Where are the Delphi Attributes Real World Examples?

Question

I know by TMS Aurelius that we can use the \"new\" 2010 attributes feature to serialize database table fields into object properties at run-time, for example, and I am not an ex

Answer 1

If you want to declare you own attribute, you can do it like this:

type
  TDisplayLabelAttribute = class(TCustomAttribute)
  private
    FText: string;
  public
    constructor Create(const aText: string);
    property Text: string read FText write FText;
  end;

An attribute is a regular class, that has the TCustomAttribute as its ancestor. You implement it as usual:

implementation

constructor TDisplayLabelAttribute.Create(const aText: string);
begin
  FText := aText;
end;

Now the attribute is declared and implemented, you can just use it:

[DisplayLabel('My Class')]
TMyClass = class
end;

So now you have an attribute declared and implemented and you have used it to add a display label to some class. The final phase is to use that attribute, since you have a class decorated with it. The code that uses the attribute does not resides in the attribute nor the decorated class, it is implemented in the service layer that will use the decoration.

Let's say we have a class that returns a possible display label for a class:

type
  TArtifactInspector = class
  public
    class function DisplayLabelFor(aClass: TClass): string;
  end;

That method will inspect a class and return its display label, given it exists. Otherwise it returns an empty string:

implementation

uses
  Rtti;

class function TArtifactInspector.DisplayLabelFor(aClass: TClass): string;
var
  rttiContext: TRttiContext;
  rttiType: TRttiType;
  attribute: TCustomAttribute;
begin
  rttiContext := TRttiContext.Create;
  try
    rttiType := rttiContext.GetType(aClass);
    for attribute in rttiType.GetAttributes do
      if attribute is TDisplayLabelAttribute then
        Exit(TDisplayLabelAttribute(attribute).Text);
    Result := '';
  finally
    rttiContext.Free;
  end; // try to recover and return the DisplayLabel
end;