发掘ListBox的潜力（一）：自动调整横向滚动条宽度

unit kktListBox;

{========================================================================
  DESIGN BY :  彭国辉
  DATE:        2004-12-24
  SITE:        http://kacarton.yeah.net/
  BLOG:        http://blog.csdn.net/nhconch
  EMAIL:       kacarton#sohu.com

  文章为作者原创，转载前请先与本人联系，转载请注明文章出处、保留作者信息，谢谢支持！
=========================================================================}

interface

uses
  Windows, Messages, SysUtils, Classes, Controls, StdCtrls, Commctrl;

type
  TkktListBox = class(TListBox)
  private
    MaxLenItemIndex: Integer;
    FScrollWidth: Integer;
    procedure LBAddString(var Message: TMessage); message LB_ADDSTRING;
    procedure LBInsertString(var Message: TMessage); message LB_INSERTSTRING;
    procedure LBDeleteString(var Message: TMessage); message LB_DELETESTRING;
    procedure CMFontChanged(var Message: TMessage); message CM_FONTCHANGED;
    procedure AdjuctScrollWidth(Message: TMessage);
    procedure ResetScrollWidth;

  protected

  public
    constructor Create(AOwner: TComponent); override;
    procedure CreateWnd; override;

  end;

procedure Register;

implementation

{ TkktListBox }

constructor TkktListBox.Create(AOwner: TComponent);
begin
  inherited Create(AOwner);
  MaxLenItemIndex := -1;
  FScrollWidth := 0;
end;

procedure TkktListBox.CreateWnd;
begin
  inherited CreateWnd;
  Canvas.Font := Font;
end;

procedure TkktListBox.LBAddString(var Message: TMessage);
begin
  inherited;
  if Message.Result = LB_ERR then Exit;
  AdjuctScrollWidth(Message);
end;

procedure TkktListBox.LBInsertString(var Message: TMessage);
begin
  inherited;
  if Message.Result = LB_ERR then Exit;
  if Message.WParam <= MaxLenItemIndex then MaxLenItemIndex := MaxLenItemIndex + 1;
  AdjuctScrollWidth(Message);
end;

procedure TkktListBox.LBDeleteString(var Message: TMessage);
begin
  inherited;
  if Message.Result = LB_ERR then Exit;
  if Message.WParam = MaxLenItemIndex then ResetScrollWidth;
end;

procedure TkktListBox.CMFontChanged(var Message: TMessage);
var
  sz: SIZE;
begin
  inherited;
  if MaxLenItemIndex = -1 then Exit;
  //这里没有使用TextWidth，而是用GetTextExtentPoint32函数，如果你有兴趣的话
  //可以跟踪一个TextWidth函数，它最终是调用GetTextExtentPoint32实现的
  GetTextExtentPoint32(Canvas.Handle, PChar(Items[MaxLenItemIndex]), Length(Items[MaxLenItemIndex]), sz);
  FScrollWidth := sz.cx + 4;
  Perform(LB_SETHORIZONTALEXTENT, FScrollWidth, 0);
end;

procedure TkktListBox.AdjuctScrollWidth(Message: TMessage);
var
  sz: SIZE;
begin
  GetTextExtentPoint32(Canvas.Handle, PChar(Message.LParam), StrLen(PChar(Message.LParam)), sz);
  if sz.cx + 4 > FScrollWidth then begin
    FScrollWidth := sz.cx + 4;
    Perform(LB_SETHORIZONTALEXTENT, FScrollWidth, 0);
    MaxLenItemIndex := Message.Result;
  end;
end;

procedure TkktListBox.ResetScrollWidth;
var
  i, MaxWidth: Integer;
  sz: SIZE;
begin
  MaxWidth := 0;
  i := Items.Count - 1;
  MaxLenItemIndex := -1;
  while i>=0 do begin
    sz.cx := 0;
    GetTextExtentPoint32(Canvas.Handle, PChar(Items[i]), Length(Items[i]), sz);
    if sz.cx + 4 > MaxWidth then begin
      MaxWidth := sz.cx + 4;
      MaxLenItemIndex := i;
    end;
    Dec(i);
  end;
  FScrollWidth := MaxWidth;
  Perform(LB_SETHORIZONTALEXTENT, FScrollWidth, 0);
end;

procedure Register;
begin
  RegisterComponents(‘Kacarton‘, [TkktListBox]);
end;

end.