Title: Implementing TCollection
Question: This document is intended for those needing to descend from a class that manages an array of lightweight persistent objects of the same type. The class that best accomplishes this is TCollection and TCollectionItem. For example, TCollection is used to manage Panels in a TStatusBar, Columns in a TDBGrid, or Constraints in a TTable.
This document begins with a discussion of the expected behavior of TCollection descendants, followed by a listing of the minimal steps necessary to implement a TCollection descendant, a listing of the component source, and finally some notes on design decisions and ideas for expansion of your TCollection descendant component.
Answer:
General Discussion
------------------
To become familiar with the default behavior of TCollection, try adding a TStatusBar component to a form, click the ellipses of the Panels property, press the Add button of the "Editing Panels".
This last step adds a TStatusPanel to the editor. Click on
on the TStatusPanel item in the editor and notice the change in
the object inspector. Instead of seeing TStatusBar now you will
see StatusBar1.Panels[0] reflected in the Object Inspector.
There are three major players involved with collections. A
collection item (TCollectionItem) descendant, a TCollection that
manages the list of TCollectionItems, and a component that contains the TCollection as one of it's properties. In our above example of TStatusBar, TStatusBar contains a descendant of TCollection called TPanels and TPanels manages a list of TCollectionItem descendants called TPanel. Notice that each TCollectionItem contains one or more properties; for instance, TPanels contains Alignment, Bevel, Style, Text, and Width properties. This list changes depending on the definition of your TCollectionItem descendant.
Creating a Minimal TCollection Implementation
---------------------------------------------
In a new unit you must first define three new descendant classes
from TCollectionItem, TCollection and a TComponent.
TMyCollectionItem = class(TCollectionItem)
TMyCollection = class(TCollection)
TMyComponent = class(TComponent)
To make TMyCollectionItem functional, you need to define
one or more properties to contain information to be tracked
by the collection mechanism. The example defines a Text and
a MoreStuff integer property. You will also need to override
the GetDisplayName method to supply the string shown for each
item in the collection property editor:
TMyCollectionItem = class(TCollectionItem)
private
FText: string;
FMoreStuff: LongInt;
function GetDisplayName: string; override;
procedure SetText(const Value: string);
procedure SetMoreStuff(const Value: LongInt);
published
property Text: string read FText write SetText;
property MoreStuff: LongInt
read FMoreStuff write SetMoreStuff;
end;
Next, define the TCollection descendant. This class will
keep track of the component the collection belongs to,
override the GetOwner method to accomodate streaming, and
manage an array of the previously defined TCollectionItem
descendants.
You will need to define a new static constructor. The parameter passed in this constructor is the reference to the component that contains the collection. Also in the constructor you need to populate the ItemClass property with the class of your TCollection item descendant. Note: ItemClass returns the class (descended from TCollectionItem) to which the items in the
collection belong.
TMyCollection = class(TCollection)
private
FMyComponent: TMyComponent;
function GetItem(Index: Integer): TMyCollectionItem;
procedure SetItem(Index: Integer; Value: TMyCollectionItem);
protected
function GetOwner: TPersistent; override;
public
constructor Create(MyComponent: TMyComponent);
function Add: TMyCollectionItem;
property Items[Index: Integer]: TMyCollectionItem
read GetItem write SetItem; default;
end;
Finally, define the component that will contain the collection.
The component will contain a property descended from the
TCollection type defined previously. The TCollection property
will need a private field, an access method to the private field,
and storage allocated in the constructor and freed in the
destructor.
Note: See The Developers Guide for more information on creating
custom components.
TMyComponent = class(TComponent)
private
FItems: TMyCollection;
procedure SetItems(Value: TMyCollection);
public
constructor Create(AOwner: TComponent); override;
destructor Destroy; override;
published
property Items: TMyCollection
read FItems write SetItems;
end;
Complete Unit Listing
---------------------
unit Collec1;
interface
// Note: TCollection and TCollectionItem are defined in Classes.Pas.
uses Classes;
type
TMyComponent = class;
TMyCollectionItem = class(TCollectionItem)
private
FText: string;
FMoreStuff: LongInt;
function GetDisplayName: string; override;
procedure SetText(const Value: string);
procedure SetMoreStuff(const Value: LongInt);
public
published
property Text: string read FText write SetText;
property MoreStuff: LongInt read FMoreStuff write SetMoreStuff;
end;
TMyCollection = class(TCollection)
private
FMyComponent: TMyComponent;
function GetItem(Index: Integer): TMyCollectionItem;
procedure SetItem(Index: Integer; Value: TMyCollectionItem);
protected
function GetOwner: TPersistent; override;
public
constructor Create(MyComponent: TMyComponent);
function Add: TMyCollectionItem;
property Items[Index: Integer]: TMyCollectionItem
read GetItem write SetItem; default;
end;
TMyComponent = class(TComponent)
private
FItems: TMyCollection;
procedure SetItems(Value: TMyCollection);
public
constructor Create(AOwner: TComponent); override;
destructor Destroy; override;
published
property Items: TMyCollection read FItems write SetItems;
end;
procedure Register;
implementation
procedure Register;
begin
RegisterComponents('Sample', [TMyComponent]);
end;
{ TMyCollectionItem }
// Note: Inherited default behavior of GetDisplayName is to
// return the classname.
function TMyCollectionItem.GetDisplayName: string;
begin
Result := Text;
if Result = '' then Result := inherited GetDisplayName;
end;
procedure TMyCollectionItem.SetText(const Value: string);
begin
if FText Value then
FText := Value;
end;
procedure TMyCollectionItem.SetMoreStuff(const Value: LongInt);
begin
if FMoreStuff Value then
FMoreStuff:= Value;
end;
{ TMyCollection }
constructor TMyCollection.Create(MyComponent: TMyComponent);
begin
inherited Create(TMyCollectionItem);
FMyComponent := MyComponent;
end;
function TMyCollection.Add: TMyCollectionItem;
begin
Result := TMyCollectionItem(inherited Add);
end;
function TMyCollection.GetItem(Index: Integer): TMyCollectionItem;
begin
Result := TMyCollectionItem(inherited GetItem(Index));
end;
procedure TMyCollection.SetItem(Index: Integer;
Value: TMyCollectionItem);
begin
inherited SetItem(Index, Value);
end;
// Note: You must override GetOwner in Delphi 3.x to get
// correct streaming behavior.
function TMyCollection.GetOwner: TPersistent;
begin
Result := FMyComponent;
end;
{ TMyComponent }
constructor TMyComponent.Create(AOwner: TComponent);
begin
inherited Create(AOwner);
FItems := TMyCollection.Create(Self);
end;
destructor TMyComponent.Destroy;
begin
FItems.Free;
inherited Destroy;
end;
procedure TMyComponent.SetItems(Value: TMyCollection);
begin
FItems.Assign(Value);
end;
end.
{---------------------------------------------------------------}
Notes
-----
In this minimal example we didn't override the Assign method for the TCollectionItem, but this method should have further support. Here's an example of how you might implement Assign in the above project:
procedure TMyCollectionItem.Assign(Source: TPersistent);
begin
if Source is TMyCollectionItem then
begin
Text := TMyCollectionItem(Source).Text;
MoreStuff := TMyCollectionItem(Source).MoreStuff;
Exit;
end;
inherited Assign(Source);
end;
Also not included in the above project is the logic needed to
notify the TCollection class when one of it's contained items has
changed. This could be particularly important in a visual control such as TStatusBar. TCollection supplies a virtual Update method for handling this behavior. See TStatusBar or THeaderControl in \source\vcl\commctrls.pas for further examples.