Something missing about packages

Problem/Question/Abstract:

Packages are a great feature of Delphi. You can put not only components into packages but also everything you want. This way you can build modular, customizable applications.
Many programmers do not use packages because they modify VCL units and they do not have VCL packages source code in order to rebuild them.

Answer:

Introduction

Packages are a great feature of Delphi. You can put not only components into packages but also everything you want. This way you can build modular, customizable applications.
Many programmers do not use packages because they modify VCL units and they do not have VCL packages source code in order to rebuild them.
In this article you will find instructions for using packages anytime, anywhere.

How do packages work

By default, when you compile your project every VCL unit required by your project is compiled into the generated .EXE file. This way a simple Delphi project has at least 300 KB. If you modify one line of one unit then you need to recompile the entire project. These kinds of applications are difficult to modularize. If you have more than one application running on the same computer then you are consuming more RAM than you need.
If you select Project | Options and go to the Packages tab you can instruct Delphi to use Runtime packages.
This way the .EXE file size decrease because the VCL units are not compiled into it. Using runtime packages VCL units are kept into VCL packages and you need to distribute them with your .EXE file. VCL packages have the .BPL extension and they are a special kind of dynamic link libraries (DLL). Delphi installs .BPL files in Windows\System32 directory.
When you use runtime packages Delphi uses .DCP files to build the .EXE file. These .DCP files are to .DPK files (packages source code) what .DCU files are to .PAS files. When you build a package Delphi puts all the .DCU files into a single .DCP file. Then, when you compile a project that uses runtime packages Delphi uses the .DCP files instead of .PAS or .DCU files. So, what happened if you modify, for example, ActnList.pas. If you want to use runtime packages then you need to rebuild VCL package, which includes this unit. And because VCL package is required by almost all the other VCL packages, then you need to rebuild them all.

How can you rebuild all the VCL packages?

Delphi includes a package for user components. The name of this package is dlcusr.dpk and it is located in the Delphi\Lib directory. Open it. In the package editor you can see the Contains and Requires clauses. Select the Requires clause and if there is not any package do the following:

Click the Add button.
Type vcl.dcp in the package name edit control in the Add dialog box
Click the OK button.

Now select the vcl.dcp package or any other VCL package and right click on it. From the popup menu select Open.
The VCL package is generated and now you can build it. Because this is a generated package you need to save it with a different name. Now you can build your project using the new generated package.

Your own VCL packages

Delphi packages were built according to Delphi needs. Your application's needs could be different. Maybe you need different packages. VCL.BPL is a big file (1.3 MB or so). With this trick now you know in which package each unit lives. So you can create your own VCL packages containing only the VCL units your project use.