使窗体尺寸最小

 当用户改变窗体的尺寸时,这个面板总位于窗体的中心,并且不允许把窗体设得比面板还小.
unit BlueBackFrm;
interface
uses
  SysUtils, Windows, Messages, Classes, Graphics, Controls, Forms, Dialogs,
  StdCtrls, Buttons, ExtCtrls;
type
  TBlueBackForm = class(TForm)
    pnlMain: TPanel;
    bbtnOK: TBitBtn;
    bbtnCancel: TBitBtn;
    procedure FormResize(Sender: TObject);
  private
    Procedure CenterPanel;
    { Create a message handler for the WM_WINDOWPOSCHANGING message }
    procedure WMWindowPosChanging(var Msg: TWMWindowPosChanging);
       message WM_WINDOWPOSCHANGING;
  end;
var
  BlueBackForm: TBlueBackForm;
implementation
uses Math;
{$R *.DFM}
procedure TBlueBackForm.CenterPanel;
{ This procedure centers the main panel horizontally and
  vertically inside the form's client area
}
begin
  { Center horizontally }
  if pnlMain.Width < ClientWidth then
    pnlMain.Left := (ClientWidth - pnlMain.Width) div 2
  else
    pnlMain.Left := 0;
  { Center vertically }
  if pnlMain.Height < ClientHeight then
    pnlMain.Top := (ClientHeight - pnlMain.Height) div 2
  else
    pnlMain.Top := 0;
end;
procedure TBlueBackForm.WMWindowPosChanging(var Msg: TWMWindowPosChanging);
var
  CaptionHeight: integer;
begin
  { Calculate the caption height }
  CaptionHeight := GetSystemMetrics(SM_CYCAPTION);
  { This procedure does not take into account the width and
    height of the form's frame. You can use
    GetSystemMetrics() to obtain these values. }

  // Prevent window from shrinking smaller then MainPanel's width
  Msg.WindowPos^.cx := Max(Msg.WindowPos^.cx, pnlMain.Width+20);
  // Prevent window from shrinking smaller then MainPanel's width
  Msg.WindowPos^.cy := Max(Msg.WindowPos^.cy, pnlMain.Height+20+CaptionHeight);
  inherited;
end;
procedure TBlueBackForm.FormResize(Sender: TObject);
begin
  CenterPanel; // Center MainPanel when the form is resized.
end;
end.