PC Plus SuperCD (UK) 2000 March/pcp161b.iso/full/delphi/DELPHI16/TECHINFO/DELPHI/FAQS/DELSEC07.FAQ

Text File | 1995-08-24 | 24.0 KB | 762 lines

SECTION 7 - Object Pascal

This document contains information that is most often provided
to users of this section.  There is a listing of common
Technical Information Documents that can be downloaded from the
libraries, and a listing of the most frequently asked
questions and their answers.

Technical Information Documents related to Delphi Object Pascal:

TI1171  Bug Report Form
TI2841  Delphi Consultants and Training Centers

Index Of Questions
------------------

1) I am using a form from the gallery and I am getting an 'unknown identifier'
   error when I compile.  What is wrong?
2) How can I do standard output with writeln?
3) How to I cast a floating point number (e.g. real) to an integer?
4) How do I make it so that when the user hits <enter>, it goes to the next
   object as though he had hit the tab key? (Note: This will not work within
   a DBGrid, since the next field is not a separate object.)
5) In a DBGrid, how do I Tab to the next field when the Enter Key is pressed?
6) How do I process command line parameters in a Delphi app?
7) How do you select a specific field on a TDBGrid to get focus?
8) I have a paradox table that uses a password.  How do I make it so that the
   form that uses the table comes up without prompting the user for the
   password?
9) How do I design a form so that the display is the same no matter what the
   resolution is?
10) How do I do date math on calculated fields?
11) Where is the exponent function?
12) How do I keep the form in icon form when I run it?
13) I have a form that is a sort of template.  I want to be able to call the
    same form several times (with different data displayed).  How do I use
    one form several times?
14) How can the component that was right clicked be determined while in an
    event handler of a popup MenuItem?
15) How can I capture windows messages and process them before my
    application.run line is executed?
16) How do I use a case statement to determine which object calls the procedure?
17) How can you do scrolling functions in a TForm component using keyboard
    commands?  For example, scrolling up and down when a  PgUp or PgDown is
    pressed.  Is there some simple way to do this or does it have to be
    programmed by capturing the keystrokes and manually responding to them?
18) How do I transfer the text in a TMemo component on a form to a TMemofield
    in a Paradox table?
19) How do I make an Fixed Length ASCII text table from a table?
20) How do I set a format for a data field?
21) How do I format a number to place commas as a thousanth's separator?
22) I have a CPU intensive function.  I want to be able to check to see if the
    user wants to cancel.  How do I do that?
23) How do you extract the high or low order byte from a word?  How do you
    insert it?
24) I have an OBJ file that has several assembler routines compiled into it.
    I wouldn't want to have to rewrite them.  Is there a way to use them in
    a Delphi app?
26) If a component doesn't handle the windows messages that I need it to
    handle, do I have to write my own version that passes the messages that
    I need, or is there way to tap (from a TForm or else where) into the
    message loop, and grap what I need?
27) Borland decided that accessing environment variables from Windows
    Programs is a Bad Thing.  Why do they force you to use the "obsolete"
    WinDos unit?
28) How would I write a basic screen saver (like, blank the screen) using Delphi?
29) How can I determine the Length in pixels of a string after a specific font has
    been aplied to it?
30) Where is the best place to open a splash screen on program start up?
31) How do I call a function from a DLL?
32) What is a Callback function, and how do I create one?


Questions and Answers
---------------------

1) Q:  I am using a form from the gallery and I am getting an 'unknown
       identifier' error when I compile.  What is wrong?

   A:  If you change the name of some item that has code associated with it, that
       code will not be changed by Delphi.  Delphi does not go through your code
       and second guess your intent.  The code that you may assume is generated
       by Delphi is actually programmed into the form by the designer of that
       form. Just go through the code and make the modifications.


2) Q:  How can I do standard output with writeln?

   A:  Include WinCRT in your uses statement.


3) Q:  How to I cast a floating point number (e.g. real) to an integer?

   A:  There are a couple of methods.

           var
      i: integer;
      r: real;

    begin
      r := 34.56;
      i := trunc(r);  {i = 34}  {First Method}
      i := round(r);  {i = 35}  {Second Method}
    end;


4) Q:  How do I make it so that when the user hits <enter>, it goes to the next
       object as though he had hit the tab key? (Note: This will not work within
       a DBGrid, since the next field is not a separate object.)

