cdkDrag does nothing when inside an *ngFor looping through a returned 2 dimensional array

[wshaver]

What is the expected behavior?

cdkDrag works from inside an ngFor looping through an array with a nested array.

What is the current behavior?

Nothing happens. cdkDrag does not move or drag.

What are the steps to reproduce?

https://stackblitz.com/edit/angular-a4ftm7-oectv4?file=app%2Fcdk-drag-drop-connected-sorting-example.html

<div *ngFor="let i of getNestedArray()">
  complete failure :(
  <div cdkDrag>1</div>
  <div cdkDrag>2</div>
  <div cdkDrag>3</div>
</div>

 getNestedArray(){
   return [[]];
 }

Which versions of Angular, Material, OS, TypeScript, browsers are affected?

chrome browser, angular 7.1.0 and 7.2.0, material 7.1.0 and 7.3.7

Is there anything else we should know?

Happy to explain more if this isn't clear. Had me stumped for hours. :(

[Airblader]

This is because calling getNestedArray, a function, creates the nested array from scratch on every change detection cycle. While this is also true for getSimpleArray, the difference is that the inner array is also recreated everytime, meaning it's a new reference.

Because of that, the draggable item is recreated on every CD cycle, which resets its position. This can be proven easily by making the component OnPush making the items draggable.

So this is actually not a bug, but just incorrect usage. Generally you just shouldn't call functions from templates, but if you must, specifying a trackBy function will help you here.

[crisbeto]

@Airblader's explanation is correct. Closing since this works as expected.

[wshaver]

Thanks for the detailed explanation, makes sense.

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