Delphi Virtual Constructor - Felix John COLIBRI.

• abstract : VIRTUAL CONSTRUCTORS together with CLASS references and dynamic Packages allow the separation between a main project and modules compiled and linked in later.
  
• key words : VIRTUAL CONSTRUCTORS - CLASS references - dynamic Packages - LoadLibrary - RegisterClass - object oriented programming
  
• software used : Windows XP, Delphi 6
  
• hardware used : Pentium 2.800Mhz, 512 M memory, 140 G hard disc
  
• scope : Delphi 5, Delphi 6, Delphi 7, Delphi 2005, Delphi 2006, Turbo Delphi, Delphi 2007
  
• level : Delphi developer
  
• plan :
  
  • Why Virtual Constructors ?
    
  • Virtual Constructor, Class Reference, Package
    
  • Application Frameworks and Plugins
    
  • Download the Sources
    

1 - Why Virtual Constructors ?

When Delphi came along, Virtual Constructors were possible. We were too busy at the time with so many other things to understand and master, that for a long period all my CLASS's CONSTRUCTOR were PUBLIC VIRTUAL, which did not seem to hurt.

Later some papers told us that this VIRTUAL CONSTRUCTOR business was at the heart of the IDE machinery, allowing the developer to add new components without having to recompile the IDE.

I only understood the working of this concept when we reengineered our old DOS drawing tool to upgrade it to a full fledged Palette / Inspector / Design surface vector graphic and UML editor.

The objective of this paper is to present a minimal example of how Delphi is able to instantiate new components without having to recompile the IDE. And of course, the whole thing is linked to VIRTUAL CONSTRUCTORs !

2 - Virtual Constructor, Class Reference, Package

2.1 - The Example

We will present two versions:

• the first solution will simply gather all the required UNITs and CLASSes, every pieces being linked together by USES clauses.
  
• the second solution will present an application containing the ancestor CLASSes. And the different figures will be appended later, without touching (recompiling) the drawing application.
  

2.2 - The Tiny Shape Editor

And here is the definition of one of the descendent shape:

The main program contains:

• a drawing surface (a tPaintBox)
  
• 3 tSpeedButtons allowing to select one of the three possible shapes
  
• the MouseDown and MouseUp events allow to draw the chosen shapes
  

2.3 - Using CLASS reference

We can also use a CLASS reference. In the base figure unit, we declare the c_base_figure_ref as a CLASS OF type:

And in the main program:

• we declare a variable of type c_base_figure_ref

  var g_c_base_figure_ref: c_base_figure_ref= Nil;

  This variable can be assigned any CLASS TYPE in the c_base_figure hierarchy: c_base_figure, or c_ellipse_figure, c_rectangle_figure, c_triangle_figure, or any of a later descendent.

• for each tSpeedButton click, we assign to this variable the shape type

  procedure TForm1.ellipse_speedbutton_Click(Sender: TObject);   begin     g_c_base_figure_ref:= c_ellipse_figure;   end; // ellipse_speedbutton_Click

• we use this type to create the corresponding shape

  procedure TForm1.PaintBox1MouseUp(Sender: TObject; Button: TMouseButton;     Shift: TShiftState; X, Y: Integer);   var l_c_current_figure: c_base_figure;   begin     l_c_current_figure:= g_c_base_figure_ref.create_figure('fig',         g_start_x, g_start_y, X+ 1- g_start_x, Y+ 1- g_start_y);     l_c_current_figure.draw_figure(PaintBox1.Canvas);     l_c_current_figure.Free;   end; // PaintBox1MouseUp

  In fact, the g_c_base_figure_ref simply replaced the CLASS TYPE, which was c_ellipse_figure in our first trial
  

However we have not yet achieved complete separation between the main program and all the U_C_xxx_FIGURE units, since they have all been imported in the USES clause of the main tForm.

2.4 - Using dynamic Packages

2.4.1 - The basic concept

So

• we create a Package which encapsulates our shape UNITs (separately or several in one Package)
  
• we place on disk a .TXT file containing the list of all package names
  
• the main project reads the Package list, and
  
  • loads the Package
    
  • for each shape, creates a SpeedButton with a link to the c_xxx_figure shape CLASS
    

2.4.2 - The c_class_list

Here is the definition of our c_class_list, using the usual tStringList encapsulation technique:

Two methods are of interest here:

• the function which returns the c_base_figure_ref:

  function c_class_list.f_c_base_figure_ref(p_class_name: String): c_base_figure_ref;   var l_c_class: c_class;   begin     l_c_class:= f_c_find_by_class(p_class_name);     Result:= l_c_class.m_c_base_figure_ref;   end; // f_c_base_figure_ref

  When we ask for the 'c_ellipse_figure', the function returns the reference to the c_ellipse_figure CLASS, which in fact converts a string into a CLASS reference