   A:  You need to trap the keystroke and set up your own response to it.  Try
       this:

    procedure TMainForm.FormCreate(Sender: TObject);
    begin
      keyPreview := true; {To turn the event "ON".}
    end;

    procedure TMainForm.FormKeyPress(Sender: TObject; var Key: Char);
    begin
      if Key = #13 then
      begin
        Key := #0;
        PostMessage(Handle, WM_NEXTDLGCTL, 0, 0);
      end;
    end;

5) Q: In a DBGrid, how do I Tab to the next field when the Enter Key is pressed?

   A: The action of the tab key is acted upon after the OnKeyDown event but the
      Enter key is acted on in the OnKeyPress.  Therefore, the Enter key has to
      be accounted for in both areas for the desired effect to occur. Try this:

    procedure TForm1.FormKeyDown(Sender: TObject; var Key: Word; Shift:
     TShiftState);
    begin
        if Key = VK_RETURN then Key = VK_TAB;
    end;

    procedure TForm1.FormKeyPress(Sender: TObject; var Key: Char);
    begin
        if Key = #13 then Key := #9;
    end;


6) Q:  How do I process command line parameters in a Delphi app?

   A:  Use the ParamCount and ParamString methods.


7) Q:  How do you select a specific field on a TDBGrid to get focus?

   A:  Using this code:

    DBGrid1.SelectedField := Table1SomeField;
    DBGrid1.SetFocus;


8) Q:  I have a paradox table that uses a password.  How do I make it so that the
       form that uses the table comes up without prompting the user for the
       password?

   A:  The table component's ACTIVE property must be set to FALSE.  Then, put
       this code on the form's create event:

    session.AddPassword('My secret password');
    table1.active := true;


9) Q:  How do I design a form so that the display is the same no matter what the
       resolution is?

   A:  Here is some code to show how it is done:

    const
      ScreenHeight: real = 800; {I designed my form in 800x600 mode.}
      ScreenWidth: real = 600;

    procedure TForm1.FormCreate(Sender: TObject);
    var
      x, y: LongInt; {Integers will not not a large enough value.}

    begin
      form1.scaled := true;
      x := getSystemMetrics(SM_CXSCREEN);
      y := getSystemMetrics(SM_CYSCREEN);
      if (x <> ScreenHeight) or (y <> ScreenWidth) then
        begin
          form1.height := form1.height * x DIV ScreenHeight;
          form1.width := form1.width * y DIV ScreenWidth;
          scaleBy(x, ScreenHeight);
        end;
    end;

       Note: you may want to add code that will change the font sizes
       proportionally.


10) Q:  How do I do date math on calculated fields?

    A:  When doing date math on calculated fields, it is important to ensure that
        all values being used are properly matched as to type.  The double method
        (not in the docs) casts the value to a useable type.


        In the following method, d1, and d2 (part of table1) can be of either
        date or dateTime type and d3 is an integer field.

    procedure TForm1.Table1CalcFields(DataSet: TDataset);
    var
      t1, t2: tDateTime;
    begin
      table1d1.asDateTime := Date + 2; {or table1d1.value := date + 2;}
      table1d2.asDateTime := Date - 2;
      t1 := table1d1.asDateTime;
      t2 := table1d2.asDateTime;
      table1d3.asInteger := trunc(double(t1) - double(t2));
    end;


11) Q:  Where is the exponent function?

    A:  For ExpXY(X, Y) (i.e. Y^X), try:

        ExpXY := Exp(Y * Ln(X))

        To use a generic function, declare the formal parameters and function
        result as Extended, and the conversions from the actual parameters and
        back to your result variable will be done automatically.  (Note:  Ln is
        the natural logarithm of a number).


12) Q:  How do I keep the form in icon form when I run it?

    A:  In the private section of the form object's declaration, put:

    PROCEDURE WMQueryOpen(VAR Msg : TWMQueryOpen); message WM_QUERYOPEN;

        In the implementation section, put this method:

    PROCEDURE TForm1.WMQueryOpen(VAR Msg : TWMQueryOpen);
    begin
      Msg.Result := 0;
    end;

    That's it! The form will always remain iconic. OBTW, of course you must
    set WindowState to wsMinimized in the form's properties initially.


13) Q:  I have a form that is a sort of template.  I want to be able to call the
    same form several times (with different data displayed).  How do I use
    one form several times?

    A:  You need to make modeless window like this:

    with TMyForm.create(self) do show;

    Now we want to control it.  Here is a way to change the caption of each
    form that is created.  You have access to it through the form's component
    array.  This example uses an about box (named "box") as the other form.

    procedure TForm1.Button1Click(Sender: TObject);

    begin
      with tBox.create(self) do show;
      {ComponentCount-1 is needed because it is zero based.}
      (form1.Components[form1.ComponentCount-1] as tForm).caption :=
        'About Box # ' + intToStr(form1.ComponentCount-2);
    end;


14) Q:  How can the component that was right clicked be determined while in an
    event handler of a popup MenuItem?

