How to explicitly open or close cell group?

Question

There is a "OpenCloseGroup" in FrontEndTokenExecute. It act like toggle. If the selected cell group is open, FrontEndTokenExecute["OpenCloseGroup"] will close cell group. If the selected cell group is closed, FrontEndTokenExecute["OpenCloseGroup"] will open cell group.

But I want a way to explicitly open or close cell group. It is not like FrontEndTokenExecute["SelectionOpenAllGroups"] or FrontEndTokenExecute["SelectionCloseAllGroups"] which will open or close all sub cell groups in any level. I want open and close one level like the FrontEndTokenExecute["OpenCloseGroup"] does.

Update

The problem is actually equivalent to seeking an efficient way to get the open status of a cell group, then we can use FrontEndTokenExecute["OpenCloseGroup"]. Thanks to Alexey Popkov for many information. I found currently the most reliable way is to NotebookRead cell data to check if it is closed or not. Here is a way

ClearAll[openSelectedCellGroup];
openSelectedCellGroup[nb_NotebookObject]:=Module[{},
If[MatchQ[NotebookRead[nb],Cell[CellGroupData[_,Closed]]],
FrontEndTokenExecute[nb,"OpenCloseGroup"]];

Though NotebookRead seems to be inefficient if the cell group contains large data. However, my experience is that it is still workable. But again, there should better be a more efficient way to check the open-close state of cell group. Hope anyone knows.

Update 2

Alexey Popkov found a way using CurrentValue[cellObj, "CellGroupOpen"]. Now we finally have an efficient tool for explicitly open and close cell groups! Like this function:

ClearAll[openSelectedCellGroup];
openSelectedCellGroup[nb_NotebookObject]:=Module[{},
If[CurrentValue[First@Cells[NotebookSelection[nb]],"CellGroupOpen"]===Closed,
FrontEndTokenExecute[nb,"OpenCloseGroup"]];

See my real applications here

Answer 1

In this answer b3m2a1 demonstrated how to attach a GroupOpener to a CellGroup using FrontEndResource["FEExpressions", "cellGroupOpenerInline"]. Evaluating the latter, we can find that it is based upon CurrentValue[cellObj, "CellGroupOpen"] which can be used as follows:

Dynamic@CurrentValue[cellObj, "CellGroupOpen"]

CellPrint@Cell[
   CellGroupData[{Cell["Close me", "Subsubsection"], 
     Cell["goodbye cruel notebook", "Text"], Cell["I'll disappear too!", "Text"]}]];
cellObj = NextCell[];

Now collapse the CellGroup by hand:

We see that CurrentValue[cellObj, "CellGroupOpen"] allows determining the current state of the CellGroup.

Unfortunately, it doesn't allow to set the state directly: CurrentValue[cellObj, "CellGroupOpen"] = Closed doesn't work.

But there is an undocumented way to do this, which was uncovered by b3m2a1 in this answer:

MathLink`CallFrontEnd@FrontEnd`Value@FEPrivate`SetCellGroupOpen[cellObj, Closed]