如何取消FMX ListBox选中项默认0.7的背景不透明度限制？

核心问题原因 #

FMX框架中TCustomListBox默认定义了TCustomListBox_UnfocusedSelectionOpacity和TCustomListBox_ExtendedSelectionOpacity两个常量，值均为0.7，导致无论自定义选中颜色为何，都会叠加该透明度，无法显示纯色效果。

可行解决方案：重写ListBox的选中绘制逻辑 #

通过继承TListBox（或TCustomListBox），重写DrawSelection方法，直接指定选中背景的不透明度为1.0（完全不透明），避开框架默认的常量限制。

Delphi代码示例

type
  TMyListBox = class(TListBox)
  protected
    procedure DrawSelection(const ARect: TRectF; const AState: TCustomListBox.TItemState); override;
  end;

procedure TMyListBox.DrawSelection(const ARect: TRectF; const AState: TCustomListBox.TItemState);
var
  LOpacity: Single;
begin
  case AState of
    TCustomListBox.TItemState.isSelected,
    TCustomListBox.TItemState.isSelectedUnfocused,
    TCustomListBox.TItemState.isSelectedExtended:
      LOpacity := 1.0; // 设置为完全不透明，按需调整
  else
    LOpacity := 0;
  end;
  Canvas.Fill.Color := SelectedColor;
  Canvas.Fill.Kind := TBrushKind.Solid;
  Canvas.Fill.Opacity := LOpacity;
  Canvas.FillRect(ARect, 0, 0, [], 1);
end;

C++Builder代码示例

class TMyListBox : public TListBox
{
protected:
    virtual void __fastcall DrawSelection(const TRectF &ARect, const TCustomListBox::TItemState AState);
};

void __fastcall TMyListBox::DrawSelection(const TRectF &ARect, const TCustomListBox::TItemState AState)
{
    float LOpacity = 0.0f;
    switch(AState)
    {
        case TCustomListBox::TItemState::isSelected:
        case TCustomListBox::TItemState::isSelectedUnfocused:
        case TCustomListBox::TItemState::isSelectedExtended:
            LOpacity = 1.0f; // 完全不透明，可根据需求修改
            break;
        default:
            LOpacity = 0.0f;
            break;
    }
    Canvas->Fill->Color = SelectedColor;
    Canvas->Fill->Kind = TBrushKind::Solid;
    Canvas->Fill->Opacity = LOpacity;
    Canvas->FillRect(ARect, 0, 0, nullptr, 1);
}

适配ComboBox的场景 #

若需解决ComboBox下拉列表的同样问题，可继承TComboBox，重写CreateListBox方法，替换内部列表为自定义的TMyListBox：

Delphi代码示例

type
  TMyComboBox = class(TComboBox)
  protected
    function CreateListBox: TCustomListBox; override;
  end;

function TMyComboBox.CreateListBox: TCustomListBox;
begin
  Result := TMyListBox.Create(Self);
  Result.Parent := Self;
end;

C++Builder代码示例

class TMyComboBox : public TComboBox
{
protected:
    virtual TCustomListBox* __fastcall CreateListBox(void);
};

TCustomListBox* __fastcall TMyComboBox::CreateListBox(void)
{
    TMyListBox* ListBox = new TMyListBox(this);
    ListBox->Parent = this;
    return ListBox;
}

方案优势 #

• 完全保留Selected属性的正常功能，不会破坏原有选中状态逻辑
• 直接从绘制层解决问题，无需通过取消选中状态等hack方式实现效果
• 可灵活调整不透明度值，满足不同场景需求

内容的提问来源于stack exchange，提问作者Bruce