How-to-implement-the-Choice-Pattern


            Title: How to implement the Choice Pattern
IChoicePattern = interface
 procedure doPatternSearch;
 end;
{
 2. We declare 2 or n classes. A class from an Interface can support/implement multiple
 interfaces. TInterfacedObject implements the methods of IUnknown, so TInterfacedObject
 automatically handles reference counting and memory management of interfaced objects.
 One of the concepts behind the design of interfaces is ensuring the lifetime management of the
 objects that implement them. The AddRef and Release methods of IUnknown provide a way of
 implementing this functionality.
}
 TCheckFormatA = class(TInterfacedObject, IChoicePattern)
 public
 procedure doPatternSearch;
 end;
 TCheckFormatB = class(TInterfacedObject, IChoicePattern)
 public
 procedure doPatternSearch;
 end;
// 3. We need a worker-class which calls the runtime Interface-methods:
 TDirWorker = class
 procedure CallCheck(myInst: IChoicePattern);
 end;
// 4. Now we implement the Interface Classes and the Worker Class too:
procedure TCheckFormatA.doPatternSearch;
var
 ldbPath: string;
begin
 if FileExists(extractFileDir(Application.exeName) + '\' + DBNAME) then
 ldbPath := extractFileDir(Application.exeName) + '\' + DBNAME;
 MessageDlg('formatASearch doing', mtInformation, [mbOK], 0);
end;
procedure TCheckFormatB.doPatternSearch;
var
 ldbPath: string;
begin
 if OpenDialog1.Execute then
 ldbPath := OpenDialog1.FileName;
 MessageDlg('formatBSearch doing', mtInformation, [mbOK], 0);
end;
procedure TDirWorker.CallCheck(myInst: IChoicePattern);
begin
 myInst.doPatternSearch;
 MessageDlg('do_some_Work', mtInformation, [mbOK], 0);
end;
{
 Here we can see, no myInst.Free is needed. Each object from TCheckFormatA or
 TCheckFormatB is automatically destroyed. Interfaces track the lifetime of an object by
 incrementing the reference count on the object when an interface reference is passed, and will
 destroy the object when that reference count is zero.
}
// 5. at least the client calls the Choice Pattern and
// every object is local at runtime:
procedure TMainFrm.Button1Click(Sender: TObject);
begin
 with(TDirWorker.Create) do
 begin
 CallCheck(TCheckFormatA.Create);
 CallCheck(TCheckFormatB.Create);
 Free;
 end;
end;
// Have fun and choice with OP (Delphi, Kylix and FreePascal)
OOP Delphi
