Add a Time-Based Debouncer to update Screen Readers at most once per second

src/browser/AccessibilityManager.ts

@@ -4,10 +4,10 @@
  */
 
 import * as Strings from 'browser/LocalizableStrings';
-import { ITerminal } from 'browser/Types';
+import { ITerminal, IRenderDebouncer } from 'browser/Types';
 import { IBuffer } from 'common/buffer/Types';
 import { isMac } from 'common/Platform';
-import { RenderDebouncer } from 'browser/RenderDebouncer';
+import { TimeBasedDebouncer } from 'browser/TimeBasedDebouncer';
 import { addDisposableDomListener } from 'browser/Lifecycle';
 import { Disposable } from 'common/Lifecycle';
 import { ScreenDprMonitor } from 'browser/ScreenDprMonitor';
@@ -28,7 +28,7 @@ export class AccessibilityManager extends Disposable {
   private _liveRegion: HTMLElement;
   private _liveRegionLineCount: number = 0;
 
-  private _renderRowsDebouncer: RenderDebouncer;
+  private _renderRowsDebouncer: IRenderDebouncer;
   private _screenDprMonitor: ScreenDprMonitor;
 
   private _topBoundaryFocusListener: (e: FocusEvent) => void;
@@ -72,7 +72,7 @@ export class AccessibilityManager extends Disposable {
     this._refreshRowsDimensions();
     this._accessibilityTreeRoot.appendChild(this._rowContainer);
 
-    this._renderRowsDebouncer = new RenderDebouncer(this._renderRows.bind(this));
+    this._renderRowsDebouncer = new TimeBasedDebouncer(this._renderRows.bind(this));
     this._refreshRows();
 
     this._liveRegion = document.createElement('div');

src/browser/RenderDebouncer.ts

@@ -3,12 +3,12 @@
  * @license MIT
  */
 
-import { IDisposable } from 'common/Types';
+import { IRenderDebouncer } from 'browser/Types';
 
 /**
  * Debounces calls to render terminal rows using animation frames.
  */
-export class RenderDebouncer implements IDisposable {
+export class RenderDebouncer implements IRenderDebouncer {
   private _rowStart: number | undefined;
   private _rowEnd: number | undefined;
   private _rowCount: number | undefined;

src/browser/TimeBasedDebouncer.ts

@@ -0,0 +1,86 @@
+/**
+ * Copyright (c) 2018 The xterm.js authors. All rights reserved.
+ * @license MIT
+ */
+
+const RENDER_DEBOUNCE_THRESHOLD_MS = 1000; // 1 Second
+
+import { IRenderDebouncer } from 'browser/Types';
+
+/**
+ * Debounces calls to update screen readers to update at most once configurable interval of time.
+ */
+export class TimeBasedDebouncer implements IRenderDebouncer {
+  private _rowStart: number | undefined;
+  private _rowEnd: number | undefined;
+  private _rowCount: number | undefined;
+
+  // The last moment that the Terminal was refreshed at
+  private _lastRefreshMs = 0;
+  // Whether a trailing refresh should be triggered due to a refresh request that was throttled
+  private _additionalRefreshRequested = false;
+
+  private _refreshTimeoutID: number | undefined;
+
+  constructor(
+    private _renderCallback: (start: number, end: number) => void,
+    private readonly _debounceThresholdMS = RENDER_DEBOUNCE_THRESHOLD_MS
+  ) {
+  }
+
+  public dispose(): void {
+    if (this._refreshTimeoutID) {
+      clearTimeout(this._refreshTimeoutID);
+    }
+  }
+
+  public refresh(rowStart: number | undefined, rowEnd: number | undefined, rowCount: number): void {
+    this._rowCount = rowCount;
+    // Get the min/max row start/end for the arg values
+    rowStart = rowStart !== undefined ? rowStart : 0;
+    rowEnd = rowEnd !== undefined ? rowEnd : this._rowCount - 1;
+    // Set the properties to the updated values
+    this._rowStart = this._rowStart !== undefined ? Math.min(this._rowStart, rowStart) : rowStart;
+    this._rowEnd = this._rowEnd !== undefined ? Math.max(this._rowEnd, rowEnd) : rowEnd;
+
+    // Only refresh if the time since last refresh is above a threshold, otherwise wait for
+    // enough time to pass before refreshing again.
+    const refreshRequestTime: number = Date.now();
+    if (refreshRequestTime - this._lastRefreshMs >= this._debounceThresholdMS) {
+      // Enough time has lapsed since the last refresh; refresh immediately
+      this._lastRefreshMs = refreshRequestTime;
+      this._innerRefresh();
+    } else if (!this._additionalRefreshRequested) {
+      // This is the first additional request throttled; set up trailing refresh
+      const elapsed = refreshRequestTime - this._lastRefreshMs;
+      const waitPeriodBeforeTrailingRefresh = this._debounceThresholdMS - elapsed;
+      this._additionalRefreshRequested = true;
+
+      this._refreshTimeoutID = window.setTimeout(() => {
+        this._lastRefreshMs = Date.now();
+        this._innerRefresh();
+        this._additionalRefreshRequested = false;
+        this._refreshTimeoutID = undefined; // No longer need to clear the timeout
+      }, waitPeriodBeforeTrailingRefresh);
+    }
+  }
+
+  private _innerRefresh(): void {
+    // Make sure values are set
+    if (this._rowStart === undefined || this._rowEnd === undefined || this._rowCount === undefined) {
+      return;
+    }
+
+    // Clamp values
+    const start = Math.max(this._rowStart, 0);
+    const end = Math.min(this._rowEnd, this._rowCount - 1);
+
+    // Reset debouncer (this happens before render callback as the render could trigger it again)
+    this._rowStart = undefined;
+    this._rowEnd = undefined;
+
+    // Run render callback
+    this._renderCallback(start, end);
+  }
+}
+

src/browser/Types.d.ts

@@ -309,3 +309,7 @@ export interface ICharacterJoiner {
   id: number;
   handler: CharacterJoinerHandler;
 }
+
+export interface IRenderDebouncer extends IDisposable {
+  refresh(rowStart: number | undefined, rowEnd: number | undefined, rowCount: number): void;
+}

src/browser/services/RenderService.ts

@@ -9,7 +9,7 @@ import { EventEmitter, IEvent } from 'common/EventEmitter';
 import { Disposable } from 'common/Lifecycle';
 import { ScreenDprMonitor } from 'browser/ScreenDprMonitor';
 import { addDisposableDomListener } from 'browser/Lifecycle';
-import { IColorSet } from 'browser/Types';
+import { IColorSet, IRenderDebouncer } from 'browser/Types';
 import { IOptionsService, IBufferService } from 'common/services/Services';
 import { ICharSizeService, IRenderService } from 'browser/services/Services';
 
@@ -22,7 +22,7 @@ interface ISelectionState {
 export class RenderService extends Disposable implements IRenderService {
   public serviceBrand: undefined;
 
-  private _renderDebouncer: RenderDebouncer;
+  private _renderDebouncer: IRenderDebouncer;
   private _screenDprMonitor: ScreenDprMonitor;
 
   private _isPaused: boolean = false;