fix(overlay): unable to change CdkConnectedOverlay origin dynamically (#9358)

Fixes the `CdkConnectedOverlay` not handling changes of its `origin` after it's opened for the first time.

Fixes #9353.

src/cdk/overlay/overlay-directives.spec.ts

@@ -20,11 +20,7 @@ describe('Overlay directives', () => {
     TestBed.configureTestingModule({
       imports: [OverlayModule],
       declarations: [ConnectedOverlayDirectiveTest, ConnectedOverlayPropertyInitOrder],
-      providers: [
-        {provide: Directionality, useFactory: () => {
-          return dir = {value: 'ltr'};
-        }}
-      ],
+      providers: [{provide: Directionality, useFactory: () => dir = {value: 'ltr'}}],
     });
   });
 
@@ -250,6 +246,28 @@ describe('Overlay directives', () => {
           .toBe('105px', `Expected overlay directive to reflect new offsetY if it changes.`);
     });
 
+    it('should be able to update the origin after init', () => {
+      const testComponent = fixture.componentInstance;
+
+      testComponent.isOpen = true;
+      fixture.detectChanges();
+
+      let triggerRect = fixture.nativeElement.querySelector('#trigger').getBoundingClientRect();
+      let overlayRect = getPaneElement().getBoundingClientRect();
+
+      expect(Math.floor(triggerRect.left)).toBe(Math.floor(overlayRect.left));
+      expect(Math.floor(triggerRect.bottom)).toBe(Math.floor(overlayRect.top));
+
+      testComponent.triggerOverride = testComponent.otherTrigger;
+      fixture.detectChanges();
+
+      triggerRect = fixture.nativeElement.querySelector('#otherTrigger').getBoundingClientRect();
+      overlayRect = getPaneElement().getBoundingClientRect();
+
+      expect(Math.floor(triggerRect.left)).toBe(Math.floor(overlayRect.left));
+      expect(Math.floor(triggerRect.bottom)).toBe(Math.floor(overlayRect.top));
+    });
+
   });
 
   describe('outputs', () => {
@@ -302,9 +320,11 @@ describe('Overlay directives', () => {
 
 @Component({
   template: `
-  <button cdk-overlay-origin #trigger="cdkOverlayOrigin">Toggle menu</button>
+  <button cdk-overlay-origin id="trigger" #trigger="cdkOverlayOrigin">Toggle menu</button>
+  <button cdk-overlay-origin id="otherTrigger" #otherTrigger="cdkOverlayOrigin">Toggle menu</button>
+
   <ng-template cdk-connected-overlay [open]="isOpen" [width]="width" [height]="height"
-            [origin]="trigger"
+            [cdkConnectedOverlayOrigin]="triggerOverride || trigger"
             [hasBackdrop]="hasBackdrop" backdropClass="mat-test-class"
             (backdropClick)="backdropClicked=true" [offsetX]="offsetX" [offsetY]="offsetY"
             (positionChange)="positionChangeHandler($event)" (attach)="attachHandler()"
@@ -313,13 +333,18 @@ describe('Overlay directives', () => {
   </ng-template>`,
 })
 class ConnectedOverlayDirectiveTest {
+  @ViewChild(CdkConnectedOverlay) connectedOverlayDirective: CdkConnectedOverlay;
+  @ViewChild('trigger') trigger: CdkOverlayOrigin;
+  @ViewChild('otherTrigger') otherTrigger: CdkOverlayOrigin;
+
   isOpen = false;
   width: number | string;
   height: number | string;
   minWidth: number | string;
   minHeight: number | string;
-  offsetX: number = 0;
-  offsetY: number = 0;
+  offsetX = 0;
+  offsetY = 0;
+  triggerOverride: CdkOverlayOrigin;
   hasBackdrop: boolean;
   backdropClicked = false;
   positionChangeHandler = jasmine.createSpy('positionChangeHandler');
@@ -329,8 +354,6 @@ class ConnectedOverlayDirectiveTest {
   });
   detachHandler = jasmine.createSpy('detachHandler');
   attachResult: HTMLElement;
-
-  @ViewChild(CdkConnectedOverlay) connectedOverlayDirective: CdkConnectedOverlay;
 }
 
 @Component({

src/cdk/overlay/overlay-directives.ts

@@ -256,6 +256,14 @@ export class CdkConnectedOverlay implements OnDestroy, OnChanges {
   }
 
   ngOnChanges(changes: SimpleChanges) {
+    if ((changes['origin'] || changes['_deprecatedOrigin']) && this._position) {
+      this._position.setOrigin(this.origin.elementRef);
+
+      if (this.open) {
+        this._position.apply();
+      }
+    }
+
     if (changes['open'] || changes['_deprecatedOpen']) {
       this.open ? this._attachOverlay() : this._detachOverlay();
     }

src/cdk/overlay/position/connected-position-strategy.ts

@@ -265,6 +265,15 @@ export class ConnectedPositionStrategy implements PositionStrategy {
     return this;
   }
 
+  /**
+   * Sets the origin element, relative to which to position the overlay.
+   * @param origin Reference to the new origin element.
+   */
+  setOrigin(origin: ElementRef): this {
+    this._origin = origin.nativeElement;
+    return this;
+  }
+
   /**
    * Gets the horizontal (x) "start" dimension based on whether the overlay is in an RTL context.
    * @param rect