How to dynamically create components on form?

[TurboRascal]

Ok now, let me get off the soapbox and back to the topic

Now I'm continuing towards creating a class for the dynamic objects to create. For example, to dynamically create panels with two buttons, to avoid creating every object for itself and reparenting buttons etc. I'd create a class like this:

Code: [Select]

 
type
...
TMyPanel = class (TPanel)
    ButtonA: TButton;
    ButtonB: TButton;
  end;      
Then I create a dynamic instance of it, like before:

Code: [Select]

procedure TForm1.FormCreate(Sender: TObject);
begin
  MyPanel := TMyPanel.Create(self);
  MyPanel.Parent := Self;
end;  

Then I get a panel. What needs to be done to create / initialize buttons which don't exist as objects in that moment?

I must note that I've tried doing this by using a frame, and it's actually a perfect solution. It's created visually, then it's simply added to the uses list, created a object using the frame's class, and 'reparented' it to the control where I wanted it to be. Yet I'd still like to know how could it be done manually like above... It's easy to use frames (and maybe object lists like someone mentioned somewhere), but I believe it's beneficial to understand how to use the language fully and without aids if need be.

[Zoran]

Code: [Select]

 
type
...
  TMyPanel = class (TPanel)
    ButtonA: TButton;
    ButtonB: TButton;
  end;      

Then I create a dynamic instance of it, like before:

Code: [Select]

procedure TForm1.FormCreate(Sender: TObject);
begin
  MyPanel := TMyPanel.Create(self);
  MyPanel.Parent := Self;
end;  

Then I get a panel. What needs to be done to create / initialize buttons which don't exist as objects in that moment?

The best approach in my opinion is to add constructor and destructor in your TMyPanel class:

Code: [Select]

 TMyPanel = class(TPanel)
  private
    ButtonA: TButton;
    ButtonB: TButton;
  public
    constructor Create(TheOwner: TComponent); override;
    destructor Destroy; override;
  end;

....

constructor TMyPanel.Create(TheOwner: TComponent);
begin
  inherited Create(TheOwner);

  ButtonA := TButton.Create(Self); // Note that here "Self" is MyPanel object,
  ButtonA.Parent := Self;          // NOT form!

  ButtonB := TButton.Create(Self);
  ButtonB.Parent := Self;

// and code here to position buttons in panel...

end;

destructor TMyPanel.Destroy;
begin
  ButtonB.Free;
  ButtonA.Free;

  inherited Destroy;
end;



The application is attached!

[eny]

Another approach is applying the decorator design pattern.

[jarto]

Zoran, that is a nice and clear example. However, I think that the destructor is not needed as ButtonA and ButtonB are owned by the panel. Hence, they are automatically free'ed when the panel is free'ed.

[Zoran]

Zoran, that is a nice and clear example. However, I think that the destructor is not needed as ButtonA and ButtonB are owned by the panel. Hence, they are automatically free'ed when the panel is free'ed.

Yes, you are right. However, I prefer to free myself what I created in code.

[eny]

However, I prefer to free myself what I created in code.

Normally I'd agree: but (visual) components, being what they are, save us the trouble of having to free them manually.
That means less code and less chances to make mistakes 

[garlar27]

If you need AND ONLY IF YOU NEED to destroy a component at runtime you can do something like this:

Code: [Select]

var
   AWinControl: TWinControl;
begin
  AWinControl := ThisComponent;
  ThisCommponent.Parent.RemoveComponent(AWinControl);
  FreeAndNil(AWinControl);
end;

NOTE: MORE OR LESS THIS IS THE SAFE WAY TO REMOVE A COMPONENT. BUT I MIGHT BE FORGETTING SOMETHING, SO YOU BETTER SEARCH IN HELPS AN DOCUMENTS HOW TO DO IT. It's been a quite long time since last time I did something like that and I can't remember exactly the right way to do it.

Good luck!!!

[Zoran]

If you need AND ONLY IF YOU NEED to destroy a component at runtime you can do something like this:

Code: [Select]

var
   AWinControl: TWinControl;
begin
  AWinControl := ThisComponent;
  ThisCommponent.Parent.RemoveComponent(AWinControl);
  FreeAndNil(AWinControl);
end;

NOTE: MORE OR LESS THIS IS THE SAFE WAY TO REMOVE A COMPONENT. BUT I MIGHT BE FORGETTING SOMETHING, SO YOU BETTER SEARCH IN HELPS AN DOCUMENTS HOW TO DO IT. It's been a quite long time since last time I did something like that and I can't remember exactly the right way to do it.

