Macro zng::app::context_local

macro_rules! context_local {
    ($(
        $(#[$meta:meta])*
        $vis:vis static $IDENT:ident : $T:ty = $init:expr;
    )+) => { ... };
}

Declares new app and context local variable.

Examples

context_local! {
    /// A public documented value.
    pub static FOO: u8 = 10u8;

    // A private value.
    static BAR: String = "Into!";
}

Default Value

All contextual values must have a fallback value that is used when no context is loaded.

The default value is instantiated once per app, the expression can be any static value that converts Into<T>.

Usage

After you declare the context local you can use it by loading a contextual value for the duration of a closure call.

context_local! { static FOO: String = "default"; }

fn print_value() {
    println!("value is {}!", FOO.get());
}

let _scope = LocalContext::start_app(AppId::new_unique());

let mut value = Some(Arc::new(String::from("other")));
FOO.with_context(&mut value, || {
    print!("in context, ");
    print_value();
});

print!("out of context, ");
print_value();

The example above prints:

in context, value is other!
out of context, value is default!

See ContextLocal<T> for more details.