BorlandTalk.com Borland discussion newsgroups   Archives   FAQ   Search   Memberlist   Usergroups   Register   Profile   Log in to check your private messages   Log in  Null Values          BorlandTalk.com Forum Index -> Delphi OO design View previous topic :: View next topic   Author Message darkattraction Guest Posted: Thu Oct 16, 2003 7:56 pm    Post subject: Null Values I'm a newbie and I read a lot about Business Object (amber, Joanna Carter... ) .... and I wonder how can I handle null values if I write for example, TEmployee=class(TPersistentObject) published ... property Birthdate: TDateTime... ... end; Have I to define a class like TDateTimeProperty as to handle null value or dictate 0 is null - but how to with integers and doubles? Mark Back to top Joanna Carter Guest Posted: Thu Oct 16, 2003 10:19 pm    Post subject: Re: Null Values | Have I to define a class like TDateTimeProperty as to handle null Quote: value or dictate 0 is null - but how to with integers and doubles? I use an Attribute framework for all value types and that has the concept of a null value for any type in the system. Joanna -- Joanna Carter Consultant Software Engineer TeamBUG support for UK-BUG TeamMM support for ModelMaker Back to top Franz-Leo Chomse Guest Posted: Fri Oct 17, 2003 9:00 am    Post subject: Re: Null Values Quote: Have I to define a class like TDateTimeProperty as to handle null value or dictate 0 is null - but how to with integers and doubles? Since most data types have no specific value for NULL you have to use an additional flag. In case of database fields these flags are part of the record buffer for properties you either have to create an attribute framework or need some kind of reflection support which is not available for the normal Delphi and Kylix, but part of the .NET CLR Regards from Germany Franz-Leo Back to top darkattraction Guest Posted: Sat Oct 18, 2003 2:17 pm    Post subject: Re: Null Values I understand I have to use some kind of attribute/property framework but I don't see in which way reflection can be useful. Could you explain it? Mark "Franz-Leo Chomse" <franz-leo.chomse (AT) samac (DOT) de> wrote Quote: Have I to define a class like TDateTimeProperty as to handle null value or dictate 0 is null - but how to with integers and doubles? Since most data types have no specific value for NULL you have to use an additional flag. In case of database fields these flags are part of the record buffer for properties you either have to create an attribute framework or need some kind of reflection support which is not available for the normal Delphi and Kylix, but part of the .NET CLR Regards from Germany Franz-Leo Back to top Franz-Leo Chomse Guest Posted: Mon Oct 20, 2003 9:07 am    Post subject: Re: Null Values On Sat, 18 Oct 2003 16:17:02 +0200, "darkattraction" <darkattraction (AT) free (DOT) fr> wrote: Quote: I understand I have to use some kind of attribute/property framework but I don't see in which way reflection can be useful. Could you explain it? A powerfull reflection framework contains the attribute framework as it allows to support metadata. So if there isn't already a metadata for NULL you can add a user defined attribute for it. Regards from Germany Franz-Leo Back to top Joanna Carter Guest Posted: Mon Oct 20, 2003 12:57 pm    Post subject: Re: Null Values Franz-Leo Chomse wrote: Quote: A powerfull reflection framework contains the attribute framework as it allows to support metadata. So if there isn't already a metadata for NULL you can add a user defined attribute for it. Just a quick example: TAttribute = class .... protected function IsNull: Boolean; virtual; abstract; procedure SetNull; virtual; abstract; end; TDateAttribute = class(TAttribute) private fValue: TDateTime; .... public function IsNull: Boolean; override; procedure SetNull; override; end; TIntegerAttribute = class(TAttribute) private fIsNull: Boolean; fValue: Integer; .... public constructor Create; ... procedure SetValue(Value: Integer); function IsNull: Boolean; override; procedure SetNull; override; end; implementation const NullDateTime = -693594; function TDateAttribute.IsNull: Boolean; begin Result := fValue = NullDateTime; end; procedure TDateAttribute.SetNull; begin fValue := NullDateTime; end; constructor TIntegerAttribute.Create; begin inherited Create; SetNull; end; procedure TIntegerAttribute.SetValue(Value: Integer); begin fValue := Value; fIsNull := False; end; function TIntegerAttribute.IsNull: Boolean; begin Result := fIsNull; end; procedure