fix(cdk/drag-drop): resolve projected handles

Currently the `cdkDragHandle` directive registers itself with the parent by resolving it through DI. This doesn't work if the directive is declared in a separate embedded view (e.g. `ng-template`) that is then projected into the draggable element. It can be problematic when adding dragging support to a `mat-table`.

These changes fix the issue by falling back to resolving the draggable directive through the DOM.

Fixes angular#29475.

Author: crisbeto

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

@@ -7,6 +7,7 @@
  */
 
 import {
+  AfterViewInit,
   Directive,
   ElementRef,
   InjectionToken,
@@ -19,6 +20,7 @@ import {Subject} from 'rxjs';
 import type {CdkDrag} from './drag';
 import {CDK_DRAG_PARENT} from '../drag-parent';
 import {assertElementNode} from './assertions';
+import {DragDropRegistry} from '../drag-drop-registry';
 
 /**
  * Injection token that can be used to reference instances of `CdkDragHandle`. It serves as
@@ -35,10 +37,11 @@ export const CDK_DRAG_HANDLE = new InjectionToken<CdkDragHandle>('CdkDragHandle'
   },
   providers: [{provide: CDK_DRAG_HANDLE, useExisting: CdkDragHandle}],
 })
-export class CdkDragHandle implements OnDestroy {
+export class CdkDragHandle implements AfterViewInit, OnDestroy {
   element = inject<ElementRef<HTMLElement>>(ElementRef);
 
   private _parentDrag = inject<CdkDrag>(CDK_DRAG_PARENT, {optional: true, skipSelf: true});
+  private _dragDropRegistry = inject(DragDropRegistry);
 
   /** Emits when the state of the handle has changed. */
   readonly _stateChanges = new Subject<CdkDragHandle>();
@@ -64,6 +67,21 @@ export class CdkDragHandle implements OnDestroy {
     this._parentDrag?._addHandle(this);
   }
 
+  ngAfterViewInit() {
+    if (!this._parentDrag) {
+      let parent = this.element.nativeElement.parentElement;
+      while (parent) {
+        const ref = this._dragDropRegistry.getDragDirectiveForNode(parent);
+        if (ref) {
+          this._parentDrag = ref;
+          ref._addHandle(this);
+          break;
+        }
+        parent = parent.parentElement;
+      }
+    }
+  }
+
   ngOnDestroy() {
     this._parentDrag?._removeHandle(this);
     this._stateChanges.complete();

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

@@ -9,6 +9,7 @@ import {
   ViewEncapsulation,
   signal,
 } from '@angular/core';
+import {NgTemplateOutlet} from '@angular/common';
 import {fakeAsync, flush, tick} from '@angular/core/testing';
 import {
   dispatchEvent,
@@ -1631,6 +1632,25 @@ describe('Standalone CdkDrag', () => {
         .toBe('translate3d(50px, 100px, 0px)');
     }));
 
+    it('should be able to drag with a handle that is defined in a separate embedded view', fakeAsync(() => {
+      const fixture = createComponent(StandaloneDraggableWithExternalTemplateHandle);
+      fixture.detectChanges();
+      const dragElement = fixture.componentInstance.dragElement.nativeElement;
+      const handle = fixture.nativeElement.querySelector('.handle');
+
+      expect(dragElement.style.transform).toBeFalsy();
+      dragElementViaMouse(fixture, dragElement, 50, 100);
+
+      expect(dragElement.style.transform)
+        .withContext('Expected not to be able to drag the element by itself.')
+        .toBeFalsy();
+
+      dragElementViaMouse(fixture, handle, 50, 100);
+      expect(dragElement.style.transform)
+        .withContext('Expected to drag the element by its handle.')
+        .toBe('translate3d(50px, 100px, 0px)');
+    }));
+
     it('should disable the tap highlight while dragging via the handle', fakeAsync(() => {
       // This test is irrelevant if the browser doesn't support styling the tap highlight color.
       if (!('webkitTapHighlightColor' in document.body.style)) {
@@ -2010,3 +2030,21 @@ class DraggableNgContainerWithAlternateRoot {
 class PlainStandaloneDraggable {
   @ViewChild(CdkDrag) dragInstance: CdkDrag;
 }
+
+@Component({
+  template: `
+    <div #dragElement cdkDrag
+      style="width: 100px; height: 100px; background: red; position: relative">
+      <ng-container [ngTemplateOutlet]="template"/>
+    </div>
+
+    <ng-template #template>
+      <div cdkDragHandle class="handle" style="width: 10px; height: 10px; background: green;"></div>
+    </ng-template>
+  `,
+  standalone: true,
+  imports: [CdkDrag, CdkDragHandle, NgTemplateOutlet],
+})
+class StandaloneDraggableWithExternalTemplateHandle {
+  @ViewChild('dragElement') dragElement: ElementRef<HTMLElement>;
+}

tools/public_api_guard/cdk/drag-drop.md

@@ -150,7 +150,7 @@ export interface CdkDragExit<T = any, I = T> {
 }
 
 // @public
-export class CdkDragHandle implements OnDestroy {
+export class CdkDragHandle implements AfterViewInit, OnDestroy {
     constructor(...args: unknown[]);
     get disabled(): boolean;
     set disabled(value: boolean);
@@ -159,6 +159,8 @@ export class CdkDragHandle implements OnDestroy {
     // (undocumented)
     static ngAcceptInputType_disabled: unknown;
     // (undocumented)
+    ngAfterViewInit(): void;
+    // (undocumented)
     ngOnDestroy(): void;
     readonly _stateChanges: Subject<CdkDragHandle>;
     // (undocumented)