Item Renderers

Item renderers are framework-specific functions or components that the form engine invokes to render each item of a dropdown, list, or radiogroup. The engine forwards an ItemRenderContext<T> object that carries the raw item plus useful metadata (selected, disabled, focused, index).

The contract

Each framework adapts the same ItemRenderContext<T> to its idiomatic shape:

Framework	Type	What the engine passes
React	ReactItemRenderer<T>	A React.ComponentType<ItemRenderContext<T>> — the context fields arrive as props.
Angular	AngularItemRenderer<T>	A component class structurally matching AngularItemRenderContext<T> — one signal input() per context field.
Lit	LitItemRenderer<T>	A (ctx: ItemRenderContext<T>) => TemplateResult — called as a function, not a custom element.
Vue	VueItemRenderer<T>	A Component<Partial<ItemRenderContext<T>>> — the context fields arrive as props on the SFC.
Vanilla JS	(ctx) => HTMLElement	A plain function called with ItemRenderContext<T> that returns a DOM node — like Lit’s, but returning real DOM instead of a TemplateResult.

The shared context shape comes from @golemui/core:

export interface ItemRenderContext<T extends ItemRenderItemData> {
  template: T;
  value: string | number;
  index: number;
  selected?: boolean;
  disabled?: boolean;
  focused?: boolean;
}

template is the raw item from the widget’s items array. The other fields reflect the live state of the option in the rendered list.

Example — country renderer

React

import type { ReactItemRenderer } from '@golemui/react';

type Country = { id: string; flag: string; label: string };

export const CountryItemRenderer: ReactItemRenderer<Country> = ({
  template,
  index,
  selected,
  disabled,
  focused,
}) => (
  <div
    className={`country-item ${selected ? 'country-item--selected' : ''} ${
      disabled ? 'country-item--disabled' : ''
    } ${focused ? 'country-item--focused' : ''} ${index % 2 ? 'country-item--odd' : ''}`}
  >
    <span className="country-item__flag">{template.flag}</span>
    <span className="country-item__label">{template.label}</span>
  </div>
);

Props are the full ItemRenderContext<T> — destructure whichever fields you need.

Angular

import { Component, input } from '@angular/core';
import { type AngularItemRenderContext } from '@golemui/angular';

type Country = { id: string; flag: string; label: string };

@Component({
  standalone: true,
  selector: 'country-item-renderer',
  template: `
    <div
      class="country-item"
      [class.country-item--selected]="selected()"
      [class.country-item--disabled]="disabled()"
      [class.country-item--focused]="focused()"
      [class.country-item--odd]="index() % 2"
    >
      <span class="country-item__flag">{{ template().flag }}</span>
      <span class="country-item__label">{{ template().label }}</span>
    </div>
  `,
})
export class CountryItemRenderer implements AngularItemRenderContext<Country> {
  template = input.required<Country>();
  value = input.required<string | number>();
  index = input.required<number>();
  selected = input<boolean | undefined>(undefined);
  disabled = input<boolean | undefined>(undefined);
  focused = input<boolean | undefined>(undefined);
}

Declare one signal input per context field; the engine binds each one when rendering an item. Read each value by calling the signal — template().flag, selected(), etc.

implements AngularItemRenderContext<T> is the recommended contract for signal-based renderers: TypeScript enforces the full set of inputs and their signal types, so a missing or mistyped field is a compile error.

Backwards compatibility with @Input()

Renderers written with the classic @Input() decorator stay supported — AngularItemRenderer<T> accepts both styles. For those, keep using implements ItemRenderContext<T> from @golemui/core:

import { Component, Input } from '@angular/core';
import { type ItemRenderContext } from '@golemui/core';

@Component({ /* ... */ })
export class CountryItemRenderer implements ItemRenderContext<Country> {
  @Input({ required: true }) template!: Country;
  @Input({ required: true }) value!: string | number;
  @Input({ required: true }) index!: number;
  @Input() selected?: boolean;
  @Input() disabled?: boolean;
  @Input() focused?: boolean;
}

Reach for AngularItemRenderContext<T> only when you opt in to signal inputs — the field types are different (InputSignal<T> vs. plain T), so the two interfaces are not interchangeable on the same class.

