Debounce the layout function in the list component

CHANGELOG.md

@@ -22,6 +22,10 @@ and this project adheres to [Semantic Versioning](http://semver.org/).
 - `textfield`
   - `autoValidate` now validates on value change rather than input
 
+- `list`
+  - List items removal from DOM initiates an async layout in the managing list
+  - Added `itemsReady` promise to the list's updateComplete
+
 ## [v0.19.1] - 2020-10-08
 
 ### Fixed

packages/list/mwc-list-base.ts

@@ -29,11 +29,28 @@ import {Layoutable, ListItemBase, RequestSelectedDetail} from './mwc-list-item-b
 
 export {ActionDetail, createSetFromIndex, isEventMulti, isIndexSet, MWCListIndex, SelectedDetail} from './mwc-list-foundation';
 
+function debounceLayout(
+    callback: (updateItems: boolean) => void, waitInMS = 50) {
+  let timeoutId: number;
+  return function(updateItems = true) {
+    clearTimeout(timeoutId);
+    timeoutId =
+        setTimeout(() => callback(updateItems), waitInMS) as unknown as number;
+  };
+}
 
 const isListItem = (element: Element): element is ListItemBase => {
   return element.hasAttribute('mwc-list-item');
 };
 
+function clearAndCreateItemsReadyPromise(this: ListBase) {
+  const oldResolver = this.itemsReadyResolver;
+  this.itemsReady = new Promise((res) => {
+    return this.itemsReadyResolver = res;
+  });
+  oldResolver();
+}
+
 /**
  * @fires selected {SelectedDetail}
  * @fires action {ActionDetail}
@@ -59,7 +76,6 @@ export abstract class ListBase extends BaseElement implements Layoutable {
   })
   activatable = false;
 
-
   @property({type: Boolean})
   @observer(function(this: ListBase, newValue: boolean, oldValue: boolean) {
     if (this.mdcFoundation) {
@@ -113,6 +129,30 @@ export abstract class ListBase extends BaseElement implements Layoutable {
   })
   noninteractive = false;
 
+  debouncedLayout: (updateItems?: boolean) => void | undefined;
+  protected itemsReadyResolver:
+      (value?: (PromiseLike<never[]>|never[]|undefined)) => void =
+          (() => {
+               //
+           }) as(value?: (PromiseLike<unknown[]>|unknown[])) => void;
+
+  constructor() {
+    super();
+    const debouncedFunction = debounceLayout(this.layout.bind(this));
+    this.debouncedLayout = (updateItems = true) => {
+      clearAndCreateItemsReadyPromise.call(this);
+
+      debouncedFunction(updateItems);
+    };
+  }
+
+  itemsReady = Promise.resolve([]);
+
+  async _getUpdateComplete() {
+    await super._getUpdateComplete();
+    await this.itemsReady;
+  }
+
   protected get assignedElements(): Element[] {
     const slot = this.slotElement;
 
@@ -486,6 +526,8 @@ export abstract class ListBase extends BaseElement implements Layoutable {
         first.tabindex = 0;
       }
     }
+
+    this.itemsReadyResolver();
   }
 
   getFocusedItemIndex() {

packages/list/mwc-list-item-base.ts

@@ -31,6 +31,7 @@ export interface RequestSelectedDetail {
 
 export interface Layoutable {
   layout: (updateItems?: boolean) => void;
+  debouncedLayout?: (updateItems?: boolean) => void | undefined;
 }
 
 export type GraphicType = 'avatar'|'icon'|'medium'|'large'|'control'|null;
@@ -277,7 +278,9 @@ export class ListItemBase extends LitElement {
     }
 
     if (this._managingList) {
-      this._managingList.layout(true);
+      this._managingList.debouncedLayout ?
+          this._managingList.debouncedLayout(true) :
+          this._managingList.layout(true);
     }
   }
 

packages/list/test/mwc-list.test.ts

@@ -153,7 +153,7 @@ const createEmptyList = (emptyMessage: string|undefined) => {
 };
 
 suite('mwc-list:', () => {
-  let fixt: TestFixture;
+  let fixt: TestFixture|null;
 
   suite('mwc-list-item', () => {
     suite('initialization', () => {
@@ -1788,5 +1788,32 @@ suite('mwc-list:', () => {
         }
       });
     });
+
+    suite('performance issue', () => {
+      test(
+          'removing a list should not call layout more than once', async () => {
+            let count = 0;
+            const originalLayout = List.prototype.layout;
+            List.prototype.layout = function(update) {
+              originalLayout.call(this, update);
+              count++;
+            };
+
+            const itemsTemplates = new Array(300).fill(0).map(() => listItem());
+            fixt = await fixture(listTemplate({items: itemsTemplates}));
+            element = fixt.root.querySelector('mwc-list')!;
+
+            count = 0;
+
+            fixt.remove();
+            await element.updateComplete;
+            fixt = null;
+            assert.equal(
+                count,
+                1,
+                'list.layout ran more than once while it shouldn\'t have');
+            List.prototype.layout = originalLayout;
+          });
+    });
   });
 });