[DragDrop] drag items indexes not being updated

[LiadIdan]

What is the current behavior?

I'm using the dragDrop service to create a draggable list from content children, but the previous and current indexes not being updated after updating the list.

What is the expected behavior?

The indexes should be updated after updating the list.

What are the steps to reproduce?

https://stackblitz.com/edit/cdk-drag-drop-index-bug-vaqfmc

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

Angular - 7.2.3
Material - 7.3.1

[LiadIdan]

I found that the cause of the issue is that DropListRef._draggables not being updated after updating the ContentChildren array. As a workaround, for now, I'm calling the moveItemInArray function with DropListRef._draggables as the first argument.
https://stackblitz.com/edit/cdk-drag-drop-index-bug-q1ys6n

[crisbeto]

It's because you're passing in an array to withItems which, unlike a QueryList, won't update once you drop the item. You should be able to get it working by passing in the shuffled array back to withItems when it's dropped.

That being said, a better approach would be if the drop list adapted when the array is mutated. Currently that won't happen since we clone the array inside withItems, but looking through the code, I can't see a good reason to continue doing so.

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