Mega Code Archive
How to implement the Choice Pattern
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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)
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