chore(a11y): Allow FocusTrap to be extended (#15770)

This will be used to allow tabbing from edit lenses to another table cell in popover edit.

Author: kseamon

src/cdk/a11y/focus-trap/focus-trap.ts

@@ -37,8 +37,8 @@ export class FocusTrap {
   private _hasAttached = false;
 
   // Event listeners for the anchors. Need to be regular functions so that we can unbind them later.
-  private _startAnchorListener = () => this.focusLastTabbableElement();
-  private _endAnchorListener = () => this.focusFirstTabbableElement();
+  protected startAnchorListener = () => this.focusLastTabbableElement();
+  protected endAnchorListener = () => this.focusFirstTabbableElement();
 
   /** Whether the focus trap is active. */
   get enabled(): boolean { return this._enabled; }
@@ -70,15 +70,15 @@ export class FocusTrap {
     const endAnchor = this._endAnchor;
 
     if (startAnchor) {
-      startAnchor.removeEventListener('focus', this._startAnchorListener);
+      startAnchor.removeEventListener('focus', this.startAnchorListener);
 
       if (startAnchor.parentNode) {
         startAnchor.parentNode.removeChild(startAnchor);
       }
     }
 
     if (endAnchor) {
-      endAnchor.removeEventListener('focus', this._endAnchorListener);
+      endAnchor.removeEventListener('focus', this.endAnchorListener);
 
       if (endAnchor.parentNode) {
         endAnchor.parentNode.removeChild(endAnchor);
@@ -103,12 +103,12 @@ export class FocusTrap {
     this._ngZone.runOutsideAngular(() => {
       if (!this._startAnchor) {
         this._startAnchor = this._createAnchor();
-        this._startAnchor!.addEventListener('focus', this._startAnchorListener);
+        this._startAnchor!.addEventListener('focus', this.startAnchorListener);
       }
 
       if (!this._endAnchor) {
         this._endAnchor = this._createAnchor();
-        this._endAnchor!.addEventListener('focus', this._endAnchorListener);
+        this._endAnchor!.addEventListener('focus', this.endAnchorListener);
       }
     });
 

tools/public_api_guard/cdk/a11y.d.ts

@@ -87,6 +87,8 @@ export declare type FocusOrigin = 'touch' | 'mouse' | 'keyboard' | 'program' | n
 
 export declare class FocusTrap {
     enabled: boolean;
+    protected endAnchorListener: () => boolean;
+    protected startAnchorListener: () => boolean;
     constructor(_element: HTMLElement, _checker: InteractivityChecker, _ngZone: NgZone, _document: Document, deferAnchors?: boolean);
     attachAnchors(): boolean;
     destroy(): void;