delsec07.txt

《Delphi开发人员指南》配书原码

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.


TI1171  Bug Report Form


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;