How to make an Excel-Like Sort By A, Then By B in a TObjectList<> using multiple comparers

Question

I have just started to use generics, and I am currently having a problem doing sorting on multiple fields.

Case:
I have a PeopleList as a TObjectList

Answer 1

Put your sort criteria in a list that includes the direction to sort and the function to use to compare items. A record like this could help:

type
  TSortCriterion = record
    Ascending: Boolean;
    Comparer: IComparer;
  end;

As the user configures the desired ordering, populate the list with instances of that record.

var
  SortCriteria: TList;

The Comparer member will refer to the functions you've already written for comparing based on name and age. Now write a single comparison function that refers to that list. Something like this:

function Compare(const A, B: TPerson): Integer;
var
  Criterion: TSortCriterion;
begin
  for Criterion in SortCriteria do begin
    Result := Criterion.Comparer.Compare(A, B);
    if not Criterion.Ascending then
      Result := -Result;
    if Result <> 0 then
      Exit;
  end;
end;