Leonardo's blog

In my previous article, I wrote about a method to paginate data shown in a ListView.
Now, I want to extend its reach by replacing TDataSets by TCollections.

What is a TCollection?

A TCollection is a container, where objects of only one type can be stored in it.
The difference with other containers like TList and TObjectList is they can
contain any kind of pointer, in the case of TList, and heterogenous objects in
a TObjectList, on the other hand, in a TCollection,
only objects of a specific class can be used.

A collection of Customers

To create a collection of Customers, we have to derive a class from TCollectionItem,
for example TCustomer, and another class from TCollection, for example
TCustomers, as follows:

TCustomer = class(TCollectionItem)
private
 FCustId: Integer;
 FLastName: string;
 FFirstName: string;
public
 property CustId: Integer read FCustId write FCustId;
 property FirstName: string read FFirstName write FFirstName;
 property LastName: string read FLastName write FLastName;
end;

TCustomers = class(TCollection)
private
 function GetItem(AIndex: Integer): TCustomer;
public
 function Add: TCustomer;
 property Items[AIndex: Integer]: TCustomer read GetItem; default;
end;

Now lets define the methods Add and GetItem:

function TCustomers.GetItem(AIndex: Integer): TCustomer;
begin
  result := TCustomer(inherited GetItem(AIndex));
end;

function TCustomers.Add: TCustomer;
begin
  result := TCustomer(inherited Add);
end;

Save the above code in a unit called customers.pas, then create a new application,
and add the newly created unit to the "uses" clause of the main form, then add
the attribute FCustomers: TCustomers; to the private section of the form:

type
  TForm1 = class(TForm)
  private
    FCustomers: TCustomers;
  public
    { Public declarations }
  end;

Now, override the OnCreate and OnDestroy methods of the main form to
instantiate and free the collection:

procedure TForm1.FormCreate(Sender: TObject);
begin
  FCustomers := TCustomers.Create(TCustomer);
end;

procedure TForm1.FormDestroy(Sender: TObject);
begin
  FCustomers.Free;
end;

From database to collection

Ok, the next step is to fill the collection with data. As I explained in my last
article, place the database connectivity components in a DataModule or just
in the main form (this is just an example) and name the TQuery as IbQuery1.

Good, the next step is to adapt the FormCreate method of the last article
to this:

procedure TForm1.FormCreate(Sender: TObject);
begin
  (* Create an instance of the customers collection *)
  FCustomers := TCustomers.Create(TCustomer);
  (* Get the total amount of customers *)
  IbQuery1.Close;
  IbQuery1.SQL.Text := 'select count(*) from customers';
  IbQuery1.Open;
  ListView1.Items.Count := IbQuery1.Fields[0].Value;
  // query for the first page
  FCurrentPage := 1;
  GetCurrentPage(FCurrentPage);
end;

Now, to fill our TCollection with data from the database, just modify
our GetCurrentPage method as this:

procedure TForm1.GetCurrentPage(ACurrentPage: Integer);
var
  lFrom: Integer;
  lTo: Integer;
  I: Integer;

begin
  (* Do the query *)
  lFrom := ((ACurrentPage * cPageSize) - cPageSize) + 1;
  lTo := (ACurrentPage * cPageSize) + 1;
  IbQuery1.Close;
  IbQuery1.SQL.Text :=
   'select CustId, FirstName, LastName from customers ' +
   'rows ' + IntToStr(lFrom) + ' to ' + IntToStr(lTo);
  IbQuery1.Open;
  (* Fill the collection *)
  FCustomers.Clear;
  while not IbQuery1.Eof do
  begin
    with FCustomers.Add do
    begin
      CustId := IbQuery1.FieldByName('CustId').AsInteger;
      FirstName := IbQuery1.FieldByName('FirstName').AsString;
      LastName := IbQuery1.FieldByName('LastName').AsString;
    end;
    IbQuery1.Next;
  end;
  IbQuery1.Close;
end;

Let me tell you that assigning data by querying with FieldByName is very slow,
but simple enough for the purpouse of this article, in a future post, I'll
show an improved version.

The last step, is to place the TListView in the form, set its properties as
shown in my last article and create the new OnData method,
adapted to our TCollection:

procedure TForm1.ListView1Data(Sender: TObject; Item: TListItem);
var
 lCurrPage: Integer;
 lPos: Integer;
begin
 (* Get current page index *)
 lCurrPage := Item.Index div cPageSize;
 (* Get the position in the current page *)
 lPos := Item.Index - (lCurrPage * cPageSize);

 (* Page changed? refresh the data *)
 if FCurrentPage - 1 <> lCurrPage then
 begin
   FCurrentPage := lCurrPage + 1;
   GetCurrentPage(FCurrentPage);
 end;

 (* Paint the ListView's item with our TCollection's items *)
 Item.Caption := FCustomers[lPos].LastName;
 Item.SubItems.Add(FCustomers[lPos].FirstName);
end;

That's it.

My next article, will be focused on creating an automated object binding method
using RTTI, to avoid repetitive assignments by hand.