> Vec<(String, String)>,
    pub env: fn(&PropSet, &Context) -> Vec<(String, String)>,
}

One subject the device can be asked about.

Fields

name: &'static str

What to type after info.

prefix: &'static str

What --env prefixes this topic’s variables with.

gate: fn(&Context) -> bool

Whether this device has anything to say on the subject.

keys: fn(&Context) -> Vec<u32>

The properties the renderers below read.

render: fn(&PropSet, &Context) -> Vec<(String, String)>

env: fn(&PropSet, &Context) -> Vec<(String, String)>

NAME=VALUE pairs, unprefixed — the caller adds Self::prefix.

A component the device did not report is an absent variable rather than an empty one, so ${RADIO_SF:-} is how a script asks whether there is a spreading factor at all.