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Creating component arrays Turn on/off line numbers in source code. Switch to Orginial background IDE or DSP color Comment or reply to this aritlce/tip for discussion. Bookmark this article to my favorite article(s). Print this article
Delphi 2.x
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			Author: Lou Adler

In Visual Basic I can create component arrays, but I can't seem to find a way to in 
Delphi. Does Delphi have this capability?


Do You Really Need a Component Array?

Visual Basic has no way to share events among components other than through 
component arrays, which is why it's so easy to program in Visual Basic. But with 
Delphi, groups of components can share a common event handler merely by assigning 
the event handler code for the event in the Object Inspector. Unlike VB, which 
requires the components to be of the same type when assigning a shared event 
handler, dissimilar components in Delphi can share the same event handler.

For instance, let's say you want a TSpeedButton to behave exactly like a TMenuItem. 
If you already have code for the TMenuItem, all you have to do is select the button 
and go to its events page. Under its OnClick event, drop down the list of available 
event handlers and select the event handler for the OnClick Event of the TMenuItem.

What I discussed above is usually the reason for wanting to build a component 
array. However, there are certain circumstances under which you would want a 
component array. One of those circumstances is handling processing for a group of 
like objects in a loop. For example, let's say you have a bunch of TLabels on a 
form whose appearance and text you want to update at runtime in response to a mouse 
click. Rather than creating one big switch statement or several if..then statements 
to change the text and appearance of the TLabels, it's much easier to process the 
changes in a loop.

However, in spite of that, a component or object array could be quite useful; 
especially if you're performing the same thing on a group of the same types of 
components sequentially. A good example of this is how instantiate a series of 
threads of the same type all at once in my article Waiting for Threads. Instead of 
making a separate variable declaration for each thread that I need to create, I 
instead create an array of the threads, and instantiate them with a FOR loop. This 
is not only much more convenient, the coding it saves makes this approach much more 
efficient with respect to productivity. So there's a good argument why you might 
want to create an array of components or objects. However, keep in mind what I said 
above. If you're using an array merely to apply a group-based event handling 
mechanism, it's probably better to stick with my example below.

Creating an Array of TLabels

The first thing that needs to be done is to make declaration of the array of 
TLabels. For this example, I've dropped eight TLabel components on a form. I want 
the array index to match the TLabel indexes, so I declare the array as follows 
under the private section of my form:

arLbl: array[1..8] of TLabel;

Next, because I know I'll be needing the label array almost immediately, I'll pop 
some code into the form's OnCreate event to initialize the values of the label 
array elements:

1   procedure TForm1.FormCreate(Sender: TObject);
2   var
3     I: Integer;
4   begin
5     for I := 1 to 8 do
6       arLbl[I] := TLabel(FindComponent('Label' + IntToStr(I)));
7   end;

Notice that I performed a typecast around the FindComponent function. FindComponent 
returns the component specified by the name input into its formal parameter. 
However, without the typecast, FindComponent will return a type of TComponent, and 
a type mismatch compiler error will occur, despite the fact that it returns the 
TLabel we're looking for. So by forcing the typecast of TLabel, we ensure that the 
type assigned to the label array element is correct.

Then, because the TLabels will share the same event handler, all we need to do is 
set up an event handler for the OnClick event of one of the TLabels, then set up 
the other labels' OnClick event handlers to point to the OnClick event of the label 
we wrote the code under:

8   procedure TForm1.Label1Click(Sender: TObject);
9   var
10    I: Integer;
11  begin
13    for I := 1 to 8 do
14    begin
15      arLbl[I].Caption := TLabel(Sender).Name;
16      arLbl[I].Font.Style := [];
17      arLbl[I].Font.Color := clWindowText;
18      arLbl[I].Color := clBtnFace;
19    end;
21    with TLabel(Sender) do
22    begin
23      Font.Style := [fsBold];
24      Font.Color := clWhite;
25      Color := clBlue;
26    end;
28  end;
30  //In the OnClick event handler above, the program goes through the entire array and 
31  sets some basic properties. These are essentially the default properties of the 
32  labels when they're dropped onto the form. Then, the procedure takes the Sender, 
33  the label that fired off the event, and changes its appearance to white, bold text 
34  with a blue background. This is much simpler than doing a big case or if..then. 
35  Here' entire unit code:
37  unit lblform;
39  interface
41  uses
42    Windows, Messages, SysUtils, Classes, Graphics, Controls, Forms, Dialogs,
43    Menus, StdCtrls;
45  type
46    TForm1 = class(TForm)
47      Label1: TLabel;
48      Label2: TLabel;
49      Label3: TLabel;
50      Label4: TLabel;
51      Label5: TLabel;
52      Label6: TLabel;
53      Label7: TLabel;
54      Label8: TLabel;
55      procedure Label1Click(Sender: TObject);
56      procedure FormCreate(Sender: TObject);
57    private
58      { Private declarations }
59      arLbl: array[1..8] of TLabel;
60    public
61      { Public declarations }
62    end;
64  var
65    Form1: TForm1;
67  implementation
69  {$R *.DFM}
71  procedure TForm1.Label1Click(Sender: TObject);
72  var
73    I: Integer;
74  begin
76    for I := 1 to 8 do
77    begin
78      arLbl[I].Caption := TLabel(Sender).Name;
79      arLbl[I].Font.Style := [];
80      arLbl[I].Font.Color := clWindowText;
81      arLbl[I].Color := clBtnFace;
82    end;
84    with TLabel(Sender) do
85    begin
86      Font.Style := [fsBold];
87      Font.Color := clWhite;
88      Color := clBlue;
89    end;
91  end;
93  procedure TForm1.FormCreate(Sender: TObject);
94  var
95    I: Integer;
96  begin
97    for I := 1 to 8 do
98      arLbl[I] := TLabel(FindComponent('Label' + IntToStr(I)));
99  end;
101 end.

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