Macro link

macro_rules! link {
    (
        //. .
        ($pcname: ident = $pcval: expr), 
        //. .
        ($($input_name: ident = $input_val: expr), * $(,) ?), 
        //. .
        ($($output_name: ident = $output_val: expr), * $(,) ?), 
        //. .
        ($($name: ident = $val: expr), * $(,) ?) $(,) ? 
        //. .
        $body: block) => { ... };
}

Define a link, a callback that triggers during event graph processing if any of the inputs change. A link will also be triggered during the event the link was created in, whether inputs changed or not.

Link specification is like defining a closure, however all captures must be explicitly defined up front. The macro takes the form of a function definition with multiple parentheses for different capture groups.

let _link = link!(
  (CONTEXT KVS),
  (INPUT KVS),
  (OUTPUT KVS),
  (OTHER CAPTURE KVS) {
  BODY
});

• Each KVS group takes values in the form of x1 = x2 where x2 is an expression for a value that will be used inside the link callback, and x1 is the name it’s assigned within the callback.

• CONTEXT KVS looks like pc = pc, it takes the ProcessingContext from the current event.

• INPUT KVS is a list of input values which will trigger this callback to run. This needs at least one item.

• OUTPUT KVS is a list of values that may be modified by this callback. This can be empty if the callback doesn’t cause any downstream processing - for example, in a UI a callback that updates view state.

• OTHER CAPTURE KVS is a list of other values to capture not related to graph processing. This can be empty.

• BODY is the code of the callback. It should read from the inputs and modify the outputs. It implicitly (no explicit return needed) returns an Option<()> to help with short circuiting, for example if you use a weak reference to an input and upgrading that input results in None.

The link returns a link object. The link callback will only trigger as long as that object exists, so you need to own it as long as it’s relevant.