There is an <EuiComboBoxField /> that already takes care of serialization, deserialization, validation at the array level, validation at the item level. I would recommend using it.

This is the config (from the schema) where you can see the 2 types of validations: at the Array level (the first one), and at the item level (the second one, declaring a type: VALIDATION_TYPES.ARRAY_ITEM).

https://github.com/elastic/kibana/blob/master/x-pack/plugins/index_management/public/application/components/template_form/template_form_schemas.tsx#L100

And this is how it is simply declared

https://github.com/elastic/kibana/blob/master/x-pack/plugins/index_management/public/application/components/template_form/steps/step_logistics.tsx#L127

No need to re-write the logic of onChange, options, selectedOptions.

The important part is to not forget to declare the defaultValue as [] (empty array).

Do you prefer to declare all the configs above the component instead of inside a FormSchema? I would find it easier to scan to have all the fields declaration in a single place, but that's my preference 😊

Not sure if this field is for index pattern or not, but in case it is, know that there is an indexPatternField validator for them.

https://github.com/elastic/kibana/blob/master/src/plugins/es_ui_shared/static/forms/helpers/field_validators/index_pattern_field.ts#L26

Instead of this + having to declare a state and update it, it is better to use the FormDataProvider

return (
  <Form form={form}>
    <ProcessorTypeField initialType={processor?.type} />
    <FormDataProvider pathsToWatch="type">
      {({ type }) => {
        const FormFields = getProcessorFormDescriptor(type as any);
        // TODO: Handle this error in a different way
        if (!FormFields) {
          throw new Error(`Could not find form for type ${type}`);
        }
        return (
          <>
            <FormFields />
            <CommonProcessorFields />
            <EuiButton onClick={form.submit}>Submit</EuiButton>
          </>
        );
      }}

    </FormDataProvider>
  </Form>
);