Ns XML Custom Components

XML builder: custom components, platform blocks, and its errors

Custom components

Point an xmlns prefix at a module path (relative to app/), then use exported view classes as tags:

<Page xmlns:ui="components/rating">
  <StackLayout>
    <ui:Rating value="3" />
  </StackLayout>
</Page>

• The module's exported symbol name is the tag (export class Rating extends GridLayout).
• A component folder can be XML-first: components/card/Card.xml (+ optional co-located Card.css, code-behind, or a package.json naming the entry) — usable with no JS class at all.
• Framework modules work through the same mechanism: xmlns:c="ui/label" → <c:Label>.
• Unknown attributes on a custom component become plain expando properties (panel['myProperty'] = 'myValue') — never an error, so attribute typos fail silently. Property names are checked nowhere; test bindings visually.
• Dashed lowercase tags are first-class: <stack-layout>, <segmented-bar> — and CSS type selectors must use that dashed form.

Page-level attributes

• codeFile="~/views/shared/list-code" and import="..." are equivalent — both point the page at a different code-behind; a bad path throws at parse time.
• cssFile="~/styles/custom.css" scopes a stylesheet to the page — and the path is itself run through file-qualifier resolution (custom.land.css can override what you referenced — see ns-file-qualifiers).

Platform-conditional markup: two mechanisms, don't nest them

<TextField ios:editable="false" android:editable="true" />

<ios><TextField hint="Cupertino" /></ios>
<android><Label text="Mountain View" /></android>

Attribute prefixes (ios:/android:) and element blocks (<ios>/<android>) both work — but nesting an <android> block inside an <ios> block throws. Keep platform blocks siblings.

Reading builder errors

Building UI from XML. @app/views/feed/feed-page.xml:11:5
 > Module 'Unicorn' not found for element 'Unicorn'.

The @file:line:col points at the offending element. Two timing rules:

• Page-level errors throw when the page loads.
• Errors inside an itemTemplate are deferred: Builder.load() succeeds and the throw happens only when the template first instantiates (first row render) — a list page that "loads fine then crashes on data arrival" is usually a broken template element, and the reported line/col points inside the template.

Programmatic use: Builder.parse(xmlString, context) and Builder.load({ path, name, exports, page, attributes }).

Verified 2026-08 against @nativescript/core 9.1.0-rc.3: error format, deferred template throws, platform-block nesting throw, expando attributes, dashed tags and codeFile/cssFile behavior are asserted in apps/automated/src/xml-declaration/xml-declaration-tests.ts; not re-run standalone.