Macro when_var 

macro_rules! when_var {
    ($($tt:tt)*) => { ... };
}

Initializes a new conditional var.

A condition var updates when the first true condition changes or the mapped var for the current condition changes.

Syntax

The macro expects a list of condition-var => condition-value-var, the list is separated by comma. The last condition must be the _ token that maps to the value for when none of the conditions are true.

The condition-var must be an expression that evaluates to a Var<bool> type. The condition-value-var must by any type that implements IntoVar. All condition values must be of the same VarValue type.

Examples

let condition = var(true);
let when_false = var("condition: false".to_txt());

let t = Text!(when_var! {
    condition.clone() => "condition: true".to_txt(),
    _ => when_false.clone(),
});

In the example if condition or when_false are modified the text updates.

cfg

Every condition can be annotated with attributes, including #[cfg(..)].

let t = Text!(when_var! {
    #[cfg(some_flag)]
    condition0 => "is condition 0".to_txt(),
    #[cfg(not(some_flag))]
    condition1 => "is condition 1".to_txt(),
    _ => "is default".to_txt(),
});

In the example above only one of the conditions will be compiled, the generated variable is the same type as if you had written a single condition.

Capabilities

The when var is contextualized when needed, meaning if any input is CONTEXT at the moment the var is created it is also contextual. The full output type of this macro is a Var<T>.