Question

我正在尝试创建一个表单实例并传递一个必须特定于该实例的参数。

type
  TDataForm = class(TForm)
    { Some components}
    { Some procedures }
    constructor Create(AOwner: TComponent; Data: PData; Page: String); reintroduce;
  private
    { Private declarations }
  public
    { Public declarations }
    ViewedData: PData;
  end;

var CharacterScreen: TDataForm;

implementation

CharacterScreen是sucn实例之一的变量。当我打开实例时，我就是这样：

constructor TDataForm.Create(AOwner: TComponent; Data: PData; Page: String);
begin
inherited Create(nil);
ViewedData:=Data;
if Page='Stats'      then CharPageControl.TabIndex:=0;
if Page='Equipment'  then CharPageControl.TabIndex:=1;
if Page='Effects'    then CharPageControl.TabIndex:=2;
if Page='Statistics' then CharPageControl.TabIndex:=3;
ShowModal;
Free;
end;

CharacterScreen:=TDataForm.Create(nil,@Data,'Page 1');

Howewer，如果稍后我要为窗体调用某些程序，则会收到AV错误。在逐步调试中观看CharacterScreen变量时，它被描述为nil。

所以我的问题是：创建表单实例的正确方法是什么（以便以后我可以创建第二个实例，依此类推）？另外，这是存储变量ViewedData实例的正确方法，该实例对于表单的每个实例必须是唯一的吗？

Answer 1

问题在于您从构造函数中调用Free，因此当您关闭表单时，实例将变得不可访问。

您可以在表单中添加静态方法，然后从其他表单中调用它。

最佳做法是删除全局表单变量，以便调用代码必须声明一个局部变量（当您创建同一表单的多个实例时，该变量将变得明显）。在此示例中，我将构造函数设为私有，因此只能在该单元外部调用execute方法。

type
  TDataForm = class(TForm)
    { Some components}
    { Some procedures }
  private
    { Private declarations }
    ViewedData: PData;
    constructor Create(AOwner : TComponent);
  public
    { Public declarations }
    class procedure Execute(Data: PData; APage: String);
  end;

implementation

class function TDataForm.Execute(Data: PData; APage: String) :TDataForm;

begin
 Result := TDataForm.Create(nil);
 Result.ViewedData := PData;
 if APage='Характеристики' then Result.CharPageControl.TabIndex:=0 else
 if APage='Экипировка'     then Result.CharPageControl.TabIndex:=1 else
 if APage='Эффекты'        then Result.CharPageControl.TabIndex:=2 else
 if APage='Статистика'     then Result.CharPageControl.TabIndex:=3;
 Result.ShowModal;
end;

constructor TDataForm.Create(AOwner: TComponent);
begin
 inherited Create(nil);
end;

现在您可以这样调用表单：

var CharacterScreen : TDataForm;

CharacterScreen := TDataForm.Execute(@Data,'Page 1');
try
 // do something with CharacterScreen when it has been closed
finally
 CharacterScreen.Free;   
end;

** 更新 **

现在，如果我是你，我会做类似的事情（伪代码，我想你会明白的）：

Unit character;

type
  TCharacterData = class
  public
   // some public fields
  end;

...

Unit EditCharacter;

type 
  TDataForm = class(TForm)
    procedure OnShow(Sender : TObject);
  public
    class procedure Execute(Data : TCharacterData) : Boolean;  
  end;

implementation;

class function TDataForm.Execute(var Character: TCharacter) : Boolean;

var Frm: TDataForm ;

begin
 Result := False; 
 Frm:= TDataForm.Create(nil);
 try
  Frm.Character := Character;
  Frm.ShowModal;
  Result := Frm.ModalResult = mrOK;
 finally
  Frm.Free;
 end;
end;

procedure TDataForm.OnShow(Sender : TObject);
 begin
  if Character = TWizardCharacter then Frm.CharPageControl.TabIndex:=0 else
  if Character = TBarbarianCharacter then Frm.CharPageControl.TabIndex:=1
   ... //etc
 end;

现在调用代码可以看起来像这样

var Character : TCharacter;
...

Character := TWizardCharacter.Create;

...
// user wants to edit character

procedure EditCharacter;
begin
 if TDataForm.Execute(Character)  then
  begin
   // user has modified character, act accordingly
  end;
end;

创建后模态形式为空

1 个答案: