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

Not sure if the question is asking for real world examples of attribute use or how to serialize db tables into objects using attributes. The example below is a contrived simple one (but an example none the less) showing how to use attributes to log changes to object properties.

Define your custom attribute

//By convention attributes are *not* prefixed with a `T` 
//and have the word `Attribute` in their name
LoggableAttribute = class(TCustomAttribute)
  private
    FDescription : String;
  public
    constructor Create(Description: String);
    property Description: String read FDescription;
  end;

The "hello world" of classes TProduct using the attribute

TProduct = Class(TObject)
   private
    FPrice: Double;
    FDescription: String;
    ..
   public  
    [LoggableAttribute('Product Price')]
    property Price : Double read FPrice write SetPrice;
    [Loggable('Product Description')]   {the `Attribute` part is optional}
    property Description : String read FDescription write SetDescription;
    property IsDirty : Boolean read FIsDirty;
  End;

Any class that has a "loggable attribute" can be passed to this method to iterate through the properties and log them.

procedure LogChanges(LoggableClass: TObject);
var
 c : TRttiContext;
 t : TRttiType;
 p : TRttiProperty;
 a : TCustomAttribute;
 Value : TValue;
begin
 c := TRttiContext.Create;    
 try
   t := c.GetType(LoggableClass.ClassType);
   for p in t.getProperties do
     for a in p.GetAttributes do
       if a is TLoggableProperty then begin
         Value := p.GetValue(LoggableClass);   
         // log to db.. 
         AddLogEntry(p.Name, TLoggableProperty(a).Description, Value.ToString);
       end;
 finally
   c.Free;
 end;

end;

Example of use:

var
 P : TProduct;
begin    
 P := TProduct.Create; 
 P.LoadPropertiesFromDB;
 ...
 ... User edits price ...    
 ... 
 P.Price := 499.99;
 ...
 ... Save product to DB 
 if P.IsDirty then  // save and log
   LogChanges(P);