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Learn to Program in Delphi – Architecture and Layers of Code

Author: Pawe Gowacki

Last week we have created „DelphiSuperCalculator” project and learn how to build, run and debug it. 

Before rushing into writing code it is a good thing to step back first and think about the overall architecture of your app. This typically involves breaking your app into smaller parts and in code a typical metaphor is a “layer” or “tier” of code. Even in the simplest possible app there could be at least two layers: one for the GUI and one for the application logic. Keeping the GUI part as thin as possible is always a good idea, especially when you build different screens for the same logic. Below the application logic there is typically some kind of data or service access layer, which encapsulates all aspects of interactions between an app and services or databases it depends on. Going further there are multi-tier scalable cloud architectures where database access can be further divided into additional layers which is important to implement scalable, fault-tolerant, multi-user systems.

You may think that a simple calculator apps does not need to have a “proper architecture”. You could be wrong. There is this saying that the more into the forest the more trees there are. Sooner then you expect even the simplest app starts to be complicated. Keeping the architecture of your app clean is of the most important thing in programming. Programming is an art. Every touch matters. It is like placing stones on the board in the game of Go. Once placed they cannot be moved. You really do not want that your application looks like this:

In the case of our simple calculator app we can define just two layers. One layer with GUI – that is our main form unit, and one with the “calculation engine” that should live in a separate unit.

Let’s add a new unit to our app and save it as “uCalculator”. Here we are going to define “TCalculator” class that is going to be responsible for performing basic calculations like adding, subtracting, multiplying and dividing.

Enter the following code in the new unit:


unit uCalculator;

interface

type
  TCalculator = class
  public
    class function Add(a, b: Double): Double;
    class function Subtract(a, b: Double): Double;
    class function Multiply(a, b: Double): Double;
    class function Divide(a, b: Double): Double;
  end;

implementation

{ TCalculator }

class function TCalculator.Add(a, b: Double): Double;
begin
  Result := a + b;
end;

class function TCalculator.Divide(a, b: Double): Double;
begin
  Result := a / b;
end;

class function TCalculator.Multiply(a, b: Double): Double;
begin
  Result := a * b;
end;

class function TCalculator.Subtract(a, b: Double): Double;
begin
  Result := a - b;
end;

end.

Object Pascal language used in Delphi has been designed to teach good programming practices. It is very readable and helps develop programs in the top-down fashion. It is possible to write your code in a purely procedural fashion, ignoring completely object-oriented programming. However thinking about the problem domain in terms of objects and classes of objects proved to be a very good approach. We want to implement calculator app, so we would probably want to have a “TCalculator” class that could be used for encapsulating calculator functionality. When you define a class it serves as a blueprint for creating objects of this class. In the case of our calculator we do not really need to have an instance of a calculator. Here the class serves more as a grouping of related arithmetic operations. That is why declarations of TCalculator methods have been preceded with a “class” modifier (which corresponds to “static” in C++). Class members of a class belong to the type itself, and you do not need to instantiate an object in order to call them. That’s what we want. In our application code we would be able to type just “z := TCalculator.Add(x,y);” without creating a TCalculator object.

Next week we are going to start building the user interface for our calculator and write code that will use our TCalculator class.

Links to all blog C++Builder and Delphi blog postings in this series are available at “Learn to Program with Starter (Homepage)”.


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