Macro container_keys 

macro_rules! container_keys {
    ($(
        $(#[$meta:meta])*
        $name:ident : $ty:ty = $default:expr
    ),* $(,)?) => { ... };
}

Declare container-value keys of your own — as environment_keys!, for ContainerValues.

What a row tells the container it is in, rather than what a container tells the views inside it.

container_keys! {
    /// How much of the row's width the leading column takes.
    ratio: f64 = 0.5,
}

text("row").container_value(ratio(), 0.25)