Author: Alec Bergamini
What are the new virtual styles in the TListbox for, and whay are they important?
Answer:
One of the new features in Delphi 6 is the addition of styles lbVirtual and
lbVirtualOwnerDraw to the standard TListBox. In all the Delphi 6 “What’s new”
articles I’ve read this addition has received no more than a passing mention.
Why are Virtual List boxes important?
Have you ever been frustrated by the limitations of the TStrings' objects property
or have had a need to create a custom display string in the list. Sure, you can
assign any pointer you want to the object field of a string list but what if you
already have a list container (like a Tlist) full of data. Why should you be forced
to duplicate this data over to the TStrings associated with the TListBox. You
should be able to just use it. Well, with a virtual list box you can.
During the past week I ran into a situation where I had a TInterfaceList and wanted
to use the value returned from one of the Interface's functions as the text of the
list item. Under Delphi 5 this required that I step down through the TInterfaceList
calling the required interface method at each step to add the strings to the
TListBox. Then I would need to synchronize any movement and selection in the list
box with the TInterfaceList. Under Delphi 6, using a virtual list box, I was able
merge my TInterfaceList and the TListbox into one highly usable pseudo object. That
is, I was able to make the TListBox “aware” of the TInterfaceList directly.
(BTW, my original thought was to not use the TInterfaceList at all and just place
the references to the Interface into the object field of the TStrings member of the
list box.. As you all probably know, this was a bad idea since it totally screws
the reference counting on the interface.)
Anyway I will not go into great detail about setting up a virtual list box since
the Delphi help is pretty good on this. Basically you need to set the style to one
of the 2 virtual styles, set the list box’s count to the number of items in the
list and then fill in at few as one and as many as 3 events.
Here is a trivial sample that may help to fill in some blanks left by the lack of
sample code in the help file.
I only wish that Borland could have also added this feature the other list controls
like the comboboxes and the treeview.
1 unit Unit1;
2
3 interface
4
5 uses
6 Windows, Messages, SysUtils, Variants, Classes, Graphics, Controls, Forms,
7 Dialogs, StdCtrls;
8
9 type
10 TForm1 = class(TForm)
11 ListBox1: TListBox;
12 Button1: TButton;
13 Edit1: TEdit;
14 lblColor: TLabel;
15 lblNumber: TLabel;
16 procedure FormCreate(Sender: TObject);
17 procedure FormDestroy(Sender: TObject);
18 procedure ListBox1Data(Control: TWinControl; Index: Integer;
19 var Data: string);
20 function ListBox1DataFind(Control: TWinControl;
21 FindString: string): Integer;
22 procedure ListBox1DataObject(Control: TWinControl; Index: Integer;
23 var DataObject: TObject);
24 procedure Button1Click(Sender: TObject);
25 procedure ListBox1Click(Sender: TObject);
26 private
27 { Private declarations }
28 public
29 { Public declarations }
30 ObjList: TList;
31 end;
32
33 TMyObj = class
34 fColor: string;
35 fNumber: Integer;
36 constructor create(const color: string; const Number: Integer);
37 end;
38
39 var
40 Form1: TForm1;
41
42 implementation
43
44 {$R *.dfm}
45
46 { TMyObj }
47
48 constructor TMyObj.create(const color: string; const Number: Integer);
49 begin
50 fColor := color;
51 fNumber := number;
52 end;
53
54 { TForm1 }
55
56 procedure TForm1.FormCreate(Sender: TObject);
57 begin
58 // create a TList and add some data to it.
59 ObjList := TList.Create;
60 ObjList.Add(TMyObj.Create('Red', 1));
61 ObjList.Add(TMyObj.Create('Yellow', 15));
62 ObjList.Add(TMyObj.Create('Blue', 21));
63 ObjList.Add(TMyObj.Create('Green', 37));
64 ObjList.Add(TMyObj.Create('Brown', 16));
65 ObjList.Add(TMyObj.Create('Black', 5135));
66 ObjList.Add(TMyObj.Create('White', 4));
67 ObjList.Add(TMyObj.Create('Orange', 333));
68 // ABSOLUTELY REQUIRED Set the count of the virtual listbox
69 Listbox1.Count := ObjList.Count;
70 end;
71
72 procedure TForm1.FormDestroy(Sender: TObject);
73 var
74 I: Integer;
75 begin
76 for I := 0 to ObjList.count - 1 do
77 TMyObj(ObjList.Items[I]).free;
78 ObjList.Free;
79 end;
80
81 procedure TForm1.ListBox1Data(Control: TWinControl; Index: Integer;
82 var Data: string);
83 // REQUIRED
84 // return a string to put in the list box
85 begin
86 Data := TMyObj(ObjList.Items[Index]).fColor;
87 end;
88
89 procedure TForm1.ListBox1DataObject(Control: TWinControl; Index: Integer;
90 var DataObject: TObject);
91 // USUALLY REQUIRED
92 // return an object associated with the current selection of the list box
93 begin
94 DataObject := ObjList.Items[Index];
95 end;
96
97 function TForm1.ListBox1DataFind(Control: TWinControl;
98 FindString: string): Integer;
99 // USUALLY REQUIRED
100 // given a string FindString, return its index
101 var
102 I: Integer;
103 begin
104 // the simplest but most brain dead approach
105 result := -1;
106 for I := 0 to TListBox(Control).Count - 1 do
107 if TListBox(Control).Items[I] = FindString then
108 result := I;
109 end;
110
111 procedure TForm1.Button1Click(Sender: TObject);
112 begin
113 ListBox1.ItemIndex := ListBox1.Items.IndexOf(edit1.Text);
114 // I don't think this next should be necessary but..
115 ListBox1Click(Self);
116 end;
117
118 procedure TForm1.ListBox1Click(Sender: TObject);
119 begin
120 lblColor.Caption := TMyObj(ListBox1.Items.objects[Listbox1.ItemIndex]).fColor;
121 lblNumber.Caption :=
122 IntToStr(TMyObj(ListBox1.Items.objects[Listbox1.ItemIndex]).fNumber);
123 end;
124
125 end.
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