    A:  Use the PopupComponent property of the PopupMenu component to determine
    what control was right clicked.

    procedure TForm1.PopupItem1Click(Sender: TObject);
    begin
      Label1.Caption := PopupMenu1.PopupComponent.ClassName;
    end;

    The form's ActiveControl property can also be used, however,  the active
    control may not necessarily be the control that caused the popup menu to
    appear.


15) Q:  How can I capture windows messages and process them before my
    application.run line is executed?

    A:  The following project source demonstrates how to get Window messages
    before the application's window procedure is called. It is rare, if ever,
    that this needs to be done.  In most cases assigning a procedure to the
    Application.OnMessage will accomplish the same thing.

    program Project1;

    uses
        Forms, messages, wintypes, winprocs,
      Unit1 in 'UNIT1.PAS' {Form1};


    {$R *.RES}

    var
        OldWndProc: TFarProc;

    function NewWndProc(hWndAppl: HWnd; Msg, wParam: Word;
                     lParam: Longint): Longint; export;
      begin
        result := 0; { Default WndProc return value }
        {*** Handle messages here; The message number is in Msg ***}
        result := CallWindowProc(OldWndProc, hWndAppl, Msg,                             wParam, lParam);
      end;

    begin
        Application.CreateForm(TForm1, Form1);
        OldWndProc := TFarProc(GetWindowLong(Application.Handle,
                  GWL_WNDPROC));
        SetWindowLong(Application.Handle, GWL_WNDPROC,
                  longint(@NewWndProc));
        Application.Run;
    end.


16) Q:  How do I use a case statement to determine which object calls the
    procedure?

    A:  Use the object's TAG property.  You set the value of the tag property of
    each object so that you know what it is.  (Using constants that describe
    the object is ideal.)

    Note:  This code assumes that only a tButton will call the procedure.

    case (sender as tButton).tag of
      0:  blah;
      1:  blah_blah;
    end;


17) Q:  How can you do scrolling functions in a TForm component using keyboard
    commands?  For example, scrolling up and down when a  PgUp or PgDown is
    pressed.  Is there some simple way to do this or does it have to be
    programmed by capturing the keystrokes and manually responding to them.

    A:  Form scrolling is accomplished by modifying the VertScrollbar or
    HorzScrollbar Postion properties of the form.  The following code
    demonstrates how to do this:

      procedure TForm1.FormKeyDown(Sender: TObject; var Key: Word;
      Shift: TShiftState);
      const
        PageDelta = 10;
      begin
        With VertScrollbar do
          if Key = vk_Next then  Position := Position+PageDelta
          else if Key = vk_Prior then Position := Position-PageDelta;
      end;


18) Q:  How do I transfer the text in a TMemo component on a form to a TMemofield
    in a Paradox table?

    A:  Here is an example.

    procedure TForm1.Button1Click(Sender: TObject);
    var
      t: TTable;
    begin
    t := TTable.create(self);
    with t do
      begin
        DatabaseName := 'MyAlias'; {personal alias}
          TableName := 'MyTbl.db';
        open;
        edit;
        insert;
        fieldByName('TheField').assign(memo1.lines); {This is it!}
        post; {required!!!}
        close;
      end;  { End of the with statement. }
    end;


19) Q:  How do I make an Fixed Length ASCII text table from a table?

    A:  Like this:

    procedure TForm1.Button1Click(Sender: TObject);
    var
      t1, t2: tTable;  {t1 = PW table; t2 = ASCII version}
    begin
      t1 := tTable.create(self);
      with t1 do
      begin
        DataBaseName := 'pw'; { Personal Alias for Paradox Directory }
        tableName := 'customer.db'; { Source Table }
        open;
      end;
      t2 := tTable.create(self);
      with t2 do
      begin
        DataBaseName := 'pw'; { Personal Alias for Paradox Directory }
        tableName := 'asdf.txt';
        TableType := ttASCII;

        createTable;
        open;
        edit;
        BatchMove(t1, batCopy);
        close;
      end;
      t1.close;
    end;


20) Q:  How do I set a format for a data field?

    A:  Select the table object, and double click.  Select the field that you
    want to format.  Use the DisplayFormat and the EditFormat properties to
    do what you want.  DisplayFormat works for when the field does not have
    focus.  EditFormat works for when the field has focus.  Use the commands
    as you would for the first parameter of the FormatFloat function, but
    without the quotes.


