What validate() does

await form.validate() walks every field and every subform, concurrently and with no short-circuit, runs their async validators too, and returns false if anything ended up invalid. It neither consults the mode nor changes it: the mode you set is the mode the form keeps for its whole life.

Future<void> submit() async {
  if (await form.validate()) {
    await api.signUp(/* … */);
  }
}

Three properties

• Await it. The result is the only thing that says the values were actually checked. canSubmit is a snapshot of known errors and is true on a form nobody has checked yet. A bare validate(); statement compiles and silently ignores the result.
• Async checks are shared, sync ones are not. Every call re-runs the sync validators against the current values, so a call made right after another field changed sees the change. A second call while async checks are in flight awaits them instead of starting new ones, so a double-tapped submit button makes one set of server calls.
• The errors stay on the fields. The form-level result is just a bool — "may this submit proceed?". The widgets showing errors are already subscribed to their fields.

On a single field

field.validate() has the same signature and the same contract: sync first, always; then, if sync passed and the field has an AsyncValidation, exactly one of — flush a check waiting out its debounce, await a check already in flight, reuse a verdict that still describes the value, or run the check now. It returns the status, so a check that could not run counts as false. Async validation has the details.

What it does not do

• It does not turn validation on. 0.1.x's validate() switched autovalidate on for every field; 0.2 leaves the mode alone. For "quiet until the first submit, live afterwards", set ValidationMode.onUserInteraction after a failed submit — the wizard does this per step.
• It does not skip untouched fields. Rule 2 applies to fields validating themselves; a submit checks everything, which is how a bad prefilled value is caught.
• It does not run on a subtree with validation switched off. That subtree returns true, matching what canSubmit counts, so the two never disagree.