The Choice Pattern needs one interface, n-classes which supports the interface and a
worker-class to provide the choice of an algorithm at runtime. The Choice Pattern is like the
Strategy Pattern, but smaller and more runtime in his behaviour.
With interfaces we don't have to concern about memory management.
Interface references are managed through reference-counting, which depends on the _AddRef
and _Release methods inherited from IUnknown. When an object is referenced only through
interfaces, there is no need to destroy it manually; the object is automatically destroyed when
the last reference to it goes out of scope.
The following restrictions apply.
- The member List can include only methods and properties.
- Fields are not allowed in interfaces.
- Interfaces have no constructors or destructors. They cannot be instantiated, except through
classes that implement their methods.
- Methods cannot be declared as virtual, dynamic, abstract, or override. Since interfaces do not
implement their own methods, these bindings have no meaning. So let's practice the
Choice Pattern in 5 steps:
//1. We need an Interface in order to be type-compatible.
IChoicePattern = interface
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)
TCheckFormatB = class(TInterfacedObject, IChoicePattern)
// 3. We need a worker-class which calls the runtime Interface-methods:
TDirWorker = class
procedure CallCheck(myInst: IChoicePattern);
// 4. Now we implement the Interface Classes and the Worker Class too:
if FileExists(extractFileDir(Application.exeName) + '\' + DBNAME) then
ldbPath := extractFileDir(Application.exeName) + '\' + DBNAME;
MessageDlg('formatASearch doing', mtInformation, [mbOK], 0);
if OpenDialog1.Execute then
ldbPath := OpenDialog1.FileName;
MessageDlg('formatBSearch doing', mtInformation, [mbOK], 0);
procedure TDirWorker.CallCheck(myInst: IChoicePattern);
MessageDlg('do_some_Work', mtInformation, [mbOK], 0);
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);
// Have fun and choice with OP (Delphi, Kylix and FreePascal)
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