Merge pull request #2219 from ing-bank/feat/allowFirstSlotRenderOnCon…

…nected

feat: allow SlotRerenderObject to to first render on connectedCallbac…

.changeset/funny-numbers-prove.md

@@ -0,0 +1,5 @@
+---
+'@lion/ui': patch
+---
+
+feat: allow SlotRerenderObject to first render on connectedCallback via `firstRenderOnConnected`

docs/fundamentals/systems/core/SlotMixin.md

@@ -104,6 +104,13 @@ A `SlotRerenderObject` looks like this:
 ```ts
 {
   template: TemplateResult;
+  /**
+   * For backward compat with traditional light render methods,
+   * it might be needed to have slot contents available in `connectedCallback`.
+   * Only enable this for existing components that rely on light content availability in connectedCallback.
+   * For new components, please align with ReactiveElement/LitElement reactive cycle callbacks.
+   */
+  firstRenderOnConnected?: boolean;
 }
 ```
 

packages/ui/components/core/src/SlotMixin.js

@@ -195,6 +195,11 @@ const SlotMixinImplementation = superclass =>
         } else if (isRerenderConfig(slotFunctionResult)) {
           // Rerenderable slots are scheduled in the "updated loop"
           this.__slotsThatNeedRerender.add(slotName);
+
+          // For backw. compat, we allow a first render on connectedCallback
+          if (slotFunctionResult.firstRenderOnConnected) {
+            this.__rerenderSlot(slotName);
+          }
         } else {
           throw new Error(
             `Slot "${slotName}" configured inside "get slots()" (in prototype) of ${this.constructor.name} may return these types: TemplateResult | Node | {template:TemplateResult, afterRender?:function} | undefined.

packages/ui/components/core/test/SlotMixin.test.js

@@ -1,5 +1,5 @@
 import sinon from 'sinon';
-import { defineCE, expect, fixture, unsafeStatic, html } from '@open-wc/testing';
+import { defineCE, expect, fixture, fixtureSync, unsafeStatic, html } from '@open-wc/testing';
 import { ScopedElementsMixin } from '@open-wc/scoped-elements';
 import { SlotMixin } from '@lion/ui/core.js';
 import { LitElement } from 'lit';
@@ -300,6 +300,83 @@ describe('SlotMixin', () => {
       expect(document.activeElement).to.equal(el._focusableNode);
       expect(el._focusableNode.shadowRoot.activeElement).to.equal(el._focusableNode._buttonNode);
     });
+
+    describe('firstRenderOnConnected (for backwards compatibility)', () => {
+      it('does render on connected when firstRenderOnConnected:true', async () => {
+        // Start with elem that does not render on connectedCallback
+        const tag = defineCE(
+          // @ts-expect-error
+          class extends SlotMixin(LitElement) {
+            static properties = { currentValue: Number };
+
+            constructor() {
+              super();
+              this.currentValue = 0;
+            }
+
+            get slots() {
+              return {
+                ...super.slots,
+                template: () => ({
+                  firstRenderOnConnected: true,
+                  template: html`<span>${this.currentValue}</span> `,
+                }),
+              };
+            }
+
+            render() {
+              return html`<slot name="template"></slot>`;
+            }
+
+            get _templateNode() {
+              return /** @type HTMLSpanElement */ (
+                Array.from(this.children).find(elm => elm.slot === 'template')
+              );
+            }
+          },
+        );
+        const el = /** @type {* & SlotHost} */ (fixtureSync(`<${tag}></${tag}>`));
+        expect(el._templateNode.slot).to.equal('template');
+        expect(el._templateNode.textContent?.trim()).to.equal('0');
+      });
+
+      it('does not render on connected when firstRenderOnConnected:false', async () => {
+        // Start with elem that does not render on connectedCallback
+        const tag = defineCE(
+          // @ts-expect-error
+          class extends SlotMixin(LitElement) {
+            static properties = { currentValue: Number };
+
+            constructor() {
+              super();
+              this.currentValue = 0;
+            }
+
+            get slots() {
+              return {
+                ...super.slots,
+                template: () => ({ template: html`<span>${this.currentValue}</span> ` }),
+              };
+            }
+
+            render() {
+              return html`<slot name="template"></slot>`;
+            }
+
+            get _templateNode() {
+              return /** @type HTMLSpanElement */ (
+                Array.from(this.children).find(elm => elm.slot === 'template')
+              );
+            }
+          },
+        );
+        const el = /** @type {* & SlotHost} */ (fixtureSync(`<${tag}></${tag}>`));
+        expect(el._templateNode).to.be.undefined;
+        await el.updateComplete;
+        expect(el._templateNode.slot).to.equal('template');
+        expect(el._templateNode.textContent?.trim()).to.equal('0');
+      });
+    });
   });
 
   describe('SlotFunctionResult types', () => {

packages/ui/components/core/types/SlotMixinTypes.ts

@@ -8,9 +8,16 @@ export type SlotRerenderObject = {
   template: TemplateResult;
   /**
    * Add logic that will be performed after the render
-   * @deprecated
+   * @deprecated use regular ReactiveElement/LitElement reactive cycle callbacks instead
    */
   afterRender?: Function;
+  /**
+   * For backward compat with traditional light render methods,
+   * it might be needed to have slot contents available in `connectedCallback`.
+   * Only enable this for existing components that rely on light content availability in connectedCallback.
+   * For new components, please align with ReactiveElement/LitElement reactive cycle callbacks.
+   */
+  firstRenderOnConnected?: boolean;
 };
 
 export type SlotFunctionResult = TemplateResult | Element | SlotRerenderObject | undefined;