21) Q:  How do I format a number to place commas as a thousanth's separator?

    A:  If you are not using Delphi, try this code:

    function FormatNumber(l: longint): string;
    var
      len, count: integer;
      s: string;
    begin
      str(l, s);
      len := length(s);
      for count := ((len - 1) div 3) downto 1 do
        begin
          insert(',', s, len - (count * 3) + 1);
          len := len + 1;
        end;
      FormatNumber := s;
    end;

        If you are using Delphi, there is, of course, the easy way:

    function FormatNumber(l: longint): string;

    begin
      FormatNumber := FormatFloat('#,##0', StrToFloat(IntToStr(l)));
    end;


22) Q:  I have a CPU intensive function.  I want to be able to check to see if
    the user wants to cancel.  How do I do that?

    A:  Call Application.ProcessMessages from time to time in your code.


23) Q:  How do you extract the high or low order byte from a word?  How do you
    insert it?

    A:  There are built in methods hi() and lo() for extracting, but for those
    that want to know how to do it on their own, here it is in its most
    efficient form (if I do say so myself <G>).  The functions for inserting
    bytes are not in Delphi.

    Note:  Assembler functions return the contents of the AX register.

    function GetHiByte(w: word): byte; assembler;
    asm
       mov ax, w
       shr ax, 8
    end;

    function GetLoByte(w: word): byte; assembler;
    asm
       mov ax, w
    end;

    function SetHiByte(b: byte; w: word): word; assembler;
    asm
       xor ax, ax
       mov ax, w
       mov ah, b
    end;

    function SetLoByte(b: byte; w: word): word; assembler;
    asm
       xor ax, ax
       mov ax, w
       mov al, b
    end;

    Another way of doing it (with an example of its use):

    Type
       TWord2Byte = record
                  Lo,Hi: Byte;
           end;

    var W: Word;
         B: Byte;
    begin
       W := $1234;
       B := TWord2Byte(W).Hi;
       writeln(TWord2Byte(W).Hi);
       { going back }
       TWord2Byte(W).Lo := $67;
       TWord2Byte(W).Hi := $98; { no shl needed! }
    end.


24) Q:  I have an OBJ file that has several assembler routines compiled into it.
    I wouldn't want to have to rewrite them.  Is there a way to use them in
    a Delphi app?

    A:  You don't indicate if these return values or not (in Pascal this
        matters.)

    If they don't, include the following near the front of your code:

      Procedure TurnOn;  External;
      Procedure TurnOff; External;

    If they return values, use:

      Function TurnOn: Integer; External;
      Function TurnOff: Integer; External;

    Replace Integer with whatever datatype they return.
    In either case follow that with:

      {$L filename.obj}

    to link in the obj file.  See also the "linking external assembler code"
    in the on-line help.


25) Q:  Which memory model does Delphi use?

    A:  Delphi uses a mixed memory model, but it is very close to the "C" large model.  The defaults are:

      - Methods are far
      - Procedures in an interface section are far
      - Procedures only used in an implementation section are near
      - Heap data and all pointers in general (including class instances)
        are far
      - Global variables are near (DS based)
      - Procedure parameters and local variables are near (SS based)
      - Procedures declared FAR or EXPORT are far
      - Virtual memory tables are far for the new class model and near for
        the old

    This scheme has been used by Borland Pascal for a very long time. I find
    it flexible and efficient.

    Since all public procedures, methods and pointers are 32bit already,
    Delphi32 won't have to change any of that.  It's likely that Delphi32
    will switch to 32bit addressing for the data and stack segments too,
    but that shouldn't affect any of your code either.  What will affect it
    is the change of Integer from 16 to 32 bit.


26) Q:  If a component doesn't handle the windows messages that I need it to
    handle, do I have to write my own version that passes the messages that
    I need, or is there way to tap (from a TForm or else where) into the
    message loop, and grap what I need?

    A:  To respond to, say, the wm_size message, you would add:

     procedure WMPaint(var Message: TWMPaint); message WM_PAINT;

    to your component.  Then you could have:

     procedure TWhateverComponent.WMPaint(var Message: TWMPaint);
     begin
        {have your way with the component}
     end;

    This will work for any windows message.  Most components respond to the
    more popular messages already, and you can override their event handlers.


27) Q:  Borland decided that accessing environment variables from Windows
    Programs is a Bad Thing.  Why do they force you to use the "obsolete"
    WinDos unit?

    A:  Use the GetDOSEnvironment() call from the API.


28) Q:  How would I write a basic screen saver (like, blank the screen)
    using Delphi?

