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Adapter Design Pattern in Delphi

Applications in Delphi

A typical example of this is the wrapper Delphi generates when you import a VBX or OCX. Delphi generates a new class which translates the interface of the external control into a Pascal compatible interface. Another typical case is when you want to build a single interface to old and new systems.

Note Delphi does not allow class adaption through multiple inheritance in the way described in Design Patterns. Instead, the adapter needs to refer to a specific instance of the old class.

Implementation Example

The following example is a simple (read only) case of a new customer class, an adapter class and an old customer class. The adapter illustrates handling the year 2000 problem, translating an old customer record containing two digit years into a new date format. The client using this wrapper only knows about the new customer class.

Translation between classes is handled by the use of virtual access methods for the properties. The old customer class and adapter class are hidden in the implementation of the unit.

 unit Adapter; interface uses SysUtils, Classes; type //The new class TNewCustomer = class private FCustomerID: Longint; FFirstName: string; FLastName: string; FDOB: TDateTime; protected function GetCustomerID: Longint; virtual; function GetFirstName: string; virtual; function GetLastName: string; virtual; function GetDOB: TDateTime; virtual; public constructor Create(CustID: Longint); virtual; property CustomerID: Longint read GetCustomerID; property FirstName: string read GetFirstName; property LastName: string read GetLastName; property DOB: TDateTime read GetDOB; end; //An interface method //Lets us hide details of TOldCustomer from the client function GetCustomer(CustomerID: Longint): TNewCustomer; implementation const Last_OldCustomer_At_Year_2000 = 15722; Last_OldCustomer_In_Database = 30000; //The new class constructor TNewCustomer.Create(CustID: Longint); begin FCustomerID := CustID; FFirstName := 'A'; FLastName := 'New_Customer'; FDOB := Now; end; function TNewCustomer.GetCustomerID: Longint; begin Result := FCustomerID; end; function TNewCustomer.GetFirstName: string; begin Result := FFirstName; end; function TNewCustomer.GetLastName: string; begin Result := FLastName; end; function TNewCustomer.GetDOB: TDateTime; begin Result := FDOB; end; type //The old class TOldDOB = record Day: 0..31; Month: 1..12; Year: 0..99; end; TOldCustomer = class FCustomerID: Integer; FName: string; FDOB: TOldDOB; public constructor Create(CustID: Integer); property CustomerID: Integer read FCustomerID; property Name: string read FName; property DOB: TOldDOB read FDOB; end; constructor TOldCustomer.Create(CustID: Integer); begin FCustomerID := CustomerID; FName := 'An Old_Customer'; with FDOB do begin Day := 1; Month := 1; Year := 1; end; end; type //The Adapter class TAdaptedCustomer = class(TNewCustomer) private FOldCustomer: TOldCustomer; protected function GetCustomerID: Longint; override; function GetFirstName: string; override; function GetLastName: string; override; function GetDOB: TDateTime; override; public constructor Create(CustID: Longint); override; destructor Destroy; override; end; //The Adapter class constructor TAdaptedCustomer.Create(CustID: Longint); begin inherited Create(CustID); FOldCustomer := TOldCustomer.Create(CustID); end; destructor TAdaptedCustomer.Destroy; begin FOldCustomer.Free; inherited Destroy; end; function TAdaptedCustomer.GetCustomerID: Longint; begin Result := FOldCustomer.CustomerID; end; function TAdaptedCustomer.GetFirstName: string; var SpacePos: integer; begin SpacePos := Pos(' ', FOldCustomer.Name); if SpacePos = 0 then Result := '' else Result := Copy(FOldCustomer.Name,1,SpacePos-1); end; function TAdaptedCustomer.GetLastName: string; var SpacePos: integer; begin SpacePos := Pos(' ', FOldCustomer.Name); if SpacePos = 0 then Result := FOldCustomer.Name else Result := Copy(FOldCustomer.Name,SpacePos+1,255); end; function TAdaptedCustomer.GetDOB: TDateTime; var FullYear: Word; begin if CustomerID > Last_OldCustomer_At_Year_2000 then FullYear := 2000 + FOldCustomer.DOB.Year else FullYear := 1900 + FOldCustomer.DOB.Year; Result := EncodeDate(FullYear, FOldCustomer.DOB.Month, FOldCustomer.DOB.Day); end; function GetCustomer(CustomerID: Longint): TNewCustomer; begin if CustomerID > Last_OldCustomer_In_Database then Result := TNewCustomer.Create(CustomerID) else Result := TAdaptedCustomer.Create(CustomerID) as TNewCustomer; end; end.

Code examples

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