I've written a small COM Server in Delphi 2010 that acts as a plug-in into a retail application. The retail application looks for a "discover" interface which registers any number of additional interfaces calling TAutoObjectFactory.Create for each one. This is working just fine--all the plug-in interfaces function as designed.
But now I'd like to call a public method of one interface from another interface so I don't have to duplicate code. Seems simple enough, just call ComClassManager.ForEachFactory looking for the ClassID of the interface I need to use. Got that working, too!
But now that I found the class, I'm stumped by a seemingly trivial final step: how to use or cast the class (or class reference?) I've located to actually call one of its methods.
In the "FactoryProc" I've sent to ForEachFactory, I assume the ComClass property of TComObjectFactory is what I'm after, but it's of type TClass, a class reference to the actual class object to which it points (at least I hope I'm understanding this correctly). I'm a little fuzzy on class references and my attempts to cast or otherwise de-reference this property has resulted in access violations or compiler errors.
Any suggestions?
You're right in your comment, ComClassManager deals with classes, not instances. What you need is (your application-local implementation of) running object table (or something similar), so plugin instances can interact with each other.
How to actually implement it depends on what you really need, e.g. call methods on all running instances, or only on instances of specific classes.
Related
We have an app that makes fairly extensive use of TIniFile. In the past we created our own descendant class, let's call it TMyIniFile, that overrides WriteString. We create one instance of this that the entire app uses. That instance is passed all around through properties and parameters, but the type of all of these is still TIniFile, since that is what it was originally. This seems to work, calling our overridden method through polymorphism, even though all the variable types are still TIniFile. This seems to be proper since we descend from TIniFile.
Now we are making some changes where we want to switch TMyIniFile to descend from TMemIniFile instead of TIniFile. Those are both descendants of TCustomIniFile. We'll also probably be overriding some more methods. I'm inclined to leave all the declarations as TIniFile even though technically our class is no longer a descendant of it, just to avoid having to change a lot of source files if I don't need to.
In every tutorial example of polymorphism, the variable is declared as the base class, and an instance is created of the descendant class and assigned to the variable of the base class. So I assume this is the "right" way to do it. What I'm looking at doing now will end up having the variables declared as, what I guess you'd call a "sibling" class, so this "seems wrong". Is this a bad thing to do? Am I asking for trouble, or does polymorphism actually allow for this sort of thing?
TIniFile and TMemIniFile are distinct classes that do not derive from each other, so you simply cannot create a TMemIniFile object and assign it to a TIniFile variable, and vice versa. The compiler won't let you do that. And using a type-cast to force it will be dangerous.
You will just have to update the rest of your code to change all of the TIniFile declarations to TCustomIniFile instead, which is the common ancestor for both classes. That is the "correct" thing to do.
The compiler is your friend - why would you lie to it by using the wrong type ... and if you do lie to it why would you expect it to know what you want it to do?
You should use a base class that you derive from, like TCustomIniFile. I would expect compile issues if you are trying to make assignments which are known at compile time to be wrong.
The different classes have different signatures so the compiler needs to know which class it is using to call the correct method or access the correct property. With virtual methods the different classes setup their own implementation of those methods so that the correct one is called - so using a pointer to a base type when you call the virtual method it calls that method in the derived type because it is in the class vtable.
So if the code does compile, it's very likely that the compiler will not be doing the right thing ...
I have classes that have a constructor such as
constructor Create(Factory: IFactory<IConnection>)
When I try and register the IFactory in the container
Container.RegisterType<IFactory<IConnection>,TConnectionFactory>
or
Container.RegisterType<TConnectionFactory>.Implements<IFactory<IConnection>>
I get an error stating that the interface does not have a guid.
I don't really want to add lots of pointless interfaces such as
IConnectionFactory = interface(IFactory<IConnection>)
['{45106BA8-43E7-4D26-B0EF-1639871B93E4}']
end;
to get around this but is this the only way?
Many thanks
As you found out the generic interface can have a GUID.
That itself causes no harm until you QueryInterface/Supports IList<string> from something that is an IList<Integer> which would erroneously succeed and subsequently fail once you start working with it. FWIW since you mentioned those types while the collection interfaces all have guids there is no need to ever do such querying when using them but they are required internally which then stays within the same generic type argument.
Currently the GUID is being used to check and get the interface from the implementing object because the RTL only supports doing that with a GUID and not with typeinfo.
