Delphi and TPropertyObserver

In Delphi you can use RTTI (RunTime Type Information) to get information about objects at runtime without knowing about them at compile time. To find out more about RTTI, you should check out these articles by Robert Love.

I used the RTTI to make an observer, that informs me, whenever a specific property gets changed. It is far from being complete, there are a lot of TypeKinds that I didn't implement a compare for. The observer could also be extended to support multiple properties.

uses
  Rtti, Classes;

type
  TProc = reference to procedure;
  TPropertyObserver = class
  private
    fTerminated : boolean;
  public
    constructor Create(aInstance : TObject; aPropertyName : string; aOnChange : TProc);
    procedure Terminate;
  end;

implementation

uses
  SysUtils, TypInfo;

{ TPropertyObserver }

constructor TPropertyObserver.Create(aInstance: TObject; aPropertyName: string; aOnChange : TProc);
begin
  TThread.CreateAnonymousThread(procedure
  var
    ctx : TRttiContext;
    t : TRttiType;
    oldValue, curValue : TValue;
    b : boolean;
    p : TRttiProperty;
  begin
    ctx := TRttiContext.Create;
    try
      t := ctx.GetType(aInstance.ClassType);
      p := t.GetProperty(aPropertyName);
      oldValue := p.GetValue(aInstance);
      while not fTerminated do
      begin
        curValue := p.GetValue(aInstance);
        b := false;
        case curValue.Kind of
          tkInt64,
          tkEnumeration,
          tkInteger: b := oldValue.AsOrdinal = curValue.AsOrdinal;
          tkWChar,
          tkLString,
          tkWString,
          tkString,
          tkUString,
          tkChar: b := oldValue.AsString = curValue.AsString;
          tkFloat: b := oldValue.AsExtended = curValue.AsExtended;
          // Some of these should get a compare, too
          tkUnknown: ;
          tkSet: ;
          tkClass: ;
          tkMethod: ;
          tkVariant: ;
          tkArray: ;
          tkRecord: ;
          tkInterface: ;
          tkDynArray: ;
          tkClassRef: ;
          tkPointer: ;
          tkProcedure: ;
        end;
        if not b then
        begin
          aOnChange;
          oldValue := curValue;
        end;
        sleep(10);
      end;
      finally
      ctx.Free;
    end;
  end).Start;
end;

procedure TPropertyObserver.Terminate;
begin
  fTerminated := true;
end;

And how it is used:

procedure TForm1.FormCreate(Sender: TObject);
begin
  fObserver := TPropertyObserver.Create(self,'Left',procedure
  begin
    TThread.Synchronize(TThread.CurrentThread,procedure // Changing the VCL has to be done in the main thread
    begin
      edit1.Text := IntToStr(self.Left);
    end);
  end);
end;

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