    A:  An easy implementation would be something like this:

    var
      f: tForm;

    begin
      f := tForm.create(self);
      f.WindowState := wsMaximized;
      f.color := black;
      f.borderStyle := bsNone;
      f.show;
    end;

    There is more to do, of course.  You must have a way back from there.
    Perhaps an actual form that has a mouse click event programmed to
    f.close it.  Also, you want to hide the mouse.  Etc, etc.  But, this
    answers the question.


29) Q:  How can I determine the Length in pixels of a string after a specific
    font has been aplied to it?

    A:  The two methods, TextHeigh and TextWidth, can be used to determine
    both the text height and width of a string in pixels.  These methods
    can only be accessed through components that have a Canvas property
    such as TForm.  The TPanel component does not have access to its Canvas
    property by default because it is protected.

    If a component doesn't have a Canvas property then The following
    function will return the text width based on the font passed.

      function GetTextWidth(CanvasOWner: TForm; Text : String;
                                 TextFont :  TFont): Integer;
      var
         OldFont : TFont;
      begin
        OldFont := TFont.Create;
        try
          OldFont.Assign( CanvasOWner.Font );
          CanvasOWner.Font.Assign( TextFont );
          Result := CanvasOWner.Canvas.TextWidth(Text);
          CanvasOWner.Font.Assign( OldFont );
        finally
          OldFont.Free;
        end;
      end;


30) Q:  Where is the best place to open a splash screen on program start up?

    A:  The best place to open a splash screen is in the project source file
    after the first FormCreate and before the Run  This is  accomplished
    by creating a form on the fly and then displaying it before the app is
    actual opened.

     program Project1;

     uses Forms,  Unit1 in 'UNIT1.PAS' {Form1}, Splash;

     {$R *.RES}
     var
       SplashScreen : TSplashScreen;  {in the Splash unit}

     begin
       Application.CreateForm(TForm1, Form1);
       SplashScreen := TSplashScreen.Create(Application);
       try
         SplashScreen.Show;
         {
          do other CreatForms or any other processing
          before the app is to be opened
         }
         SplashScreen.Close;
       finally  {Make sure the splash screen gets released}
         SplashScreen.Free;
       end;
       Application.Run;
     end.


31) Q:  How do I call a function from a DLL?

    A:  In order to call a function you must know it's exact syntax. and setup
    a function type.  For example to call the CallMe function, which accepts
    two integers and returns a string, from MyTest.Dll when a button is
    pushed, you could use the following code:

    procedure TForm1.Button1Click(Sender: TObject);

    type
      TCallMeDll = function(a,b: Integer): string;

    var
      CallMeDll: TCallMeDll;
      FuncPtr: TFarProc;
      hDll: THandle;
      result: string;

    begin
      hDll:=LoadLibrary('Mytestdll.dll');
      FuncPtr:=GetProcAddress(hDLL,'CallMe');
      @CallMeDll:=FuncPtr;
      if @CallMeDll <> nil then
        result:=CallMeDll(4,5);
      FuncPtr:=nil;
      FreeLibrary(hDll);
    end;

    Note that w must first load the dll into memory, then we can obtain a
    pointer to the function, and assign that to the contents of CallMeDll.
    At this point I am checking to see if the ProcAdderss is Nil, this would
    indicate that the call to GetProcAddress failed.

    It is important to note that sometimes you might want to load the
    library, and get the procedure address once at the begenning of your
    program, and free the library once at the end.


32) Q:  What is a Callback function, and how do I create one?

    A:  A call back function is a function which you write, but is called by
    some other program or module, such as windows.  To create a callback function, you
    must first declare a function type, the funciton itself, and implement
    the function.  In the interface section:

    { In main program interface }

    type
      TCallBackFunction = function(s: string): integer;
      CallMe(s: string): integer;

    And in the Implementation section:

    { In main program implementation }

    procedure TestCallBack(CallBackFunction: TCallBackFunction); far; external 'Other';
        { Note that 'other' is a Dll containing the procedure TestCallBack }

    function CallMe(s: PChar): integer;
    begin
      { what ever you need to do }
      CallMe := 1; { What ever you need to return }
    end;

    procedure TForm1.Button1Click(Sender: TObject);
    begin
      TestCallBack(CallMe);
    end;

    Note that in 'Other' you would also declare a function type, and use it
    like this:

    { in library Other interface }

    type
      TMainFunction = function(s: string): integer;
      TestCallBack(MainFunc: TMainFunction);

    { in library Other implementation }

    TestCallBack(MainFunc: TMainFunction);
    var
      result: integer;
    begin
      result:=MainFunc('test');
    end;