Delphi: the Factory Pattern with virtual Create Constructors (via: What Design Patterns do you implement in common Delphi programming? – Stack Overflow)
Posted by Jeroen Pluimers on 2014/02/20
From long ago, but still very valid, as I recently had another question like “what design patterns does Delphi use?”.
The Delphi usage of patterns to make the VCL and your applications work is one of the reasons I like the Delphi Component Design: Danny Thorpe so much.
Do not let you scare by the book title: a lot of information in this book is much broader than designing components.
It is about why and how things are done in the RTL and VCL, and which patterns you can use yourself.
Try and git it while you can still get it. It is excellent, but rare to get as it has been out of print for a while.
Only a minority of the Delphi developers knows that every Delphi developer uses a Factory pattern (delphi.about.com has an example in “regular” Delphi), but then implemented using virtual Create constructors.
So: time to shed some light on that :-)
Virtual constructors are to classes like virtual methods are like object instances.
The whole idea of the factory pattern is that you decouple the logic that determines what kind (in this case “class”) of thing (in this case “object instance”) to create from the actual creation.
It works like this using virtual Create constructors:
TComponent has a virtual Create constructor so, which can be overridden by any descending class:
type TComponent = class(TPersistent, ...) constructor Create(AOwner: TComponent); virtual; ... end;
For instance the TDirectoryListBox.Create constructor overrides it:
type TDirectoryListBox = class(...) constructor Create(AOwner: TComponent); override; ... end;
You can store a class reference (the class analogy to an object instance reference) in a variable of type ‘class type’. For component classes, there is a predefined type TComponentClass in the Classes unit:
type TComponentClass = class of TComponent;
When you have a variable (or parameter) of type TComponentClass, you can do polymorphic construction, which is very very similar to the factory pattern:
var ClassToCreate: TComponentClass; ... procedure SomeMethodInSomeUnit; begin ClassToCreate := TButton; end; ... procedure AnotherMethodInAnotherUnit; var CreatedComponent: TComponent; begin CreatedComponent := ClassToCreate.Create(Application); ... end;
The Delphi RTL uses this for instance here:
Result := TComponentClass(FindClass(ReadStr)).Create(nil);
// create another instance of this kind of grid SubGrid := TCustomDBGrid(TComponentClass(Self.ClassType).Create(Self));
The first use in the Delphi RTL is how the whole creation process works of forms, datamodules, frames and components that are being read from a DFM file.
The form (datamodule/frame/…) classes actually have a (published) list of components that are on the form (datamodule/frame/…). That list includes for each component the instance name and the class reference.
When reading the DFM files, the Delphi RTL then:
- finds about the components instance name,
- uses that name to find the underlying class reference,
- then uses the class reference to dynamically create the correct object
A regular Delphi developer usually never sees that happen, but without it, the whole Delphi RAD experience would not exist.
Let me know if that was enough light on the virtual Create constructor topic :-)