fix(drag-drop): preview element not maintaining canvas data (#15808)

By default we generate the preview and placeholder for a `cdkDrag` using `cloneNode`, however it won't clone the content of `canvas` elements. These changes add some extra logic to transfer the canvas content over into the clones.

Fixes #15685.

src/cdk/drag-drop/directives/drag.spec.ts

@@ -1513,6 +1513,26 @@ describe('CdkDrag', () => {
       expect(preview.getAttribute('id')).toBeFalsy();
     }));
 
+    it('should clone the content of descendant canvas elements', fakeAsync(() => {
+      const fixture = createComponent(DraggableWithCanvasInDropZone);
+      fixture.detectChanges();
+      const item = fixture.componentInstance.dragItems.toArray()[1].element.nativeElement;
+      const sourceCanvas = item.querySelector('canvas') as HTMLCanvasElement;
+
+      // via https://stackoverflow.com/a/17386803/2204158
+      expect(sourceCanvas.getContext('2d')!
+        .getImageData(0, 0, sourceCanvas.width, sourceCanvas.height)
+        .data.some(channel => channel !== 0)).toBe(true, 'Expected source canvas to have data.');
+
+      startDraggingViaMouse(fixture, item);
+
+      const preview = document.querySelector('.cdk-drag-preview')! as HTMLElement;
+      const previewCanvas = preview.querySelector('canvas')!;
+
+      expect(previewCanvas.toDataURL()).toBe(sourceCanvas.toDataURL(),
+          'Expected cloned canvas to have the same content as the source.');
+    }));
+
     it('should clear the ids from descendants of the preview', fakeAsync(() => {
       const fixture = createComponent(DraggableInDropZone);
       fixture.detectChanges();
@@ -3821,6 +3841,47 @@ class ConnectedWrappedDropZones {
   done = ['Four', 'Five', 'Six'];
 }
 
+@Component({
+  template: `
+    <div
+      cdkDropList
+      style="width: 100px; background: pink;"
+      [id]="dropZoneId"
+      [cdkDropListData]="items"
+      (cdkDropListSorted)="sortedSpy($event)"
+      (cdkDropListDropped)="droppedSpy($event)">
+      <div
+        *ngFor="let item of items"
+        cdkDrag
+        [cdkDragData]="item"
+        [style.height.px]="item.height"
+        [style.margin-bottom.px]="item.margin"
+        style="width: 100%; background: red;">
+          {{item.value}}
+          <canvas width="100px" height="100px"></canvas>
+        </div>
+    </div>
+  `
+})
+class DraggableWithCanvasInDropZone extends DraggableInDropZone implements AfterViewInit {
+  constructor(private _elementRef: ElementRef<HTMLElement>) {
+    super();
+  }
+
+  ngAfterViewInit() {
+    const canvases = this._elementRef.nativeElement.querySelectorAll('canvas');
+
+    // Add a circle to all the canvases.
+    for (let i = 0; i < canvases.length; i++) {
+      const canvas = canvases[i];
+      const context = canvas.getContext('2d')!;
+      context.beginPath();
+      context.arc(50, 50, 40, 0, 2 * Math.PI);
+      context.stroke();
+    }
+  }
+}
+
 /**
  * Component that passes through whatever content is projected into it.
  * Used to test having drag elements being projected into a component.

src/cdk/drag-drop/drag-ref.ts

@@ -1049,6 +1049,7 @@ function getTransform(x: number, y: number): string {
 function deepCloneNode(node: HTMLElement): HTMLElement {
   const clone = node.cloneNode(true) as HTMLElement;
   const descendantsWithId = clone.querySelectorAll('[id]');
+  const descendantCanvases = node.querySelectorAll('canvas');
 
   // Remove the `id` to avoid having multiple elements with the same id on the page.
   clone.removeAttribute('id');
@@ -1057,6 +1058,20 @@ function deepCloneNode(node: HTMLElement): HTMLElement {
     descendantsWithId[i].removeAttribute('id');
   }
 
+  // `cloneNode` won't transfer the content of `canvas` elements so we have to do it ourselves.
+  // We match up the cloned canvas to their sources using their index in the DOM.
+  if (descendantCanvases.length) {
+    const cloneCanvases = clone.querySelectorAll('canvas');
+
+    for (let i = 0; i < descendantCanvases.length; i++) {
+      const correspondingCloneContext = cloneCanvases[i].getContext('2d');
+
+      if (correspondingCloneContext) {
+        correspondingCloneContext.drawImage(descendantCanvases[i], 0, 0);
+      }
+    }
+  }
+
   return clone;
 }