使用ownerdraw样式时，Delphi Seattle TCombobox Dropdowncount不起作用

问题描述

我在Delphi Seattle中有一个所有者绘制的 TComboBox ，它忽略了 DropDownCount 属性的任何设置.单个项目的高度为59像素，宽度为311像素.我在 Items 字符串列表中有5个条目.将值设置为低于8将导致根本不显示任何下拉菜单，而将其设置为更高的值将导致下拉菜单中仅显示一项以及垂直滚动条.我需要能够在下拉列表的项目列表中显示所有5个条目.

I have an owner-drawn TComboBox in Delphi Seattle which ignores any settings to the DropDownCount property. The height of the individual items is 59 pixels and the width is 311 pixels. I have 5 entries in the Items stringlist. Setting a value below 8 results in no dropdown being shown at all, while any higher values result in only 1 item being shown in the dropdown along with a vertical scrollbar. I need to be able to show all 5 entries in the items list in the dropdown.

这里是DFM重现该问题:

Here's the DFM to reproduce the problem:

object Form1: TForm1
  Left = 0
  Top = 0
  Caption = 'Form1'
  ClientHeight = 223
  ClientWidth = 527
  Color = clBtnFace
  Font.Charset = DEFAULT_CHARSET
  Font.Color = clWindowText
  Font.Height = -11
  Font.Name = 'Tahoma'
  Font.Style = []
  OldCreateOrder = False
  PixelsPerInch = 96
  TextHeight = 13
  object ComboBox1: TComboBox
    Left = 96
    Top = 16
    Width = 311
    Height = 22
    Style = csOwnerDrawVariable
    ItemIndex = 0
    TabOrder = 0
    Text = 'ITEM ONE'
    OnDrawItem = ComboBox1DrawItem
    OnMeasureItem = ComboBox1MeasureItem
    Items.Strings = (
      'ITEM ONE'
      'ITEM TWO'
      'ITEM THREE'
      'ITEM FOUR'
      'ITEM FIVE')
  end
end

这是PAS文件:

unit CBBoxBugUnit;

interface

uses
  Winapi.Windows, Winapi.Messages, System.SysUtils, System.Variants, System.Classes, Vcl.Graphics,
  Vcl.Controls, Vcl.Forms, Vcl.Dialogs, Vcl.StdCtrls;

type
  TForm1 = class(TForm)
    ComboBox1: TComboBox;
    procedure ComboBox1MeasureItem(Control: TWinControl; Index: Integer; var Height: Integer);
    procedure ComboBox1DrawItem(Control: TWinControl; Index: Integer; Rect: TRect; State: TOwnerDrawState);
  private
    { Private declarations }
  public
    { Public declarations }
  end;

var
  Form1: TForm1;

implementation

{$R *.dfm}

procedure TForm1.ComboBox1DrawItem(Control: TWinControl; Index: Integer; Rect: TRect; State: TOwnerDrawState);
var Working_String : String;
    Text_Width ,
    Text_Height ,
    Output_X ,
    Output_Y       : Integer;
begin
  ComboBox1.Canvas.Pen.Color := clWhite;
  ComboBox1.Canvas.Brush.Color := clGreen;
  ComboBox1.Canvas.Brush.Style := bsSolid;
  ComboBox1.Canvas.Pen.Style := psSolid;
  ComboBox1.Canvas.Pen.Width := 1;
  ComboBox1.Canvas.Font.Color := clWhite;
  ComboBox1.Canvas.Font.Name := 'ARIAL BLACK';
  ComboBox1.Canvas.Font.Size := 14;
  ComboBox1.Canvas.FillRect( Rect );
  Working_String := self.ComboBox1.Items.Strings[ Index ];
  Text_Width := self.ComboBox1.Canvas.TextWidth( Working_String );
  Text_Height := self.ComboBox1.Canvas.TextHeight( Working_String );
  Output_X := ( Rect.Width - Text_Width ) div 2;
  Output_Y := ( Rect.Height - Text_Height ) div 2;
  ComboBox1.Canvas.TextOut( Output_X , Output_Y , Working_String );
end;

procedure TForm1.ComboBox1MeasureItem(Control: TWinControl; Index: Integer; var Height: Integer);
begin
  Height := 59;
end;

end.

推荐答案

即使 TComboBox 的 ItemHeight 的nofollow noreferrer>文档状态:

Even though the documentation for ItemHeight of TComboBox states:

VCL的内部计算取决于 FItemHeight .将 ItemHeight 设置为59，将接受 DropDownCount (已经接受，但默认值为16).如果您在设计时设置该值，则该组合还会将其大小调整为运行时的大小.

VCL has internal calculations that depend on FItemHeight. Set ItemHeight to 59 and DropDownCount will be honored (it is already honored but for the default 16). If you set the value in design time, the combo will also adjust its size to that of at runtime.

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