BorlandTalk.com Borland discussion newsgroups   Archives   FAQ   Search   Memberlist   Usergroups   Register   Profile   Log in to check your private messages   Log in  Is this the right approach (singleton question) ?          BorlandTalk.com Forum Index -> Delphi OO design View previous topic :: View next topic   Author Message Mark Smith Guest Posted: Sat Nov 08, 2003 6:21 pm    Post subject: Is this the right approach (singleton question) ? As I develop my first "fully-oo" app (wow, what a difference) I am finding myself creating singleton after singleton called, for example: TWebManager - handles dispatches to website and connects the app with web resources TListManager - handles "lists" (customers, users, etc). TeEventManager - manager for internal event model TeExceptionManager - handles exceptions gracefully TLicenseManager - handles software licensing (what parts are enabled, etc) This seems to make sense, but I want to make sure I'm not just "manager-happy". Like I said, this is my first OO project, and while I've discovered the power of singletons, are there better patterns for this? Maybe I'm using singletons just because they are all I know. The "code smell" I see - and maybe there are others I don't see - is the inclusion of manager after manager in the uses clauses of my units. Should I put all managers in one unit? Or should I create a "managers.pas" file that contains lots of {$I WebMgr.pas} type statements? Or is this not really a "code smell"? Thanks in advance for any comments. Best Regards, Mark Smith Back to top Wayne Niddery [TeamB] Guest Posted: Sun Nov 09, 2003 2:08 am    Post subject: Re: Is this the right approach (singleton question) ? Mark Smith wrote: Quote: As I develop my first "fully-oo" app (wow, what a difference) I am finding myself creating singleton after singleton called, for example: TWebManager - handles dispatches to website and connects the app with web resources TListManager - handles "lists" (customers, users, etc). TeEventManager - manager for internal event model TeExceptionManager - handles exceptions gracefully TLicenseManager - handles software licensing (what parts are enabled, etc) Like I said, this is my first OO project, and while I've discovered the power of singletons, are there better patterns for this? Maybe I'm using singletons just because they are all I know. Well, I expect you know your Customer, Users, etc will not be singletons. <g> You would have to provide more detail on each of the above before anyone could really give advice. Off-hand, I'm not sure what the List manager really does for you, and I'm not sure the Web Manager should be a singleton, the web is a big place with lots of different things that can be done - what is a single instance of a single class going to do? Quote: The "code smell" I see - and maybe there are others I don't see - is the inclusion of manager after manager in the uses clauses of my units. That's alright because in future projects you may want some and not others. Quote: Should I put all managers in one unit? Absolutely not, they are not related - that they all "manage" something is not at all sufficient to group them. Quote: Or should I create a "managers.pas" file that contains lots of {$I WebMgr.pas} type statements? That would be even worse. Quote: Or is this not really a "code smell"? Can't smell anything from here at this level of detail. <g> -- Wayne Niddery - Logic Fundamentals, Inc. (www.logicfundamentals.com) RADBooks: http://www.logicfundamentals.com/RADBooks.html "It is error alone which needs the support of government. Truth can stand by itself." - Thomas Jefferson Back to top Mark Smith Guest Posted: Sun Nov 09, 2003 3:34 am    Post subject: Re: Is this the right approach (singleton question) ? Wayne, Thanks for taking the time to respond. I'll try to clarify. Quote: Off-hand, I'm not sure what the List manager really does for you As an InstantObjects user, I have access to the objects (Customers,Users) via the framework. However, when loading 1000-3000 customers (avg for my users) it is very, very slow (8 min. to iterate the entire list of 1200 in one test I did). So the list manager bypasses the framework and loads the "list-type" objects into memory tables at the start of the user's session, then when the user asks for the list it performs a SQL query to find objects with a "LastUpdated" time-stamp that is later than when the list was last loaded/refreshed. It then retrieves just those items and presents a list to the user. This results in speed increases on the order of thousands-fold. So in a nutshell, the list manager makes sure "list-type" objects are kept in memory tables and kept fresh, outside the scope of the IO framework. I hope that was clear. Quote: and I'm not sure the Web Manager should be a singleton, the web is a big place with lots of different things that can be done - what is a single instance of a single class going to do? I guess it should be called a WebsiteGateway rather than a WebManager since it really provides a single point of access for my app to connect with my website. Functions such as WebManager.SurfTo( aURL : string ) are present etc. as well as constants for the URL parameter such as URL_HOMEPAGE, URL_COMMUNITY, URL_FAQ, etc. I think it was the writing of this "manager" more than any other that prompted my posting, because it seemed as if I were moving global variables and functions into a singleton, which in itself feels like one BIG global variable. So now with my "managers" I feel as if I have 5-10 massive globals instead of 50-100 little ones. Better, but still globals. Maybe there's just no way around that. In my last version of this app I had a file called "eCommon.pas" that contained hundreds of types, consts, functions, etc that many places in the code needed to access. While the singletons clean this up by grouping related items into classes called "managers" I still think they are just globals with another name. Quote: The "code smell" I see - and maybe there are others I don't see - is the inclusion of manager after manager in the uses clauses of my units. That's alright because in future projects you may want some and not others. Good point. I haven't thought past the scope of this project at the moment. Quote: Should I put all managers in one unit? Absolutely not, they are not related - that they all "manage" something is not at all sufficient to group them. Thank you, that settles that. <g> Quote: Or should I create a "managers.pas" file that contains lots of {$I WebMgr.pas} type statements? That would be even worse. I thought so too. Quote: Can't smell anything from here at this level of detail. <g Maybe I'm too cerebral and everything's actually OK. I'm just afraid I'm missing something because I thought the "OO-way" avoided global variables. ;) Quote: You would have to provide more detail on each of the above before anyone could really give advice. The WebManager and ListManager are covered above. Here are the others: The TEventManager maintains a list of TEventDispatchers. TEvents can be fired out via the manager to all the dispatchers, which ultimately deliver them to a HandleEvent() function. So my business objects, forms, etc. need only to declare a private TEventDispatcher (which self-registers as a Subject with the TEventManager Observer) and wire it to a HandleEvent() function to receive and process system wide notifications when things happen. So if a customer is edited in one form, and the object is saved, a TeObjectChangedEvent fires, the TListManager catches it, and updates the Customer List so the window the user happens to have open is properly refreshed with the new information. The TLicenseManager is called by various parts of the program to see if they are "allowed" to run based on purchased licensing. When the program launches the license is established in the singleton and then anyone can call it. As I said before, one big global variable. The TeExceptionManager hooks into Application.OnException for global exception handling, again, it's an object doing the work of what could just as easily be a few global functions isolated in a unit. Again, thanks for your reply, I realize this is very long but I thought a complete reply was in order. 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