delsec09.txt

《Delphi开发人员指南》配书原码

SECTION 9 - Component Design.

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 Component Design:

None

Zip files related to Component Design:

TBD

Questions and answers:

Q: "Should I purchase the VCL source code?"

A: If you are interested in component design then positively,
   absolutely, one hundred percent YES!  Having the VCL source code
   allows you to examine the way that VCL components are created right
   down to the private sections.  This can be very useful when trying
   to create your own custom components.  You also have the ability to
   step through the source code with the debugger.


Q: "Okay, I purchased the VCL source code but where is the source code
   for the tabbed controls (i.e. TTabbedNotebook etc)?"

A: The source code doesn't contain the source for the tabbed controls
   because of legal reasons.  However, the interface source is provided
   in the DELPHI\DOC directory with an INT extension.

   Note:  Registered owners of the Delphi RTL source code can request
   the TTabSet and TTabbedNotebook source code from Borland Corporate
   Affairs by contacting Karen Rogers via fax at (US) 408-431-4171.


Q: "I am trying to create a custom component but I'm not sure which
   class I should decend from."

A: The easiest way is to decend from an existing component that has most
   of the characteristics you are looking for.  If you have to create a
   custom component from "scratch" then use one of the classes listed
   below.

   TComponent      - The base starting point for non-visual components.
   TWinControl     - The base starting point for components that need to
                     have window handles.
   TGraphicControl - A good starting point for visual components that
                     don't need the overhead of a window handle.  This
                     class has a Paint method, that should be overridden,
                     but no canvas.
   TCustomControl  - The most common starting point for visual components.
                     This class has a Window handle, common events and
                     properties, and most importantly a canvas with a
                     Paint() method.


Q: "How do I override the default message handler for my Application?"

A: You create a dynamic method that is indexed to the message constant
   that you want to override.  If you want to override the CM_DIALOGKEY
   message you would declare the following procedure in the public
   section of you class declaration:

   Procedure CMDialogKey(var Message: TCMDialogKey);message CM_DIALOGKEY;

   It is common practice to declare the procedure name the same as the
   message name minus the underscore.  Your message handler would look
   something like this:

   Procedure TForm1.CMDialogKey(var Message: TCMDialogKey);
   begin
     if CharCode = VK_TAB then begin
       {Process the Alt+Tab key here}
       result := 1;
       exit;
     end;
     inherited;
   end;

   Setting result to 1 stops further processing. The inherited
   statement passes control to the parent handler. Be sure to
   execute inherited for all the cases you don't want to handle.
   Do not execute it for the ones you do handle.


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
   accomplisged 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 anyother processing
      before the app is to be opened
     }
     SplashScreen.Close;
   finally               {Make sure the splash screen gets released}
     SplashScreen.Free;
   end;
   Application.Run;
   end.


Q: "What is the order of event handlers when a form is created
   and shown?"

A: When a form is created the event handlers are executed in the
   following order:  OnCreate, OnShow, OnPaint, OnActivate, OnResize
   and OnPaint again.


Q: "Why can't my program find any of the resources that I put in a
   .RES file if that .RES file is the same name as my form's unit name?"

A: If the name of an included .RES file is the same as the name
   of a .DPR file Delphi wll overwrite it with it's own .RES file.


Q: "I have derived a component from a TPanel.  In the constructor, I
   look at the BevelWidth property; it is always 1 regardless of what
   the user sets it to at design time.  Apparently the property is
   updated with the design time value after my constructor fires.
   When does this happen?  Where is a good place to trap this value
   during initialization of the component?"

A: When components are read from a stream, each component is first
   constructed, then its property values are read from the stream.  A
   component's Loaded method is called after all streaming has finished.
   You can override Loaded to do something with the component after its
   state has been loaded from the stream.


Q: "I am trying to publish an array property but I keep getting a #202
   error telling me that the property cannot be published.  I thought
   that a property that has a property editor can be published.

A: Check the online help under "published parts" to see what properties
   and fields can be published.  To stream the data for non-standard
   data types (which include array properties), override the component's
   inherited DefineProperties method to register a property reader and
   writer routine. See TStringList source for an example - it uses
   DefineProperties to register reader and writer routines to handle
   streaming the strings in its list.


Q: "I am creating a component at run time but the default values that
   I have declared don't seem to be working.  Why not?"

A: You need to make sure that you are setting the default values for
   the properties in the Create constructor for the component.  The
   "default" value for a property is not used when you create a
   component at run time.