CODEWITHSUNDEEP
formsdelphinullmodal-dialog
Alex Niman
Alex Niman

Reputation: 39

Modal form is nil after creation

I'm trying to create an instance of a form and pass a parameter, that has to be specific to that instance.

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 is a var for one of sucn instances. When i'm opening than instance, i'm doing it like this:

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, if later i'm calling some procedure for than form, i get an AV error. When watching CharacterScreen variable in step-by-step debug, it's described as nil.

So my question is: what is the correct way to create an instance of a form (so that later i can create a second instance and so on)? Also, is it a correct way to store an instance of variable ViewedData, that has to be unique to each instance of a form?

Upvotes: 0

Views: 274

Answers (1)

whosrdaddy
whosrdaddy

Reputation: 11860

The problem is that you call Free from the constructor, so the instance will become inaccessible when you close the form.

You can add a static method to the form and call that from other forms.

it is best practice to remove the global form variable so that the calling code must declare a local variable (will become apparent when you create multiple instances of the same form). In this example, I made the constructor private, so that only the execute method can be called outside this unit.

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;

now you call your form like this:

var CharacterScreen : TDataForm;

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

** UPDATE **

Now If I were you I would do something like this (pseudo code, I assume you'll get the idea) :

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;

now the calling code can look like this

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;

Upvotes: 4

Related Questions