Again over interfaces in Delphi - delphi

Question

In order to understand interfaces I've realized a small application with a form, a data module with a simple database.
here is the form
The data module contains only a connection, a table and a TDataSource component.
The interface unit is this:
unit databaseInterface;
interface
uses
MSAccess;
type
IDBTest = interface
['{5B8CF4FF-66F7-402D-8E18-0159CB22F805}']
procedure SetTable(table: TMSTable);
function SetPriorRecord: Boolean;
function SetNextRecord: Boolean;
end;
implementation
end.
and it's implementation is this:
unit databaseImplementation;
interface
uses
databaseInterface, database, MSAccess;
type
TDBTest = class(TInterfacedObject, IDBTest)
protected
DBTable: TMSTable;
FbtnPriorStatus: Boolean;
procedure SetTable(Table: TMSTable);
function SetPriorRecord: Boolean;
function SetNextRecord: Boolean;
public
property Table: TMSTable read DBTable write SetTable;
end;
implementation
{ TDBTest }
procedure TDBTest.SetTable(Table: TMSTable);
begin
if DBTable <> Table then begin
DBTable := Table;
DBTable.Open;
end;
end;
function TDBTest.SetPriorRecord: Boolean;
begin
if not DBTable.Bof then begin
DBTable.Prior;
Result := DBTable.Bof;
end else
Result := True;
end;
function TDBTest.SetNextRecord: Boolean;
begin
if not DBTable.Eof then begin
DBTable.Next;
Result := DBTable.Eof;
end else
Result := True;
end;
end.
Now, this is the question. The code of my form is as below:
unit main;
interface
uses
Winapi.Windows, Winapi.Messages, System.SysUtils, System.Variants,
System.Classes, Vcl.Graphics, Vcl.Controls, Vcl.Forms, Vcl.Dialogs,
Vcl.ExtCtrls, databaseInterface, databaseImplementation, JvExMask,
JvToolEdit, JvMaskEdit, JvCheckedMaskEdit, JvDatePickerEdit,
Vcl.StdCtrls, Vcl.Mask, Vcl.DBCtrls, JvDBDatePickerEdit, JvExControls,
JvButton, JvTransparentButton, database;
type
TfrmMain = class(TForm)
pnlCommands: TPanel;
pnlData: TPanel;
pnlMessages: TPanel;
bvlIcons: TBevel;
bvlNavigation: TBevel;
lblId: TLabel;
lblFirstName: TLabel;
lblLastName: TLabel;
lblBirthday: TLabel;
edtId: TDBEdit;
edtFirstName: TDBEdit;
edtLastName: TDBEdit;
dtpBirthday: TJvDBDatePickerEdit;
btnPrior: TJvTransparentButton;
btnNext: TJvTransparentButton;
procedure btnNextClick(Sender: TObject);
procedure btnPriorClick(Sender: TObject);
procedure FormCreate(Sender: TObject);
procedure FormActivate(Sender: TObject);
private
DBTest: IDBTest;
public
{ Public declarations }
end;
var
frmMain: TfrmMain;
implementation
{$R *.dfm}
procedure TfrmMain.FormCreate(Sender: TObject);
begin
DBTest := TDBTest.Create;
end;
procedure TfrmMain.FormActivate(Sender: TObject);
begin
DBTest.SetTable(dmAuthors.tblAuthors);
end;
procedure TfrmMain.btnPriorClick(Sender: TObject);
begin
btnPrior.Enabled := not DBTest.SetPriorRecord;
btnNext.Enabled := True;
end;
procedure TfrmMain.btnNextClick(Sender: TObject);
begin
btnNext.Enabled := not DBTest.SetNextRecord;
btnPrior.Enabled := True;
end;
end.
So I call the methods SetPriorRecord and SetNextRecord when the user click over the related button and then, accordingly with the status of the table (BOF or EOF), I disable or enable buttons.
I wonder if there is a way to set buttons status via interface, decoupling this operation from the form; for example binding buttons in any way or something else, but I don't know how to do it, if it is possible!
I hope I was clear in my explication of the problem.

Answer 1

The existing interface is not sufficient. You need to pass in some means of letting the client know the state of the table, but without exposing the TDataSet's detailed logic (preferably). A callback to an event handler would work; a way to trigger TAction would work; as would an anonymous method. You basically need to return a flag of some kind signifying BOF, EOF, or somewhere in between; possibly also a record# and record count.

Answer 2

I've modified the application interface in this way:
unit databaseInterface;
interface
uses
MSAccess;
type
IDBTest = interface
['{5B8CF4FF-66F7-402D-8E18-0159CB22F805}']
procedure SetTable(table: TMSTable);
procedure SetPriorRecord;
procedure SetNextRecord;
function GetIsBof: Boolean;
function GetIsEof: Boolean;
property IsBof: Boolean read GetIsBof;
property IsEof: Boolean read GetIsEof;
end;
implementation
end.
and this is the interface implementation:
unit databaseImplementation;
interface
uses
databaseInterface, database, MSAccess;
type
TDBTest = class(TInterfacedObject, IDBTest)
protected
DBTable: TMSTable;
FIsBof: Boolean;
FIsEof: Boolean;
procedure SetTable(Table: TMSTable);
procedure SetPriorRecord;
procedure SetNextRecord;
function GetIsBof: Boolean;
function GetIsEof: Boolean;
procedure SetCursorStatus;
public
property Table: TMSTable read DBTable write SetTable;
property IsBof: Boolean read GetIsBof;
property IsEof: Boolean read GetIsEof;
end;
implementation
{ TDBTest }
procedure TDBTest.SetTable(Table: TMSTable);
begin
if DBTable <> Table then begin
DBTable := Table;
DBTable.Open;
end;
end;
procedure TDBTest.SetPriorRecord;
begin
try
DBTable.Prior;
finally
SetCursorStatus;
end;
end;
procedure TDBTest.SetNextRecord;
begin
try
DBTable.Next;
finally
SetCursorStatus;
end;
end;
procedure TDBTest.SetCursorStatus;
begin
FIsBof := DBTable.Bof;
FIsEof := DBTable.Eof;
end;
function TDBTest.GetIsBof: Boolean;
begin
Result := FIsBof;
end;
function TDBTest.GetIsEof: Boolean;
begin
Result := FIsEof;
end;
end.
So the form code become this:
unit main;
interface
uses
Winapi.Windows, Winapi.Messages, System.SysUtils, System.Variants,
System.Classes, Vcl.Graphics, Vcl.Controls, Vcl.Forms, Vcl.Dialogs,
Vcl.ExtCtrls, Vcl.StdCtrls, Vcl.Mask, Vcl.DBCtrls, JvExMask, JvToolEdit,
JvMaskEdit, JvCheckedMaskEdit, JvDatePickerEdit, JvDBDatePickerEdit,
JvExControls, JvButton, JvTransparentButton, database, databaseInterface,
databaseImplementation;
type
TfrmMain = class(TForm)
pnlCommands: TPanel;
pnlData: TPanel;
pnlMessages: TPanel;
bvlIcons: TBevel;
bvlNavigation: TBevel;
lblId: TLabel;
lblFirstName: TLabel;
lblLastName: TLabel;
lblBirthday: TLabel;
edtId: TDBEdit;
edtFirstName: TDBEdit;
edtLastName: TDBEdit;
dtpBirthday: TJvDBDatePickerEdit;
btnPrior: TJvTransparentButton;
btnNext: TJvTransparentButton;
procedure btnNextClick(Sender: TObject);
procedure btnPriorClick(Sender: TObject);
procedure FormCreate(Sender: TObject);
procedure FormActivate(Sender: TObject);
private
DBTest: IDBTest;
procedure SetNavButtonsStatus;
public
{ Public declarations }
end;
var
frmMain: TfrmMain;
implementation
{$R *.dfm}
procedure TfrmMain.FormCreate(Sender: TObject);
begin
DBTest := TDBTest.Create;
end;
procedure TfrmMain.FormActivate(Sender: TObject);
begin
DBTest.SetTable(dmAuthors.tblAuthors);
end;
{ Begin table navigation ----------------------------------------------------- }
procedure TfrmMain.btnPriorClick(Sender: TObject);
begin
DBTest.SetPriorRecord;
SetNavButtonsStatus;
end;
procedure TfrmMain.btnNextClick(Sender: TObject);
begin
DBTest.SetNextRecord;
SetNavButtonsStatus;
end;
procedure TfrmMain.SetNavButtonsStatus;
begin
btnPrior.Enabled := not DBTest.IsBof;
btnNext.Enabled := not DBTest.IsEof
end;
{ End table navigation ------------------------------------------------------- }
end.
Now I think buttons are decoupled, but I'm not sure abot the solution I've found. Can It be good?