BorlandTalk.com Borland discussion newsgroups   Archives   FAQ   Search   Memberlist   Usergroups   Register   Profile   Log in to check your private messages   Log in  How do I add a field to an existng component?          BorlandTalk.com Forum Index -> Delphi VCL Components Writing View previous topic :: View next topic   Author Message Robert Rowlands Guest Posted: Thu Aug 07, 2003 8:43 pm    Post subject: How do I add a field to an existng component? I am not good on creating new classes. I am trying to add a new string field to an existing component. I understand I need to inherit from the component and somehow create the class (or do I register it?). I have tried the following but I get an Exception at runtime. In the Interface section of the form I have put type MyAppointment = class(TAppointment) Protected Public MyNewString : string; end; And in the Initialization initialization RegisterClass(MyAppointment); I want to be able to reference the new field as follows MyAppointment.MyNewString := 'MyText'; Could someone be kind enough to point me in the right direction? Thanks. Rob. Back to top Ignacio Vazquez Guest Posted: Thu Aug 07, 2003 8:51 pm    Post subject: Re: How do I add a field to an existng component? "Robert Rowlands" <rob (AT) notathome (DOT) com> wrote in message [email]3f32ba5e (AT) newsgroups (DOT) borland.com[/email]... Quote: MyAppointment.MyNewString := 'MyText'; From the code that you've given, MyAppointment is a class, not a class instance. You can't assign values to fields on a class, you have to use a class instance. You can create a class instance from a class by using the ..Create method. Cheers, Ignacio -- No, don't send me e-mail directly. No, just don't. Back to top Kurt Barthelmess Guest Posted: Fri Aug 08, 2003 10:34 am    Post subject: Re: How do I add a field to an existng component? "Robert Rowlands" <rob (AT) notathome (DOT) com> wrote: Quote: I am not good on creating new classes. You need to understand the difference between a class and what is called an "instance" of that class. So when you see: var Form1: TForm1; "TForm1" is a class; "Form1" will be an instance of that class. Quote: I am trying to add a new string field to an existing component. I understand I need to inherit from the component and somehow create the class (or do I register it?). type MyAppointment = class(TAppointment) Protected Public MyNewString : string; end; That's all you need to do. Whenever an instance of MyAppointment is created, it will have a field named MyNewString. Quote: And in the Initialization initialization RegisterClass(MyAppointment); Unless the new class is going to be streamed or added to the IDE's palette, it does not need to be registered. If TAppointment was registered, you'll probably want to register MyAppointment as well. btw... by convention, new classes are usually prefixed with the letter "T", as in TMyAppointment. Quote: MyAppointment.MyNewString := 'MyText'; That's the equivalent of writing: TForm1.Width := 99; which doesn't work obviously. See the distinction between a class and an instance of a class here? If you had: var OneOfMyAppointments: MyAppointment; .... OneOfMyAppointments.MyNewString := 'M yText'; that would be ok (assuming OneOfMyAppointments has been initialized.) Hope that helps. Good luck. Kurt Back to top Robert Rowlands Guest Posted: Sat Aug 09, 2003 8:17 am    Post subject: Re: How do I add a field to an existng component? Thank you Kurt. Should I be able to see all the private/public/published items of the parent Class in the Object Treeview window? When my form Unit has focus (where I have the code to create TMyAppointment), TMyAppointment is shown as a Class but I can only see the JobNo property and not all of those of the parent, TAppointment. Thanks Rob. "Kurt Barthelmess (TeamB)" <kbarthelmess (AT) compuserve (DOT) com> wrote Quote: "Robert Rowlands" <rob (AT) notathome (DOT) com> wrote: I am not good on creating new classes. You need to understand the difference between a class and what is called an "instance" of that class. So when you see: var Form1: TForm1; "TForm1" is a class; "Form1" will be an instance of that class. I am trying to add a new string field to an existing component. I understand I need to inherit from the component and somehow create the class (or do I register it?). type MyAppointment = class(TAppointment) Protected Public MyNewString : string; end; That's all you need to do. Whenever an instance of MyAppointment is created, it will have a field named MyNewString. And in the Initialization initialization RegisterClass(MyAppointment); Unless the new class is going to be streamed or added to the IDE's palette, it does not need to be registered. If TAppointment was registered, you'll probably want to register MyAppointment as well. btw... by convention, new classes are usually prefixed with the letter "T", as in TMyAppointment. MyAppointment.MyNewString := 'MyText'; That's the equivalent of writing: TForm1.Width := 99; which doesn't work obviously. See the distinction between a class and an instance of a class here? If you had: var OneOfMyAppointments: MyAppointment; ... OneOfMyAppointments.MyNewString := 'M yText'; that would be ok (assuming OneOfMyAppointments has been initialized.) Hope that helps. Good luck. Kurt Back to top Kurt Barthelmess Guest Posted: Sat Aug 09, 2003 12:01 pm    Post subject: Re: How do I add a field to an existng component? "Robert Rowlands" <rob (AT) notathome (DOT) com> wrote: Quote: Should I be able to see all the private/public/published items of the parent Class in the Object Treeview window? Only the published ones. Quote: When my form Unit has focus (where I have the code to create TMyAppointment), TMyAppointment is shown as a Class but I can only see the JobNo property and not all of those of the parent, TAppointment. Either TAppointment has to publish them, or TMyAppointment. As a descendant, TMyAppointment can "promote" a property of TAppointment to published status by simply writing: property MyAncestorsProperty; in the published section. Good luck. 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