Good luck!!!

1. RemoveComponent has nothing to do with parent, but it removes the component from owner's ComponentList. Your code is wrong, as parent and owner can be diferrent.

2. RemoveComponent is quite pointless if you destroy the component in the next line as you do.
It can be used for example if you want to destroy the owner and not this component.

[jarto]

Yes, you are right. However, I prefer to free myself what I created in code.

In that case it's a good idea to set the owner to nil in create:

ButtonA:=TButton.Create(nil);
ButtonB:=TButton.Create(nil);

[Zoran]

Yes, you are right. However, I prefer to free myself what I created in code.

In that case it's a good idea to set the owner to nil in create:

ButtonA:=TButton.Create(nil);
ButtonB:=TButton.Create(nil);

Yes, I agree that it is a better programming practise. However, I used it in my example though, just not paying attention to the "details".

BUT, thanks to this post of yours, now I have found an article which explains that it is more than that! Read this: http://delphi.about.com/od/kbcurt/ss/dynamiccreateno.htm

So, using nil is not only better programming practise, but doing otherwise should be actually regarded as bad code, as it can have serious impact on performance.

[markusloew]

I hope there will be read my late question to this topic:

How do I detect and process a mouse click or other events on this object "buttons" ?
Or more general: How do I detect events on objects having been created dynamically?

[markusloew]

Sorry for that question: The answer is so simple !!!

buttons.onclick := @buttonsclick ;

 

[xxpyton]

hello guys,
i'm new here and started learning programming a few weeks ago   i'm trying to add combo boxes to my form by pressing a button, but it doesn't work (nothing happens, only TList.Count increases):

Code: [Select]

procedure TForm1.btnAddComboBoxClick(Sender: TObject);
var
  i: integer;
begin
  cbText:= TComboBox.Create(Self);
  cbText.Parent:= Self;
  cbText.Top:= btnAddComboBox.Top + btnAddComboBox.Height + 1;
  cbText.Left:= btnAddComboBox.Left;
  cbText.Height:= btnAddComboBox.Height;
  cbText.Width:= 256;
  for i:= 0 to slData.Count-1 do
  begin
    cbText.Items[i]:= slData.Names[i];
  end;
  cbList.Add(cbText);
  TComboBox(cbList[cbList.Count-1]).Top:= TComboBox(cbList[cbList.Count-1]).Top + TComboBox(cbList[cbList.Count-1]).Height + 1 ;
end;
 
i took a look on eny's example for this and thought by creating again another TComboBox on cbText i would get a second one, but it seems to be the same box at the same address or did i access the TList wrong?

What is really new to me is this:
TComboBox(cbList[cbList.Count-1])

What happens in this expression? i get the pointer to what? cbText or a pointer to the copy of the data stored in cbText when it was added to the cbList? and why do i use the class(?) TComboBox with brackets?

[User137]

, but it doesn't work (nothing happens, only TList.Count increases):

What do you mean? What TList, what Count?

Code: [Select]

  for i:= 0 to slData.Count-1 do
  begin
    cbText.Items[i]:= slData.Names[i];
  end;

You never created the items, yet you are assigning values to uninitialized array. I assume you meant something like:

Code: [Select]

  for i:= 0 to slData.Count-1 do
  begin
    cbText.Items.Add(slData.Names[i]);
  end;
Last line seems a little confusing written like that, you can do:

Code: [Select]

with TComboBox(cbList[cbList.Count-1]) do
  Top:=Top+Height+1;
TComboBox(cbList[cbList.Count-1]) is a pointer to TComboBox component itself. Component means all the strings, position data and others.

[Blaazen]

Code: [Select]

procedure TForm1.btnAddComboBoxClick(Sender: TObject);
var
  i: integer;
begin
  cbText:= TComboBox.Create(Self);
  cbText.Parent:= Self;
  i:=cbList.Items.Count;
  cbText.Name:='Combo'+inttostr(i);
  cbText.Top:= btnAddComboBox.Top + i*(btnAddComboBox.Height + 1);
  cbText.Left:= btnAddComboBox.Left;
  cbText.Height:= btnAddComboBox.Height;
  cbText.Width:= 256;
  cbList.AddItem(cbText.Name, cbText);
end;               
You can see on the last line, you add string together with object to combobox.
You can acces this object: TComboBox(cbText.Items.Objects)

What is really new to me is this:
TComboBox(cbList[cbList.Count-1])

It is error.
You can do

Code: [Select]

cbText.Items[i]to get strings or

Code: [Select]

cbText.Items.Objects[i]to get objects as I mentioned above.