Programming in Delphi sins #3

In this serie I will tell you, what we find a sin, in Delphi programming.

Using published properties of components from another class
When you place components on forms and datamodules in Delphi they are published by default. In other words you can use components in your form that belong to other forms/datamodules. If you do this, those forms will be tightly coupled and therefor this is not quite a good OOP practice.

type
  TdmMain = class(TDataModule)
    //These components are published and direct 
    // accesable by other forms/datamodules
    SomeDataSet: TADOTable;
    adsAnotherDataSet: TADODataSet;
  private
    { Private declarations }
  public
    { Public declarations }
  end;

Suppose you have a form that uses a datamodule with some dataset on it and you want to show some field value in a label on your form. You could do it like this:

procedure TForm3.SetCaption;
begin
 SomeLabel.Caption := 
 dmMain.SomeDataSet.FieldByName('SomeField').AsString;
end;

This is bad practice. Suppose that, something simple as the name of the field changes, you would have change both your datamodule, and your form.

Solution:

Use properties and functions (getters), not properties from published components.

In the datamodule:

property SomField : string read GetSomeField;
procedure TdmMain.GetSomeField : String;
begin
  Result := 
  SomeDataSet.FieldByName('SomeField').AsString;
end;

In your form:

procedure TForm3.SetCaption;
begin
  SomeLabel.Caption := dmMain.SomeField;
end;

A more dramatic way to avoid use of published properties is to move all published properties into the private section of the class.

This gives you best practice, however if you use dataware controls, you then should couple them manually.

So for the sake of dataware controls we keep them published, but always write properties or getter functions to get values in code.