Set Parent of Component to Application.MainForm from Unit

[egnew 3]

I populate forms dynamically from units.  Attempts to create a TForm component on the Application.MainForm but has failed.  My current workaround is to drop a "Host" panel on the MainForm and use it as the parent.

procedure MakeSurface;

MyComponent[0] := TPanel.Create;

TPanel(MyComponent[0]).Parent := Application.MainForm;

MyParent := MyComponent[0];

 

procedure MakeTopPanel

MyComponent[1] := TPanfel.Create;

TPanel(MyComponent[1]).Align := alTop;

TPanel(MyComponent[1].Parent := MyParent;

TPanel(MyComponent[1].Name := 'TopPanel';

 

Application.MainForm.FormCreate

inherited;

MakeSurface;

MakeTopPanel;

This fails with the message "Control 'TopPanel' has no parent window.  Path: TopPanel."

 

Application.MainForm.FormCreate

inherited;

MakeSurface;

MyComponent[0].Parent := Self;

MakeTopPanel;

This works without a problem.

 

 

 

[Lajos Juhász 293]

A simple debug would tell you the reason. At the moment of Application.MainForm.Formcreate the Application.MainForm property is nil. 

[Pat Foley 51]

Delete   

 

 

[egnew 3]

Lajos Juhász - Thanks.

 

I called inherited in the OnCreate event and never thought Delphi would set the MainForm property later.   I moved the code to OnShow, and it works fine now.

 

Thanks

 

Edited March 24, 2023 by egnew

[Stano 143]

2 minutes ago, egnew said:

I called inherited in the OnCreate event and never thought Delphi would set the MainForm property later.   I moved the code to OnShow, and it works fine now.

I wouldn't do that. You or user can call an OnShow event repeatedly. It will create objects each time.

[Lajos Juhász 293]

TApplication.CreateForm in the first place must create the instance (when the formcreate is called) and just after can assign it to mainform.

 

You can do

 

TMainForm.FormCreate(Sender: TObject);

begin

 .....

  MakeSurface(self);

 

For me it's always a code smell when somebody uses a global variable or reference.

[egnew 3]

On 3/24/2023 at 10:50 AM, Stano said:

I wouldn't do that. You or user can call an OnShow event repeatedly. It will create objects each time.

This is not an issue because the first statement in the OnShow event is "OnShow := nil;" which keeps it from being called again.  I would never call the OnShow event explicitly.

 

[programmerdelphi2k 237]

this would works for you?

 

unit uMyControlsBeforeFormCreate;

interface

uses
  System.SysUtils,
  System.Classes,
  Vcl.Controls,
  Vcl.Forms,
  Vcl.StdCtrls,
  Vcl.ExtCtrls,
  Vcl.Dialogs;

procedure CreateMyPanelFromDPR(AOwner: TComponent; AParent: TWinControl; APanelName: string);
procedure ComponentsOnForm1(const AForm: TForm);

implementation

procedure CreateMyPanelFromDPR(AOwner: TComponent; AParent: TWinControl; APanelName: string);
var
  APanel: TPanel;
begin
  if (AOwner = nil) then
    AOwner := Application;
  //
  if (AParent = nil) then
    AParent := Application.MainForm;
  //
  if (APanelName.IsEmpty) then
    APanelName := 'PanelHello';
  //
  APanel        := TPanel.Create(AOwner);
  APanel.Name   := APanelName;
  APanel.Parent := AParent;
  //
  APanel.Left   := 10;
  APanel.Top    := 10;
  APanel.Width  := 200;
  APanel.Height := 200;
  //
  APanel.Caption := AOwner.ToString + AParent.ToString;
end;

procedure ComponentsOnForm1(const AForm: TForm);
var
  LMemo: TComponent;
begin
  if (AForm = nil) and not(AForm is TForm) then
    exit;
  //
  LMemo := AForm.FindComponent('Memo1');
  //
  if (LMemo <> nil) and (LMemo is TMemo) then
    begin
      TMemo(LMemo).Text := 'Application components';
      //
      for var i: integer := 0 to (Application.ComponentCount - 1) do
        TMemo(LMemo).Lines.Add(Application.Components[i].ToString);
      //
      TMemo(LMemo).Lines.Add(slinebreak + 'Form1 components');
      //
      for var i: integer := 0 to (AForm.ComponentCount - 1) do
        TMemo(LMemo).Lines.Add(AForm.Components[i].ToString);
      //
      TMemo(LMemo).Lines.Add(slinebreak + 'Form1 controls');
      //
      for var i: integer := 0 to (AForm.ControlCount - 1) do
        TMemo(LMemo).Lines.Add(AForm.Controls[i].ToString);
    end;
end;

end.

program Project1;

uses
  Vcl.Forms,
  uFormMain in 'uFormMain.pas' {Form1} ,
  uMyControlsBeforeFormCreate in 'uMyControlsBeforeFormCreate.pas';

{$R *.res}

begin
  ReportMemoryLeaksOnShutdown := true;
  //
  Application.Initialize;
  Application.MainFormOnTaskbar := true;
  Application.CreateForm(TForm1, Form1);
  //
  CreateMyPanelFromDPR( { Application, Form1 } Form1, { Form1 } nil, 'PanelHello');
  //
  ComponentsOnForm1(Form1);
  //
  Application.Run;

end.

type
  TForm1 = class(TForm)
    Memo1: TMemo;
    procedure FormShow(Sender: TObject);
  private
    procedure MyPanelClick(Sender: TObject);
  public
    { Public declarations }
  end;

var
  Form1: TForm1;

implementation

{$R *.dfm}

procedure TForm1.FormShow(Sender: TObject);
var
  LCmp: TComponent;
begin
  LCmp := FindComponent('PanelHello');
  //
  if (LCmp <> nil) and (LCmp is TPanel) then
    TPanel(LCmp).OnClick := MyPanelClick;
end;

procedure TForm1.MyPanelClick(Sender: TObject);
begin
  ShowMessage('Hello Panel');
end;

end.

 

Edited March 30, 2023 by programmerdelphi2k

[Stano 143]

To my knowledge, unless it's changed, OnShow is called on every new form view.
MyForm.Hide -> MyForm.Show(Modal)

[programmerdelphi2k 237]

YOU can call "Form1.FormShow(Self)" before "SHOW" your forms, to execute this event!!! 

no needs wait that it be on the screen!

var
  LFlagFormShow: boolean = true; // false  // global var no necessary good for all, then, try a property of the object!

procedure TForm1.FormShow(Sender: TObject);
begin
  if not LFlagFormShow then
    exit;
  // ...
end;