feat(material/autocomplete): test harness should throw when options are requested but panel is closed

The autocomplete harness has methods for retrieving the options of an
autocomplete. When this method is invoked, the options are determined
from the overlay DOM element, if available. If the panel is not in the DOM
(due to the autocomplete not being focused), the method will just return
an empty array, indicating that there are no options.

This is confusing for test authors and currently not well-defined, potentially
causing invalid tests that just pass by coincidence.

We want to throw when the panel is not open, raising awareness for such invalid
tests. In the future we may want to auto-focus the autocomplete to open the
panel (like `selectOption`).

Author: devversion

src/material/autocomplete/testing/autocomplete-harness.ts

@@ -12,6 +12,7 @@ import {
   ComponentHarness,
   ComponentHarnessConstructor,
   HarnessPredicate,
+  TestElement,
 } from '@angular/cdk/testing';
 import {
   MatOptgroupHarness,
@@ -71,6 +72,10 @@ export abstract class _MatAutocompleteHarnessBase<
 
   /** Gets the options inside the autocomplete panel. */
   async getOptions(filters?: Omit<OptionFilters, 'ancestor'>): Promise<Option[]> {
+    if (!(await this.isOpen())) {
+      throw new Error('Unable to retrieve options for autocomplete. Autocomplete panel is closed.');
+    }
+
     return this._documentRootLocator.locatorForAll(
       this._optionClass.with({
         ...(filters || {}),
@@ -81,6 +86,12 @@ export abstract class _MatAutocompleteHarnessBase<
 
   /** Gets the option groups inside the autocomplete panel. */
   async getOptionGroups(filters?: Omit<OptionGroupFilters, 'ancestor'>): Promise<OptionGroup[]> {
+    if (!(await this.isOpen())) {
+      throw new Error(
+        'Unable to retrieve option groups for autocomplete. Autocomplete panel is closed.',
+      );
+    }
+
     return this._documentRootLocator.locatorForAll(
       this._optionGroupClass.with({
         ...(filters || {}),
@@ -106,7 +117,7 @@ export abstract class _MatAutocompleteHarnessBase<
   }
 
   /** Gets the panel associated with this autocomplete trigger. */
-  private async _getPanel() {
+  private async _getPanel(): Promise<TestElement | null> {
     // Technically this is static, but it needs to be in a
     // function, because the autocomplete's panel ID can changed.
     return this._documentRootLocator.locatorForOptional(await this._getPanelSelector())();