TIntegerAttribute.SetNull; begin fValue := 0; fIsNull := True; end; Joanna -- Joanna Carter Consultant Software Engineer TeamBUG support for UK-BUG TeamMM support for ModelMaker Back to top Ritchie Annand Guest Posted: Tue Oct 21, 2003 9:40 am    Post subject: Re: Null Values In article <3f93dc5a$1 (AT) newsgroups (DOT) borland.com>, [email]joannac (AT) btinternet (DOT) com[/email] says... Quote: Franz-Leo Chomse wrote: | A powerfull reflection framework contains the attribute framework as | it allows to support metadata. So if there isn't already a metadata | for NULL you can add a user defined attribute for it. Just a quick example: TAttribute = class ... protected function IsNull: Boolean; virtual; abstract; procedure SetNull; virtual; abstract; end; They apparently were concerned about this in .NET, too, to read the docs. They made an entire separate tree for values in SQL, the only difference being that the SQL/DB-related ones had an extra property for null. http://msdn.microsoft.com/library/default.asp?url=/library/en- us/cpref/html/frlrfsystemdatasqltypes.asp They all implement INullable, too. Whee :) -- Ritchie Annand Senior Software Architect Malibu Software & Engineering Ltd. Business: http://www.malibugroup.com Personal: http://nimble.nimblebrain.net Wiki: http://wiki.nimblebrain.net Back to top Aivar Annamaa Guest Posted: Wed Oct 29, 2003 8:36 am    Post subject: Re: Null Values Joanna Carter wrote: Quote: TIntegerAttribute = class(TAttribute) private fIsNull: Boolean; fValue: Integer; .... procedure SetValue(Value: Integer); If I understand correctly, then declaration of a domain class would be similar to this: TPerson = class published property Weight:TIntegerAttribute read getWeight write setWeight; .... end; But that way TPerson class can't really control changing the value of Weight in SetWeight, because client can use person.Weight.setValue(whaterver); For this reason it seems to me very cumbersome to use objects for representing primitive values. What about using Variants instead? One could define several "variant types" like this TIntegerVariant = Variant; TStringVariant = Variant; .... to allow showing info about the real attribute type in class declaration. Aivar Back to top Joanna Carter Guest Posted: Wed Oct 29, 2003 9:12 am    Post subject: Re: Null Values Aivar Annamaa wrote: Quote: If I understand correctly, then declaration of a domain class would be similar to this: TPerson = class published property Weight:TIntegerAttribute read getWeight write setWeight; ... end; Not quite... TAttributeList = class procedure Add(const Attrib: TAttribute); .... end; TObjectAttribute = class(TAttribute) private fAttributes: TAttributeList; protected function GetAttributes: TAttributeList; .... end; TPerson = class(TObjectAttribute) public constructor Create; property Weight:Integer read GetWeight write SetWeight; .... end; constructor TPerson.Create; begin inherited Create; GetAttributes.Add(TIntegerAttribute.Create('Weight')); ... end; function GetWeight: Integer; begin Result := TIntegerAttribute(GetAttributes['Weight']).GetValue; end; procedure SetWeight(Value: Integer); begin TIntegerAttribute(GetAttributes['Weight']).SetValue(Value); end; Joanna -- Joanna Carter Consultant Software Engineer TeamBUG support for UK-BUG TeamMM support for ModelMaker Back to top Solerman Kaplon Guest Posted: Wed Oct 29, 2003 11:07 am    Post subject: Re: Null Values Hi, There are many possible variations of knowing when an attibute changes, would be a good thing to use the observer pattern on the attribute to know when it changes? possible allowing some kind of constraint/validation manager connect to it and provide ways of validating data. Is this the right place to do it? I mean, the data may get an invalid status, and the presentation layer can observe the validation manager to determine what to do. The validation can take place before the data gets assigned to the internal fields, providing a just-in-time validation when appopriate. Do you see any performance problems in this approach? just wondering... :) Solerman Joanna Carter wrote: Quote: Not quite... TAttributeList = class procedure Add(const Attrib: TAttribute); ... end; TObjectAttribute = class(TAttribute) private fAttributes: TAttributeList; protected function GetAttributes: TAttributeList; ... end; TPerson = class(TObjectAttribute) public constructor Create; property Weight:Integer read GetWeight write SetWeight; ... end; constructor TPerson.Create; begin inherited