Add support for constant in ha-form (#6324)

Co-authored-by: Paulus Schoutsen <balloob@gmail.com>

src/components/ha-form/ha-form-constant.ts

@@ -0,0 +1,47 @@
+import {
+  customElement,
+  html,
+  LitElement,
+  property,
+  TemplateResult,
+  CSSResult,
+  css,
+  PropertyValues,
+} from "lit-element";
+import { HaFormElement, HaFormConstantSchema } from "./ha-form";
+import { fireEvent } from "../../common/dom/fire_event";
+
+@customElement("ha-form-constant")
+export class HaFormConstant extends LitElement implements HaFormElement {
+  @property({ attribute: false }) public schema!: HaFormConstantSchema;
+
+  @property() public label!: string;
+
+  protected firstUpdated(changedProps: PropertyValues) {
+    super.firstUpdated(changedProps);
+    fireEvent(this, "value-changed", {
+      value: this.schema.value,
+    });
+  }
+
+  protected render(): TemplateResult {
+    return html`<span class="label">${this.label}</span>: ${this.schema.value}`;
+  }
+
+  static get styles(): CSSResult {
+    return css`
+      :host {
+        display: block;
+      }
+      .label {
+        font-weight: 500;
+      }
+    `;
+  }
+}
+
+declare global {
+  interface HTMLElementTagNameMap {
+    "ha-form-constant": HaFormConstant;
+  }
+}

src/components/ha-form/ha-form.ts

@@ -15,8 +15,10 @@ import "./ha-form-multi_select";
 import "./ha-form-positive_time_period_dict";
 import "./ha-form-select";
 import "./ha-form-string";
+import "./ha-form-constant";
 
 export type HaFormSchema =
+  | HaFormConstantSchema
   | HaFormStringSchema
   | HaFormIntegerSchema
   | HaFormFloatSchema
@@ -33,6 +35,11 @@ export interface HaFormBaseSchema {
   description?: { suffix?: string; suggested_value?: HaFormData };
 }
 
+export interface HaFormConstantSchema extends HaFormBaseSchema {
+  type: "constant";
+  value: string;
+}
+
 export interface HaFormIntegerSchema extends HaFormBaseSchema {
   type: "integer";
   default?: HaFormIntegerData;