fix: ensure StyleElementCustomPropertyManager works with disconnected elements

packages/web-components/fast-foundation/docs/api-report.md

@@ -274,7 +274,7 @@ export interface CustomPropertyManager {
 }
 
 // @public
-export interface CustomPropertyManagerClient extends FASTElement {
+export interface CustomPropertyManagerClient extends FASTElement, HTMLElement {
     cssCustomPropertyDefinitions: Map<string, CSSCustomPropertyDefinition>;
     evaluate(definition: CSSCustomPropertyDefinition): string;
 }
@@ -782,7 +782,7 @@ export class StyleElementCustomPropertyManager extends CustomPropertyManagerBase
     // (undocumented)
     protected customPropertyTarget: CSSStyleDeclaration;
     // (undocumented)
-    readonly sheet: CSSStyleSheet;
+    get sheet(): CSSStyleSheet | null;
     // (undocumented)
     readonly styles: HTMLStyleElement;
 }

packages/web-components/fast-foundation/src/custom-properties/manager.spec.ts

@@ -155,6 +155,39 @@ describe("StyleElementCustomPropertyManager", () => {
         await disconnect();
     });
 
+    it("should not throw when constructed with a disconnected element", () => {
+        const element = document.createElement("fast-client") as Client;
+
+        expect(() => {
+            new StyleElementCustomPropertyManager(
+                document.createElement("style"),
+                element
+            );
+        }).not.to.throw();
+    });
+
+    it("should queue and apply properties set prior to connection once connected", () => {
+        const element = document.createElement("fast-client") as Client;
+        element.cssCustomPropertyDefinitions.set("my-property", {
+            name: "my-property",
+            value: "value",
+        });
+        const client = new StyleElementCustomPropertyManager(
+            document.createElement("style"),
+            element
+        );
+
+        assert.equal(
+            window.getComputedStyle(element).getPropertyValue("--my-property"),
+            ""
+        );
+        document.body.appendChild(element);
+        assert.equal(
+            window.getComputedStyle(element).getPropertyValue("--my-property"),
+            "value"
+        );
+    });
+
     it("should connect the style element to the DOM during construction", async () => {
         const { element, connect, disconnect } = await setup();
 

packages/web-components/fast-foundation/src/custom-properties/manager.ts

@@ -1,4 +1,10 @@
-import { DOM, ElementStyles, FASTElement } from "@microsoft/fast-element";
+import {
+    DOM,
+    ElementStyles,
+    FASTElement,
+    Observable,
+    observable,
+} from "@microsoft/fast-element";
 import { CSSCustomPropertyDefinition } from "./behavior";
 
 const hostSelector = ":host{}";
@@ -9,7 +15,7 @@ const hostSelector = ":host{}";
  *
  * @public
  */
-export interface CustomPropertyManagerClient extends FASTElement {
+export interface CustomPropertyManagerClient extends FASTElement, HTMLElement {
     /**
      * All registered CSSCustomPropertyDefinitions.
      */
@@ -87,10 +93,15 @@ export interface CustomPropertyManager {
 }
 
 abstract class CustomPropertyManagerBase implements CustomPropertyManager {
+    /**
+     * A queue of additions and deletions. Operations will be queued when customPropertyTarget is null
+     */
+    protected queue: Set<Function> = new Set();
+
     /**
      * The CSSStyleDeclaration to which all CSS custom properties are written
      */
-    protected abstract customPropertyTarget: CSSStyleDeclaration;
+    protected abstract customPropertyTarget: CSSStyleDeclaration | null = null;
 
     /**
      * {@inheritdoc CustomPropertyManager.owner}
@@ -153,25 +164,29 @@ abstract class CustomPropertyManagerBase implements CustomPropertyManager {
     /**
      * {@inheritdoc CustomPropertyManager.set}
      */
-    public set = (definition: CSSCustomPropertyDefinition) => {
+    public set(definition: CSSCustomPropertyDefinition) {
         if (this.owner) {
-            this.customPropertyTarget.setProperty(
-                `--${definition.name}`,
-                this.owner.evaluate(definition)
-            );
+            this.customPropertyTarget
+                ? this.customPropertyTarget.setProperty(
+                      `--${definition.name}`,
+                      this.owner.evaluate(definition)
+                  )
+                : this.queue.add(this.set.bind(this, definition));
         }
-    };
+    }
 
     /**
      * Removes a CSS custom property from the provider.
      * @param name - the name of the property to remove
      */
-    public remove = (name: string): void => {
-        this.customPropertyTarget.removeProperty(`--${name}`);
-    };
+    public remove(name: string): void {
+        this.customPropertyTarget
+            ? this.customPropertyTarget.removeProperty(`--${name}`)
+            : this.queue.add(this.remove.bind(this, name));
+    }
 
     /**
-     * {@inheritdoc CustomPropertyManager.set}
+     * {@inheritdoc CustomPropertyManager.setAll}
      */
     public setAll() {
         if (this.ticking) {
@@ -262,28 +277,64 @@ export class ConstructableStylesCustomPropertyManager extends CustomPropertyMana
  * @public
  */
 export class StyleElementCustomPropertyManager extends CustomPropertyManagerBase {
-    public readonly sheet: CSSStyleSheet;
+    private _sheet: CSSStyleSheet | null = null;
+    public get sheet(): CSSStyleSheet | null {
+        return this._sheet;
+    }
+
+    @observable
     protected customPropertyTarget: CSSStyleDeclaration;
+    private customPropertyTargetChanged(
+        prev: CSSStyleDeclaration | null,
+        next: CSSStyleDeclaration | null
+    ) {
+        if (!prev && this.queue.size) {
+            this.queue.forEach(fn => fn());
+            this.queue.clear();
+        }
+    }
+
     public readonly styles: HTMLStyleElement;
 
     constructor(style: HTMLStyleElement, client: CustomPropertyManagerClient) {
         super();
+        const controller = client.$fastController;
 
         // For HTMLStyleElements we need to attach the element
         // to the DOM prior to accessing the HTMLStyleElement.sheet
         // because the property evaluates null if disconnected
-        client.$fastController.addStyles(style);
-        this.sheet = style.sheet!;
-        this.styles = style;
-
-        this.customPropertyTarget = (this.sheet.rules[
-            this.sheet.insertRule(hostSelector)
-        ] as CSSStyleRule).style;
+        controller.addStyles(style);
 
+        this.styles = style;
         this._owner = client;
 
+        // If the element isn't connected when the manager is created, the sheet can be null.
+        // In those cases, set up notifier for when the element is connected and set up the customPropertyTarget
+        // then.
+        client.isConnected
+            ? this.handleConnection.handleChange()
+            : Observable.getNotifier(controller).subscribe(
+                  this.handleConnection,
+                  "isConnected"
+              );
+
         client.cssCustomPropertyDefinitions.forEach(def => {
             this.register(def);
         });
     }
+
+    private handleConnection = {
+        handleChange: () => {
+            this._sheet = this.styles.sheet!;
+
+            this.customPropertyTarget = (this.sheet!.rules[
+                this.sheet!.insertRule(hostSelector)
+            ] as CSSStyleRule).style;
+
+            Observable.getNotifier(this._owner?.$fastController).unsubscribe(
+                this.handleConnection,
+                "isConnected"
+            );
+        },
+    };
 }