Delphi - Managing Forms

• Introduction to forms
• Creating a form at run
• Closing a form
• Splash screen at start up with progress bar
• Preventing more than one instance of application from running
• Form shapes
• Status bar hints with multiple forms
• Always on Top
• Making a secondary form the top form instead of the main form
• Making a form come to top of system even when application is minimised
• Making application invisible
• Form sizing
• Hiding Windows TaskBar temporarily
• see also:
  • Delphi Programming Main Index

Introduction to forms:

• a form is the Delphi equivalent of a window
• the "Main Form":
  • an application can have only 1 "MainForm" which is the 1st form to be created in the project source file using the Application.CreateForm method
    • thus to create a splash screen BEFORE the main form is created, you must use TSplashForm.Create(nil).
      • NB. user cannot interact with splash screen before Application.run occurs, although the user can see the splash screen if "show" & "update" are used.
  • if this main form is freed/destroyed then the application will terminate
  • you cannot change which form is the main form at runtime as this property is read only!
• a datamodule is a specialised non-visual version of a form which can be used to maintain datasets and their business rules, but note that if a form references datasets in a datamodule, that datamodule must already have been created, otherwise an access error will result!!

Creating a form at run time:

• normally form creation is automatically performed by Delphi at application startup via the project source code (appname.dpr). However, this can cause application startup time to be prolonged, esp. if form includes its own database access.
• To get around this, use Project:Options to determine which forms you wish to be auto-created at startup and which forms will only be available to be manually created when you so wish.
• You can then create a form using the following code:
  • Application.CreateForm(FormClass,FormName);
  • FormVariableName := FormClass.Create(Application); //this syntax is preferred to the above which should only be used in DPR file.
• Using manually created forms like this improves performance BUT can cause 2 main types of problems for the programmer:
  1. referencing an object in a form that has not yet been created will cause an access violation & crash your system (unless embedded in try..except clause)
  2. multiple instances of a form may be inadvertently be created causing unpredictable results
• To solve these problems, use any of the following techniques to create a form manually:
  • iterating through Application.Components:
    • function TfrmMain.ShowForm(formName: string) : boolean;
      • {Determine whether a form has been created or not}
      • var i : integer;
      • begin
        • result := False;
        • for i := 0 to screen.formCount -1 do
        • begin
        • if (screen.Forms[i].name = formName) then
          • begin result := True;Break;end;
        • end;
      • end;
    • to show form then use:
      • try
      • if not IsForm('Form2') then Form2 := TForm2.Create(Application);
      • Form2.show;
      • except on E:exception do Showmessage(E.message); end;
    • this is simplified by using gaTools.pas & calling:
      • if LoadForm(Form2,TForm2,'Form2') then Form2.show;
  • testing for form var = nil:
    • Most forms have a var at the beginning of the form, just after the class declaration, eg:
      • TForm2=class(TForm)
      • ...
      • end
      • var Form2: TForm2;
    • So you could test whether or not the form was nil, eg
      • if Form2<> nil then Form2.show else Form2 := TForm2.Create(Application);
    • However, the instance variable for Form2 is NOT set to nil upon the Form's destruction, if the form was NOT auto-created by the application, thus to get around this problem, one can use the form.tag property to store a pointer to the form variable as follows:
      • Mainform.OnCreate event:
        • Form2 := nil; //explicitly set all non-auto-created form's instances to nil
      • Show form via:
        • if Form2 = nil then begin
          • Form2 := TForm2.Create(Self);
          • Form2.Tag := Longint(Addr(Form2));
          • Form2.Show;
          • end;
      • On Form2's OnDestroy event:
        • TForm2(Pointer(TForm2(Sender).Tag)^) := nil; //this dereferences the pointer to the form variable & sets it to nil.
  • MDI style:
    • function TfBaseApplication.CreateMDIForm(InstanceClass: ComponentClass):TForm;
    • var Reference: TfBaseMDIForm; i: integer;
    • begin
    • for i:=0 to MDIChildCount-1 do begin
      • if MDIChildren[i].ClassName=InstanceClass.ClassName then begin
        • Result:=TfBaseMDIForm(MDIChildren[i]); exit;end;
      • end;
    • try Screen.Cursor := crHourglass; Application.CreateForm(InstanceClass,Reference); Result:=Reference;
    • finally Screen.Cursor := crDefault;end;
    • end;

Closing a form:

• a user can close a form by activating the form.OnClose event by either:
  • clicking on the X in the top right corner of the form
  • pressing Ctrl-F4
  • clicking a button or menu item which calls Form.close;
• in either case the default action is for the form to hide (ie. become invisible) but still exist in memory and can be re-displayed by calling Form.Show or Form.ShowModal.
• to remove a form from memory (ie. destroy it) then either:
  • add this line to its OnClose event:
    • action := caFree; //the default action is caHide; to prevent a form from closing, use caNone;
  • or call Form.free;

Splash screen at start up with progress bar:

• Design a form with a progress bar on it (in this case its Form5) then manually modify the project source file to resemble:
  • begin
  • Application.Initialize;
  • with TForm5.Create(nil) do // the spash screen
  • try
  • ProgressBar1.Max := 100;
  • Show; // show a splash screen contain ProgressBar control
  • Update; // force display of Form5
  • Application.CreateForm(TForm1, Form1);
  • ProgressBar1.StepBy(25);
  • Application.CreateForm(TForm2, Form2);
  • ProgressBar1.StepBy(25);
  • Application.CreateForm(TForm3, Form3);
  • ProgressBar1.StepBy(25);
  • Application.CreateForm(TForm4, Form4);
  • ProgressBar1.StepBy(25);
  • finally Free; end;
  • Application.Run;
  • end.

