BorlandTalk.com Borland discussion newsgroups   Archives   FAQ   Search   Memberlist   Usergroups   Register   Profile   Log in to check your private messages   Log in  Re: OO Newby - Class diagram problem          BorlandTalk.com Forum Index -> Delphi OO design View previous topic :: View next topic   Author Message Bob Dawson Guest Posted: Wed Jul 09, 2003 1:44 pm    Post subject: Re: OO Newby - Class diagram problem "William Buchanan" wrote Quote: which spans over a couple of tables. The users are going to be able to open each email and edit all the data on it then save the data to the db. Then we're reading and writing properties of the TEmail object, rather than of a recordset or table drectory--good. Quote: I was stuck on trying to decide if the GUI button to, for example load an email, would call a method of the user to load the email, which would in turn call a method of TEmail to load the data from the db, or would the GUI button call the load method of TEmail directly.... or am I completely on the wrong track. For now, I'd say let the GUI call the email's load method. I don't see any reason to couple the user object to the email object--they don't have to know about each other. General rule of thumb here is that each object touches/knows about the least number of other objects possible (called the 'Law of Demeter'). As for the GUI, first think about your application as a series of layers: GUI Quote: business objects storage Higher layers can know about lower layers, but not the reverse, so there's really nothing wrong with having the a button call the AnEmail.Load() method, so long as the TEmail object doesn't know about the GUI (The form writes to the email object and reads from it, never the reverse.) In a more formal/complex system, we create portal layers-- GUI presenter or controller //handles transfers between your business objects and //your GUI, and may contain logic for dynamic object //interaction / business objects data access //handles transfers between your business objects and //your storage, and may contain optimizing logic (object //caches, metadata objects, SQL generators, etc.) / persistence storage The presenter or controller is part of a model-view-controller/presenter (MVC or MVP) pattern, and the data access layer is the center of a similar three-part pattern known as an OPF (object persistence framework) Because user types may change viewing or editing rights, you need to think about that as a general problem and write a security manager object to handle how rights are negotiated. That object could provide you with either a boolean or scaled response to the 'Can object X do Y to object C?' question in a general way that might look something like this: public function HasRights(Actor : ISecurityRequestor; DirectObject : ISecuredEntity; const Activity : string) : boolean; Any user, machine, or even program process could implement the ISecurityRequestor interface, and any business object implementing ISecuredEntity can be controlled for available behaviour. Using a rights manager with interface (or could be ancestor) parameters allows your user and email objects to determine security levels without any actual dependencies on each other. Quote: I hope this explains my problem (which i'm sure is a pretty stupid one). It's a core question--not a stupid one at all. And I hope this helps to answer it. Here's an older paper about layered architecture that's pretty good: http://members.aol.com/kgb1001001/Articles/LAYERS/appmod.htm If you need information about MVC/MVP patterns or OPFs, they're sort of favorite topics around here... :-) bobD Back to top William Buchanan Guest Posted: Wed Jul 09, 2003 1:46 pm    Post subject: Re: OO Newby - Class diagram problem Hi Phil Again, many thanks. That clears up my dilemma, and also answers a couple of things i've been pondering over! Regards Will "Phil Shrimpton" <phil (AT) nospam (DOT) co.uk> wrote Quote: In article <3f0bc7f1$1 (AT) newsgroups (DOT) borland.com>, William Buchanan says... Hi, I am designing a system which will send marketing email to people who subscribe to our service. I will have different levels of user with different rights, e.g. designer, authoriser, progress tracker. I was stuck on trying to decide if the GUI button to, for example load an email, would call a method of the user to load the email, which would in turn call a method of TEmail to load the data from the db, or would the GUI button call the load method of TEmail directly.... or am I completely on the wrong track. OK, so you TUser is a class that holds information about the current 'person' using the app? If that is correct, I don't thing the loadEmail() method should be part of the TUser class. What I would do is call the TEmail.load() method from the UI, and depending on how you have implemented your 'user rights' either pass it a role or the TUser class itself. For instance... If your TUser class has a 'Role' property (e.g. containing 'authoriser'), you could.... MyEmail := TEmail.load(CurrentUser.getRole()); If you have a separate 'rights management class', you could... MyEmail := TEmail.load(CurrentUser); ..then