Lit

import { html, TemplateResult } from 'lit';
import { classMap } from 'lit/directives/class-map.js';
import { ItemRenderContext } from '@golemui/core';

type Country = { id: string; flag: string; label: string };

export const countryItemRenderer = (
  ctx: ItemRenderContext<Country>,
): TemplateResult => {
  const classes = {
    'country-item': true,
    'country-item--selected': !!ctx.selected,
    'country-item--disabled': !!ctx.disabled,
    'country-item--focused': !!ctx.focused,
    'country-item--odd': ctx.index % 2 !== 0,
  };

  return html`
    <div class=${classMap(classes)}>
      <span class="country-item__flag">${ctx.template.flag}</span>
      <span class="country-item__label">${ctx.template.label}</span>
    </div>
  `;
};

Lit renderers are functions — the engine invokes them with an ItemRenderContext<T> argument and inlines the returned template into the dropdown / list. A LitElement class with an @property() item field will not be picked up by the engine.

Vue

CountryItemRenderer.vue

<script setup lang="ts">
import type { ListItemRendererProps } from '@golemui/gui-vue';

type Country = { id: string; flag: string; label: string };

defineProps<ListItemRendererProps<Country>>();
</script>

<template>
  <div
    :class="{
      'country-item': true,
      'country-item--selected': !!selected,
      'country-item--disabled': !!disabled,
      'country-item--focused': !!focused,
      'country-item--odd': index % 2 !== 0,
    }"
  >
    <span class="country-item__flag">{{ template?.flag }}</span>
    <span class="country-item__label">{{ template?.label }}</span>
  </div>
</template>

Vue renderers are SFCs. Use optional chaining on template?.flag because the props can be undefined while items are loading (ListItemRendererProps<T> marks template and value optional for that reason).

Vanilla JS

country-item-renderer.js

export const countryItemRenderer = (ctx) => {
  const root = document.createElement('div');
  root.className = 'country-item';
  if (ctx.selected) root.classList.add('country-item--selected');
  if (ctx.disabled) root.classList.add('country-item--disabled');
  if (ctx.focused) root.classList.add('country-item--focused');
  if (ctx.index % 2 !== 0) root.classList.add('country-item--odd');

  const flag = document.createElement('span');
  flag.className = 'country-item__flag';
  flag.textContent = ctx.template.flag;

  const label = document.createElement('span');
  label.className = 'country-item__label';
  label.textContent = ctx.template.label;

  root.append(flag, label);
  return root;
};

Vanilla renderers are plain functions called with an ItemRenderContext<T> — just like Lit’s, but they return a real DOM node rather than a TemplateResult. The engine inserts the returned element directly into the dropdown / list.

Registering the renderer

The widget references the renderer by name through its itemRenderer prop. Where the renderer map lives depends on the path:

In the Programmatic API the renderer map goes inside formConfig.itemRenderers:

import { gui } from '@golemui/gui-shared';
import { CountryItemRenderer } from './country-item-renderer';

const config = {
  formDef: [
    gui.inputs.dropdown('country', {
      items: [
        /* … */
      ],
      itemRenderer: 'countryItemRenderer',
    }),
  ],
  formConfig: { itemRenderers: { countryItemRenderer: CountryItemRenderer } },
};

// <GuiForm config={config} />

In the JSON path the renderer map is a top-level property of the config object — there is no formConfig wrapper for JSON forms. The JSON references the renderer by name on the widget.

{
  "form": [
    {
      "kind": "input",
      "type": "dropdown",
      "path": "country",
      "items": [],
      "itemRenderer": "countryItemRenderer"
    }
  ]
}

import { CountryItemRenderer } from './country-item-renderer';
import formDef from './my-form.json';

const config = {
  formDef,
  itemRenderers: { countryItemRenderer: CountryItemRenderer },
};

// <GuiForm config={config} />

See also

• Features / Item Renderers — wiring renderers in formConfig.
• Getting Started / Custom renderer for the car list — a full step-by-step example.