Recipe 2 — Cross-field rule

Goal: validate a constraint that spans two fields — "password == confirm-password", or "start date ≤ end date" — and surface it as a single form-level error, not anchored to either field.

FormValidator::rule(predicate, message, rule_id) is exactly this: the predicate is a bool() closure that may read any number of Properties. The emitted Error has an empty field_path and source = "FormValidator" (so it routes as a form-level message, not a per-field one), keyed by ValidationKey{"", rule_id}.

#include "aria/binding/form.hpp"
#include "aria/property.hpp"
 
aria::Property<std::string> password{""};
aria::Property<std::string> confirm{""};
 
aria::FormValidator form;
 
// Cross-field: passwords must match.
form.rule(
    [&]{ return password.get() == confirm.get(); },
    "Passwords do not match",
    "password_match");
 
// Another cross-field rule on a date range.
aria::Property<int> start_day{1};
aria::Property<int> end_day{1};
form.rule(
    [&]{ return start_day.get() <= end_day.get(); },
    "Start must be on or before end",
    "date_order");

The rule re-evaluates whenever any tracked field changes (wire the fields with form.track(field) so their on_changed feeds recompute_()), and the first failing rule populates form.first_error / form.first_error_full:

auto err_sub = form.first_error.on_changed([](const std::string& msg){
    banner.text    = msg;
    banner.visible = !msg.empty();
});

Notes

• Order matters for the headline. first_error reports the first failing rule in registration order — register the most important rule first if you want it to win the banner.
• Keep predicates pure and cheap. They run on every change of any tracked field; do not perform I/O here (use an AsyncValidator — Recipe 1 — for that).
• Routing. Because the error's field_path is empty and source == "FormValidator", a diagnostics filter on source cleanly separates form-level rules from per-field ones (see docs/reference/diagnostics.md).