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How to trap changes of the clipboard content 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
28-Aug-02
Category
Win API
Language
Delphi 2.x
Views
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Publisher:
DSP, Administrator
Reference URL:
DKB
			Author: Tomas Rutkauskas 

Is there a way to use an OnChange event for the clipboard? I want to avoid to check 
for a change of the clipboard content every millisecond.

Answer:

An application can register itself in the clipboard viewer chain. The first window 
in this chain always receives the messages. Every window is responsible to pass the 
messages on to the next one.

1   unit Unit1;
2   
3   interface
4   
5   uses
6     Windows, Messages, Forms, Classes, Controls, StdCtrls;
7   
8   type
9     TForm1 = class(TForm)
10      Button1: TButton;
11      Button2: TButton;
12      procedure Button1Click(Sender: TObject);
13      procedure Button2Click(Sender: TObject);
14      procedure FormCreate(Sender: TObject);
15      procedure FormDestroy(Sender: TObject);
16    private
17      FNextClipboardViewer: HWND;
18      procedure WMChangeCBChain(var Msg: TWMChangeCBChain); message WM_CHANGECBCHAIN;
19      procedure WMDrawClipboard(var Msg: TWMDrawClipboard); message WM_DRAWCLIPBOARD;
20    end;
21  
22  var
23    Form1: TForm1;
24  
25  implementation
26  
27  {$R *.DFM}
28  
29  procedure TForm1.FormCreate(Sender: TObject);
30  begin
31    { Initialize variable }
32    FNextClipboardViewer := 0;
33  end;
34  
35  procedure TForm1.Button1Click(Sender: TObject);
36  begin
37    if FNextClipboardViewer <> 0 then
38      MessageBox(0, 'This window is already registered!', nil, 0)
39    else
40      { Add to clipboard chain }
41      FNextClipboardViewer := SetClipboardViewer(Handle);
42  end;
43  
44  procedure TForm1.Button2Click(Sender: TObject);
45  begin
46    { Remove from clipboard chain }
47    ChangeClipboardChain(Handle, FNextClipboardViewer);
48    FNextClipboardViewer := 0;
49  end;
50  
51  procedure TForm1.WMChangeCBChain(var Msg: TWMChangeCBChain);
52  begin
53    inherited;
54    { mark message as done }
55    Msg.Result := 0;
56    { the chain has changed }
57    if Msg.Remove = FNextClipboardViewer then
58      { The next window in the clipboard viewer chain had been removed. We recreate 
59  it. }
60      FNextClipboardViewer := Msg.Next
61    else
62      { Inform the next window in the clipboard viewer chain }
63      SendMessage(FNextClipboardViewer, WM_CHANGECBCHAIN, Msg.Remove, Msg.Next);
64  end;
65  
66  procedure TForm1.WMDrawClipboard(var Msg: TWMDrawClipboard);
67  begin
68    inherited;
69    { Clipboard content has changed }
70    try
71      MessageBox(0, 'Clipboard content has changed!', 'Clipboard Viewer', 
72  MB_ICONINFORMATION);
73    finally
74      { Inform the next window in the clipboard viewer chain }
75      SendMessage(FNextClipboardViewer, WM_DRAWCLIPBOARD, 0, 0);
76    end;
77  end;
78  
79  procedure TForm1.FormDestroy(Sender: TObject);
80  begin
81    if FNextClipboardViewer <> 0 then
82    begin
83      { Remove from clipboard chain }
84      ChangeClipboardChain(Handle, FNextClipboardViewer);
85      FNextClipboardViewer := 0;
86    end;
87  end;
88  
89  end.


			
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