Preventing more than one instance of application from running:

• Option 1:
  • Try this in the project source file:
    • {usual stuff at the top of the project source file
    • var hwnd: Word;
    • begin
    • if hPrevInst = 0 then
    • begin
    • Application.CreateForm(TForm1, Form1);
    • Application.Run;
    • end
    • else
    • begin
    • hwnd := FindWindow('TForm1', nil);
    • if (not IsWindowVisible(hwnd)) then
    • begin
    • ShowWindow(hwnd, sw_ShowNormal);
    • PostMessage(hwnd, wm_User, 0, 0);
    • end;
    • else
    • SetWindowPos(hwnd, HWND_TOP, 0,0,0,0,SWP_NOSIZE or SWP_NOMOVE);
    • end;
    • end.
  • In the form's PAS file add a message response function for the wm_User message.
    • {in the form declaration}
    • public
    • procedure WMUser(var msg: TMessage); message wm_User;
    • {in the implementation section}
    • procedure TForm1.WMUser(var msg: TMessage);
    • begin
    • Application.Restore;
    • end;
• Option 2:
  • in project source file:
    • if hprevinst <> 0 then
      • begin
      • bringwindowtotop(findwindow('TForm1', nil));
      • halt;
      • end;
    • Application.CreateForm(TForm1, Form1);
    • Application.Run;

Form shapes:

• the default Window's form is rectangular, however there are various ways in which you can create other shaped forms:
  • use Woll2Woll's 1st Class components
  • Neil Rubenking has given code for a donut-shaped splash form where clicking on hollow centre activates programs visible there
    • see Delphi Super Recall - DBK - #741

Status bar hints with multiple forms:

• see Delphi Super Recall - DBK - #735 for technique using same named status bar on each form then just creating one DisplayHint procedure on the main form only.
•  

Always on Top:

• To use AlwaysOnTop windows like several toolbars etc, then you can specify FormStyle := fsStayOnTop; But if you want to switch your window to top and to normal, then this flickers your form.A function that does it w/o flickering:
  • function PutOnTop(Form: TForm; ToTop: boolean): boolean;
  • var Status: Integer;
  • begin
    • if ToTop then Status := HWND_TOPMOST else Status := HWND_NOTOPMOST;
    • Result := SetWindowPos(Form.Handle, Status, 0, 0, 0, 0, SWP_NOMOVE or SWP_NOSIZE)
  • end;
• You can use it like PutOnTop(Form1, true); and if you want to normalize it, specify false as second param.

Making a secondary form the top form instead of the main form:

• Try this in any secondary window that you DON'T want dragging the program along:
  • private {This goes in the form's type declaration.}
  • { Private declarations }
  • procedure CreateParams(VAR Params: TCreateParams); override;
  • ...
  • procedure TForm2.CreateParams(VAR Params: TCreateParams);
  • begin
  • Inherited CreateParams(Params);
  • Params.WndParent := GetDesktopWindow;
  • end;
  • By setting the form's parent window handle to the desktop, you remove the link that would normally force the whole application to come to the top when this form comes to the top.

Making a form come to top of system even when application is minimised:

• Check out API-function SetSysModalWindow
• Check out API-function BringWindowToTop
• If you just want a simple dialog with a bit of text, an optional icon and some buttons, use MessageBox in WinProcs and use MB_SYSTEMMODAL in the TextType parameter.
• Otherewise, use the SetSysModalWindow() Windows API function. The following code snippet demonstrates its use. There can be only one system-modal window at a time, and that handle is returned by SetSysModalWindow(). I suggest that you save that return value and then reset it when you are finished, as demonstrated below.
  • procedure TForm1.Button1Click(Sender: TObject);
  • var x : word ;
  • begin
  • x := SetSysModalWindow(AboutBox.handle) ;
  • AboutBox.showmodal ;
  • SetSysModalWindow(x) ;
  • end;

Making application invisible:

• make an app that has an icon in the notification area with a popupmenu. However the application is still visible on the taskbar. Using Application.ShowMainForm:=False; is not enough.This little bit of code works great.
  • procedure TMainForm.FormCreate(Sender: TObject);
  • begin
  • Application.OnMinimize:=AppMinimize;
  • Application.OnRestore:=AppMinimize;
  • Application.Minimize;
  • AppMinimize(@Self);
  • end;
  • procedure TMainForm.AppMinimize(Sender: TObject);
  • begin
  • ShowWindow(Application.Handle, SW_HIDE);
  • end;

Form sizing:

• you can set form size using:
  • Form.height / Form.width properties
  • Form.WindowState property (wsNormal, wsMaximized, wsMinimized)
• but sometimes you can still have problems with the Windows task bar so to work out its current height:
  • Simple method to determine task bar height if docked top/bottom:
    • Screen.Height - GetSystemMetrics(SM_CYMAXIMIZED)
  • other way is to dig into AppBar messages. But taskbar may be docked to left/right too, so you must determine position first and then calculate. Actually there is special SHAppBarMessage but it is more complex. Other alternative is to hide the taskbar.
• see also:
  • controlling form sizing - DI Nov 1997 p64; Apr, May1998

Hiding Windows TaskBar temporarily:

• 1) First declare a variable of type HWND to store the Window handle of the Windows 95 taskbar.
  • TForm1 = class(TForm)
    • ...
    • private
    • hTaskBar: HWND;
    • ...
    • end;
• 2) In your main form's OnCreate() event handler, place some code that resembles:
  • hTaskBar := FindWindow('Shell_TrayWnd', nil);
  • ShowWindow(hTaskBar, SW_HIDE);
• 3) Finally, in your main form's OnDestroy() event handler, code something like:
  • ShowWindow(hTaskBar, SW_SHOW);