• the initialization of the list from a .TXT file:

  procedure c_class_list.load_packages_register_classes(p_full_file_name: String);   type t_pr_register_figure= Procedure(p_c_class_list: c_class_list);   var l_package_file_name: String;       l_class_index: Integer;       l_package_handle: tHandle;       l_pr_register_figure: t_pr_register_figure;   begin     with tStringList.Create do     begin       LoadFromFile(p_full_file_name);       for l_class_index:= 0 to Count- 1 do       begin         l_package_file_name:= 'pk_'+ Strings[l_class_index]+ '_figure.bpl';         l_package_handle:= LoadPackage(l_package_file_name);         l_pr_register_figure:=             GetProcAddress(l_package_handle, 'register_figure');         // -- this will add the <name, class_ref> item to the c_class_list         l_pr_register_figure(Self);       end; // for l_class_index       Free;     end; // with tStringList   end; // load_packages_register_classes

  This works like this:

  • on disk, we save a file with the shape names:

    ellipse     rectangle     triangle

  • this file is loaded by our method, and for each xxx shape name
    
    • a package PK_xxx_FIGURE.BPL is loaded
      
    • the address of the register_figure procedure is computed
      
    • this procedure is called
      
    In each package, the register_figure adds the package's figure CLASSes to the c_class_list. Here is the register_figure in the PK_ELLIPSE_FIGURE package:

    procedure register_figure(p_c_class_list: c_class_list);   begin     p_c_class_list.f_c_add_class('c_ellipse_figure', c_ellipse_figure);   end; // register_figure

    
    

2.4.3 - The figure Packages

And the figure UNITs are those of our first trial, but with the register_figure procedure:

2.4.4 - The main tForm

• we use a tButton to create the tSpeedButton and build the c_class_list
  
• each clic will provide a figure name, which will be used to create the shape
  

And:

• the tSpeedButton are created using the CLASS_LIST.TXT file. Each tSpeedButton
  • has a Name which is the 'c_xxx_figure' string
    
  • the glyph is loaded from an XXX.BMP image
    
  • the OnClick event is tied to a generic figure_speedbutton_click event which returns the Name of the Sender parameter
    
• the 'c_xxx_figure' string is used to locate the c_xxx_figure CLASS reference, which will create the corresponding shape
  

and the same after we clicked "load_packages"

2.4.5 - The Overall Picture

• a main tForm which only imports an ancestor c_base_figure CLASS, and a c_class_list. This tForm loads a .TXT file which can be written after the compilation
  
• several Packages, which can be compiled much later, even without the sources of the main project.
  

To drive the point home

• the main project can create any c_base_figure descendent because it can instantiate such a descendent using a CLASS reference
  
• the CLASS reference is searched in a <string, class_ref> list which can be build at runtime, using information updated after the main project compilation
  
• we can add any kind of c_base_figure descendent which contains:
  
  • the VIRTUAL methods of the ancestor
    
  • a register_figure method which will be used to initialize the <string, class_ref> link
    
• a .TXT file is used as the link between the main project and all the descendent shapes
  

• once we hold a CLASS reference

  my_c_figure_ref:= c_ellipse_figure;

• we can create the descendent CLASS using a VIRTUAL constructor:

  my_c_figure:= my_c_figure_ref.create_figure(g_figure_name,             g_start_x, g_start_y, X+ 1- g_start_x, Y+ 1- g_start_y);

  
  

3 - Application Frameworks and Plugins

• the resizing plots
  
• the mouse movements
  
• links between the shapes
  
• an Object Inspector
  
• a figure container (a tree in our case)
  
• a streaming mechanism
  

To establish the link between the CLASS name and a CLASS reference, Delphi offers the RegisterClass procedure. To qualify for this procedure, a CLASS must descend from tComponent, which is not the case in our example. But there is no reason why this could not be changed. But using more primitive CLASSes and a homegrown c_class_list registering mechanism was more instructive.

Finally, the separation of a main project and several UNITs compiled later but used by the main project without any recompilation is at the heart of many useful techniques:

• the split of huge .EXE into separated modules, which can shorten link time, and also make deployment much easier (you only ship the new .BPLs)
  
• the split of an application into conceptual modules which can be developed by different teams. This can be called an "Application Framework", where the main project simply loads the different Packages
  

4 - Download the Sources

• virtual_constructor.zip : the first trial with the project and the shape units (12 K)
  
• dynamic_loading.zip : the project and the separated packages and shape units (15 K)
  To use this .ZIP
  • compile the .DPR
    
  • compile the three .DPK (making sure the .BPL will be found by the .EXE)
    
  • execute the .EXE
    

The .ZIP file(s) contain:

• the main program (.DPR, .DOF, .RES), the main form (.PAS, .DFM), and any other auxiliary form
  
• any .TXT for parameters, samples, test data
  
• all units (.PAS) for units
  

• are self-contained: you will not need any other product (unless expressly mentioned).
  
• for Delphi 6 projects, can be used from any folder (the pathes are RELATIVE)
  
• will not modify your PC in any way beyond the path where you placed the .ZIP (no registry changes, no path creation etc).
  

• create or select any folder of your choice
  
• unzip the downloaded file
  
• using Delphi, compile and execute
  

The Pascal code uses the Alsacian notation, which prefixes identifier by program area: K_onstant, T_ype, G_lobal, L_ocal, P_arametre, F_unction, C_lasse etc. This notation is presented in the Alsacian Notation paper.

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5 - The author