feat(cdk/drag-drop): Allow dynamic drop lists creation while dragging an item

[fmancuso]

Feature Description

I have noticed that if I try to create a drop list dynamically while I am dragging an item, the drop list is created graphically but it is not possible to drop the item I am dragging into that drop list. This seems to happen because the "_siblings" are created at drag initiation and do not notice if a row is created at runtime. It start working after a few drop list exit or enter but not if you directly go to that drop list.

Would it be possible to have this feature? Or if it is already possible could you tell me how to do it? For the moment I solved it by adding this method to my code, but it seems to me a very dirty solution.

private refreshDropLists() {
    // do it only if the component rows and the library rows are not sync
    if (this.dropLists.toArray().length === this.configuration.length) {
      // loop over the droplists
      this.dropLists.toArray().forEach((dropList) => {
        // force to recalculate the row positions
        // @ts-ignore
        dropList._dropListRef._cacheParentPositions();
        // force the link between each row except for the row itself
        dropList._dropListRef.connectedTo(
          this.dropLists
            .toArray()
            .filter((dropList2) => dropList.id !== dropList2.id)
            .map((droplist2) => droplist2._dropListRef)
        );
      });
    }
  }

An example can be viewed at the following link: https://stackblitz.com/edit/stackblitz-starters-7xvrf3?file=src%2Fapp%2Fapp.component.ts

If you want to make it work where to uncomment line 30 and 31 of the file "app.component.ts"

Use Case

A useful use case would be, as with the linked stackblitz, creating a new droplists from the element I am dragging. So if I have as in my case a structure of rows and columns, I want to be able to drag my column to a new row that doesn't exist.

[tkiefer24]

I use also dynamic creating while dragging over a three structure and open it, to show the child elements. My thoughts are to just expose an API to refresh the cache, would be a first step for this feature. I mean these both functions, that are currently private:

dropListRef['_sortStrategy']._cacheItemPositions();
dropListRef['_cacheParentPositions']();

Thanks