Delphi 2009:组件对象属性默认值

Question

如何设置组件对象属性值的默认值？

组件类代码:

unit CustomClass;

interface

uses
    Classes;

type
  TCustomClass = class;

  TConfiguration = class;

  TCustomClass = class (TComponent)
  private
    FConfiguration: TConfiguration;
    procedure SetConfiguration(const Value: TConfiguration);
  published
    property Configuration: TConfiguration read FConfiguration write SetConfiguration;
  public
    constructor Create(AOwner: TComponent); override;
    destructor Destroy; override;
  end;

  TConfiguration = class (TPersistent)
  private
    FDelay: Integer;
    FSize: Integer;
    procedure SetDelay(const Value: Integer);
    procedure SetSize(const Value: Integer);
  published
    property Delay: Integer read FDelay write SetDelay;
    property Size: Integer read FSize write SetSize;
  public
    procedure Assign(Source: TPersistent); override;
  end;

implementation

{ TCustomClass }

constructor TCustomClass.Create(AOwner: TComponent);
begin
  inherited;
  Configuration.FileName:= 'FileName';
  Configuration.Size := 10;
end;

destructor TCustomClass.Destroy;
begin
  Configuration.Free;
  inherited;
end;

procedure TCustomClass.SetConfiguration(const Value: TConfiguration);
begin
  FConfiguration.Assign(Value);
end;

{ TConfiguration }

procedure TConfiguration.Assign(Source: TPersistent);
begin
  inherited;
  Delay := (Source as TConfiguration).Delay;
  Size := (Source as TConfiguration).Size;
end;

procedure TConfiguration.SetDelay(const Value: Integer);
begin
  FDelay := Value;
end;

procedure TConfiguration.SetSize(const Value: Integer);
begin
  FSize := Value;
end;

end.

在我的IDE中,它将显示为对象属性被修改(粗体蓝色).

我想在TConfiguration类属性上创建默认和存储函数,如下所示:

TConfiguration

interface

private
  function FileNameStored: Boolean;
published
  property FileName: string read FFileName write SetFileName stored FileNameStored;
  property Size: Integer read FSize write SetSize default 10;

implementation

function TConfiguration.FileNameStored: Boolean;
begin
  Result := FileName <> 'FileName';
end;

它以正常蓝色为TConfiguration的属性着色,但不是TCustomClass的Configuration属性,而不是我想要设置其默认值,在TCustomClass上,因为TConfiguration可能在另一个组件中使用.

然后,我还想:

TCustomClass

interface

private
  function ConfigurationStored: Boolean;
published
  property Configuration: TConfiguration read FConfiguration write SetConfiguration stored ConfigurationStored;

implementation

function TCustomClass.ConfigurationStored: Boolean;
begin
  Result := Configuration.FileName <> 'FileName' and
    Configuration.Size <> 10;
end;

但是,这只将TCustomClass配置属性颜色设置为普通蓝色,而不是其属性.

回答

正如@RemyLebeau指出的那样(在第一个和最顶层的答案中),代码中存在错误.在该组件和那种情况下,我决定不为属性设置任何默认值.

Answer 1

您的代码中有几个错误.

1. 你的TCustomClass构造函数没有创建TConfiguration对象.你需要添加:

   constructor TCustomClass.Create(AOwner: TComponent);
   begin
     inherited;
     FConfiguration := TConfiguration.Create; // <-- add this
     FConfiguration.FileName := 'FileName';
     FConfiguration.Size := 10;
   end;
   

   话虽这么说,FileName和Size属性的赋值应该可以移动到TConfiguration构造函数而不是TCustomClass构造函数.

2. 一个String属性不能用一个用户定义的默认值来定义,默认的始终是一个空字符串.因此,在FileName创建组件时,您的属性将始终显示为已修改,因为构造函数正在为其分配非默认值.你的stored方法是解决这个问题的正确方法.或者,只是不指定默认值FileName at all,将其留空.如果用户未指定显式FileName,则代码可以根据需要采用默认值.

   的Integer属性,在另一方面,可以与用户定义的默认值来定义.但是,你的Size财产不是这样做的.它应该是(特别是如果您将默认值的赋值移动到TConfiguration构造函数中):

   property Size: Integer read FSize write SetSize default 10;
   

3. 您的TConfiguration.Assign()方法实现不正确.通过Assign()在复制值之前调用继承的方法,EConvertError如果调用者将一个TConfiguration对象分配给另一个对象,则代码将始终在运行时引发异常.原因是因为基类TPersistent.Assign()实现只是调用Source.AssignTo(Self),并且TConfiguration不会覆盖该AssignTo()方法,因此TPersistent.AssignTo()调用它,它只是调用Dest.AssignError(Self),从而引发异常.所以,Assign()如果Source它实际上是一个TConfiguration对象,请不要调用继承的方法:

   procedure TConfiguration.Assign(Source: TPersistent);
   begin
     if Source is TConfiguration then
     begin
       FDelay := TConfiguration(Source).Delay;
       FSize := TConfiguration(Source).Size;
     end else
       inherited;
   end;