in the load method, you could do... CurrentRight = TRightsManager.getLevel(CurrentUser); case CurrentRight of rAuthoriser : DoSomething(); Cheers Phil Back to top William Buchanan Guest Posted: Wed Jul 09, 2003 2:00 pm    Post subject: Re: OO Newby - Class diagram problem Hi Bob Again, many thanks for taking the time to reply.... That's very interesting, and as a result I will need to do some major changes! I have just linked classes together as needed but can see that a good structure is required. Will have a go at that! Regards Will "Bob Dawson" <bdawson (AT) idtdna (DOT) com> wrote Quote: "William Buchanan" wrote which spans over a couple of tables. The users are going to be able to open each email and edit all the data on it then save the data to the db. Then we're reading and writing properties of the TEmail object, rather than of a recordset or table drectory--good. I was stuck on trying to decide if the GUI button to, for example load an email, would call a method of the user to load the email, which would in turn call a method of TEmail to load the data from the db, or would the GUI button call the load method of TEmail directly.... or am I completely on the wrong track. For now, I'd say let the GUI call the email's load method. I don't see any reason to couple the user object to the email object--they don't have to know about each other. General rule of thumb here is that each object touches/knows about the least number of other objects possible (called the 'Law of Demeter'). As for the GUI, first think about your application as a series of layers: GUI | business objects | storage Higher layers can know about lower layers, but not the reverse, so there's really nothing wrong with having the a button call the AnEmail.Load() method, so long as the TEmail object doesn't know about the GUI (The form writes to the email object and reads from it, never the reverse.) In a more formal/complex system, we create portal layers-- GUI presenter or controller //handles transfers between your business objects and //your GUI, and may contain logic for dynamic object //interaction / business objects data access //handles transfers between your business objects and //your storage, and may contain optimizing logic (object //caches, metadata objects, SQL generators, etc.) / persistence storage The presenter or controller is part of a model-view-controller/presenter (MVC or MVP) pattern, and the data access layer is the center of a similar three-part pattern known as an OPF (object persistence framework) Because user types may change viewing or editing rights, you need to think about that as a general problem and write a security manager object to handle how rights are negotiated. That object could provide you with either a boolean or scaled response to the 'Can object X do Y to object C?' question in a general way that might look something like this: public function HasRights(Actor : ISecurityRequestor; DirectObject : ISecuredEntity; const Activity : string) : boolean; Any user, machine, or even program process could implement the ISecurityRequestor interface, and any business object implementing ISecuredEntity can be controlled for available behaviour. Using a rights manager with interface (or could be ancestor) parameters allows your user and email objects to determine security levels without any actual dependencies on each other. I hope this explains my problem (which i'm sure is a pretty stupid one). It's a core question--not a stupid one at all. And I hope this helps to answer it. Here's an older paper about layered architecture that's pretty good: http://members.aol.com/kgb1001001/Articles/LAYERS/appmod.htm If you need information about MVC/MVP patterns or OPFs, they're sort of favorite topics around here... :-) bobD Back to top Bob Dawson Guest Posted: Wed Jul 09, 2003 2:54 pm    Post subject: Re: OO Newby - Class diagram problem "William Buchanan" wrote Quote: That's very interesting, and as a result I will need to do some major changes! I have just linked classes together as needed but can see that a good structure is required. Will have a go at that! Last piece of advice--OO and architectural thinking are there to help manage and contain complexity--don't go overboard making everything exactly 'correct' if the actual problem is really simple--building the Library of Congress cataloging system helps keep a library managable, but it's a bit much to undertake if what you really have is the bookshelf next to your desk. Pragmatics always. Happy to help, and good luck. bobD Back to top Phil Shrimpton Guest Posted: Wed Jul 09, 2003 4:11 pm    Post subject: Re: OO Newby - Class diagram problem In article <3f0c1c96 (AT) newsgroups (DOT) borland.com>, William Buchanan says... Hi, Quote: That clears up my dilemma, and also answers a couple of things i've been pondering over! At least it does not create more questions <g> Phil Back to top Marjan Venema Guest Posted: Wed Jul 09, 2003 5:19 pm    Post subject: Re: OO Newby - Class diagram problem Quote: At least it does not create more questions <g Just wait a bit longer... 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