feat(drag-utils): add utility function for cloning array items from one array to another (#13743)

* feat(drag-utils): add utility function for cloning array items from one array to another

creates a utility helper for cloning an array item into the array of the drop container
compliments moveItemInArray and transferArrayItem

partially closes #13100

* feat(drag-utils): add utility function for cloning array items from one array to another

resolves issues surfaced from code review

* feat(drag-utils): add utility function for cloning array items from one array to another

cuts extra space between parens

* removes link in comments, and clarifies function definition as recommended in review

* resolves nits

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

@@ -1,4 +1,4 @@
-import {moveItemInArray, transferArrayItem} from './drag-utils';
+import {moveItemInArray, transferArrayItem, copyArrayItem} from './drag-utils';
 
 describe('dragging utilities', () => {
   describe('moveItemInArray', () => {
@@ -66,4 +66,45 @@ describe('dragging utilities', () => {
     });
 
   });
+
+  describe('copyArrayItem', () => {
+    it('should be able to copy an item from one array to another', () => {
+      const a = [0, 1, 2];
+      const b = [3, 4, 5];
+
+      copyArrayItem(a, b, 1, 2);
+      expect(a).toEqual([0, 1, 2 ]);
+      expect(b).toEqual([3, 4, 1, 5]);
+    });
+
+    it('should handle an index greater than the target array length', () => {
+      const a = [0, 1, 2];
+      const b = [3, 4, 5];
+
+      copyArrayItem(a, b, 0, 7);
+
+      expect(a).toEqual([0, 1, 2]);
+      expect(b).toEqual([3, 4, 5, 0]);
+    });
+
+    it('should handle an index less than zero', () => {
+      const a = [0, 1, 2];
+      const b = [3, 4, 5];
+
+      copyArrayItem(a, b, 2, -1);
+      expect(a).toEqual([0, 1, 2]);
+      expect(b).toEqual([2, 3, 4, 5]);
+    });
+
+    it('should not do anything if the source array is empty', () => {
+      const a: number[] = [];
+      const b = [3, 4, 5];
+
+      copyArrayItem(a, b, 0, 0);
+      expect(a).toEqual([]);
+      expect(b).toEqual([3, 4, 5]);
+    });
+
+  });
+
 });

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

@@ -42,7 +42,6 @@ export function transferArrayItem<T = any>(currentArray: T[],
                                            targetArray: T[],
                                            currentIndex: number,
                                            targetIndex: number): void {
-
   const from = clamp(currentIndex, currentArray.length - 1);
   const to = clamp(targetIndex, targetArray.length);
 
@@ -51,6 +50,26 @@ export function transferArrayItem<T = any>(currentArray: T[],
   }
 }
 
+/**
+ * Copies an item from one array to another, leaving it in its
+ * original position in current array.
+ * @param currentArray Array from which to copy the item.
+ * @param targetArray Array into which is copy the item.
+ * @param currentIndex Index of the item in its current array.
+ * @param targetIndex Index at which to insert the item.
+ *
+ */
+export function copyArrayItem<T = any>(currentArray: T[],
+                                       targetArray: T[],
+                                       currentIndex: number,
+                                       targetIndex: number): void {
+  const to = clamp(targetIndex, targetArray.length);
+
+  if (currentArray.length) {
+    targetArray.splice(to, 0, currentArray[currentIndex]);
+  }
+}
+
 /** Clamps a number between zero and a maximum. */
 function clamp(value: number, max: number): number {
   return Math.max(0, Math.min(max, value));

src/demo-app/drag-drop/drag-drop-demo.html

@@ -1,6 +1,10 @@
 <div>
   <div class="demo-list">
+
     <h2>To do</h2>
+    <mat-slide-toggle color="primary" [(ngModel)]="cloneMode">
+      Clone Mode
+    </mat-slide-toggle>
     <div
       cdkDropList
       #one="cdkDropList"

src/demo-app/drag-drop/drag-drop-demo.ts

@@ -9,7 +9,12 @@
 import {Component, ViewEncapsulation} from '@angular/core';
 import {MatIconRegistry} from '@angular/material/icon';
 import {DomSanitizer} from '@angular/platform-browser';
-import {CdkDragDrop, moveItemInArray, transferArrayItem} from '@angular/cdk/drag-drop';
+import {
+  CdkDragDrop,
+  moveItemInArray,
+  transferArrayItem,
+  copyArrayItem
+} from '@angular/cdk/drag-drop';
 
 @Component({
   moduleId: module.id,
@@ -19,6 +24,8 @@ import {CdkDragDrop, moveItemInArray, transferArrayItem} from '@angular/cdk/drag
   encapsulation: ViewEncapsulation.None,
 })
 export class DragAndDropDemo {
+
+  cloneMode = false;
   axisLock: 'x' | 'y';
   todo = [
     'Go out for Lunch',
@@ -56,10 +63,19 @@ export class DragAndDropDemo {
     if (event.previousContainer === event.container) {
       moveItemInArray(event.container.data, event.previousIndex, event.currentIndex);
     } else {
-      transferArrayItem(event.previousContainer.data,
-                        event.container.data,
-                        event.previousIndex,
-                        event.currentIndex);
+      if (this.cloneMode) {
+        copyArrayItem(
+          event.previousContainer.data,
+          event.container.data,
+          event.previousIndex,
+          event.currentIndex);
+      } else {
+        transferArrayItem(
+          event.previousContainer.data,
+          event.container.data,
+          event.previousIndex,
+          event.currentIndex);
+      }
     }
   }
 }