Create; GetAttributes.Add(TIntegerAttribute.Create('Weight')); ... end; function GetWeight: Integer; begin Result := TIntegerAttribute(GetAttributes['Weight']).GetValue; end; procedure SetWeight(Value: Integer); begin TIntegerAttribute(GetAttributes['Weight']).SetValue(Value); end; Back to top Aivar Annamaa Guest Posted: Wed Oct 29, 2003 11:21 am    Post subject: Re: Null Values Thanks Joanna! So, you are using a collection of attributes in your BO-s and provide direct links to the items of that collection via properties, right? And such way one doesn't have to mess with RTTI but can use GetAttributes to inspect a BO of unknown type (when persisting or displaying an object), yes? But still, about assigning null to a person's weight, it seems that it's possible only via attributes collection, not via respective property. And if client wants to read "weight" it has to check from the attribute collection if the weight property represents currently real weight or the actual value is not known? If so, then whats the use of this integer property at all? (besides that it communicates the structure of the object) Aivar Back to top Joanna Carter Guest Posted: Wed Oct 29, 2003 12:00 pm    Post subject: Re: Null Values Solerman Kaplon wrote: Quote: There are many possible variations of knowing when an attibute changes, would be a good thing to use the observer pattern on the attribute to know when it changes? possible allowing some kind of constraint/validation manager connect to it and provide ways of validating data. Is this the right place to do it? I mean, the data may get an invalid status, and the presentation layer can observe the validation manager to determine what to do. The validation can take place before the data gets assigned to the internal fields, providing a just-in-time validation when appopriate. Do you see any performance problems in this approach? just wondering... You seem to have got the right idea. Performance is only going to be a problem in batch update situations, but time between single updates usually measured in seconds due to user inefficiency rather than in milliseconds due to extra code. Joanna -- Joanna Carter Consultant Software Engineer TeamBUG support for UK-BUG TeamMM support for ModelMaker Back to top Joanna Carter Guest Posted: Wed Oct 29, 2003 12:07 pm    Post subject: Re: Null Values Aivar Annamaa wrote: Quote: So, you are using a collection of attributes in your BO-s and provide direct links to the items of that collection via properties, right? Right Quote: And such way one doesn't have to mess with RTTI but can use GetAttributes to inspect a BO of unknown type (when persisting or displaying an object), yes? Absolutely Quote: But still, about assigning null to a person's weight, it seems that it's possible only via attributes collection, not via respective property. Assigning null to an attribute is not going to be something that a user can do direcly. It is usually something that would be done in response to a user clearing an edit. If you use a GUI framework like MVP, then that is something that the Interactopr wouold detect and then use the metadata provided by the attribute framework to call the attribute's SetNull method. Quote: And if client wants to read "weight" it has to check from the attribute collection if the weight property represents currently real weight or the actual value is not known? If so, then whats the use of this integer property at all? (besides that it communicates the structure of the object) The only clients that are really going to have to do any talking to the IsNull state of an attribute are the OPF and GUI and both of these. Talking to the properties of a BO when the state of the underlying attribute is null would usually yield the default nil/zero value. Unless you can think of a better way of doing it?? Joanna -- Joanna Carter Consultant Software Engineer TeamBUG support for UK-BUG TeamMM support for ModelMaker Back to top Aivar Annamaa Guest Posted: Wed Oct 29, 2003 1:01 pm    Post subject: Re: Null Values Joanna Carter wrote: Quote: Unless you can think of a better way of doing it?? Unfourtunately not :) If attributes were variants, then one could use same property both for setting/getting nulls and other values. PHP is my main language, so I'm used with loose types, but in Delphi I don't feel right when using Variants. So, I'm not happy with using variant attributes and I will consider using a collection of attributes. Thanks for help! 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