In fact following code would not raise an exception during the registration but eventually cause defects when being resolved:
RegisterType<IFactory<IConnection>, TSomeFactory> where TSomeFactory implements IFactory<ISomethingElse>
However there is some rather hidden typeinfo available (see the commented line in System.TInterfaceTable) that has the exact typeinfo of the implemented interfaces. Spring4D internally uses that at some places, for example Spring.Reflection.TRttiTypeHelper.GetInterfaces.
That could be used but then there is another catch: generic types across multiple modules have different typeinfo. So it's not so easy to simply use that information to validate during registration and query the interface from the implementing class because right now the container (via some extension) supports registrations and dependencies across multiple modules.
Making the registration more robust and if possible remove the requirement for having a GUID on the interface is something I have on my list for the container refactoring which is planned for later this year.
i added map(), reduce() and where(qlint : string) to a Spring4D fork of mine.
While i was programming these functions, i found out that there is a differnce in the behaviour of the lists, when they are created in different ways.
If i create them with TList<TSomeClass>.create the objects in the enumerables are of the type TSomeClass.
If i create them with TCollections.CreateList<TSomeClass> the objects in the enumerables are of the type TObject.
So the question is:
Is there a downside by using TList<TSomeClass>.create ?
Or in other words: Why should i use TCollections.CreateList<TSomeClass> ?
btw: with TCollections.CreateList i got a TObjectList and not a TList. So it should be called TCollections.CreateObjectList... but that's another story.
Depending on the compiler version many of the Spring.Collections.TCollections.Create methods are applying what the compiler is unable to: folding the implementation into only a very slim generic class. Some methods are doing that from XE on, some only since XE7 ( GetTypeKind intrinsic function makes it possible to do the type resolution at compile time - see the parameterless TCollections.CreateList<T> for example).
This greatly reduces the binary size if you are creating many different types of IList<T> (where T are classes or interfaces) because it folds them into TFolded(Object|Interface)List<T>. However via the interface you are accessing the items as what you specified them and also the ElementType property returns the correct type and not only TObject or IInterface. On Berlin it adds less than 1K for every different object list while it would add around 80K if the folding is not applied due to all the internal classes involved for the different operations you can call on an IList<T>.
As for TCollections.CreateList<T> returning an IList<T> that is backed by a TFoldedObjectList<T> when T is a class that is completely as designed. Since the OwnsObject was passed as False it has the exact same behavior as a TList<T>.
The Spring4D collections are interface based so it does not matter what class is behind an interface as long as it behaves accordingly to the contract of the interface.
Make sure that you only carry the lists around as IList<T> and not TList<T> - you can create them both ways (with the benefits I mentioned before when using the TCollections methods). In our own application some places are still using the constructor of the classes while many other places are using the static methods from Spring.Collections.TCollections.
BTW:
I saw the activity in your fork and imo there is no need to implement Map/Reduce because that is already there. Since the Spring4D collections are modelled after .NET they are called Select and Aggregate (see Spring.Collections.TEnumerable). They are not available on IEnumerable<T> directly though because interfaces must not have generic parameterized methods.
Suppose:
1) HelpfulUserAtSO answers my SO question with a snippet copied from his production code:
type
IReqBase = Interface(IInterface)
['{B71BD1C3-CE4C-438A-8090-DA6AACF0B3C4}']
procedure FillWithTemplateData;
end;
2) I think Great answer! and blindly copy this into my production code.
3a) We both distribute our apps and user X wants to install both executables on his computer.
What are the consequences?
3b) I buy up HelpfulUserAtSO's company and want to integrate his code (containing the interface definition) into mine (containing the copy. Assume no scoping conflicts).
What are the consequences?
After all a GUID should be 'globally unique'...
If the same GUID are used not within the same process, this is safe to have the same GUID defined. But if, e.g. you access them via COM, it will definitively be confusing.
If you use diverse interfaces with the same GUID in the same process, e.g. by sharing Delphi code units, you may definitively have issues. By convention, an unique GUID should define an unique signature (i.e. set of methods), so the code may think that a given class instance implements all methods of the interface, and it won't be the case. As a result, the internal execution lookup tables (IMT) won't match. You will get a lot of A/V when calling methods.
Take a look at this very complete article for details about how interfaces work, and what is this internal IMT lookup table. The same GUID would mean the same IMT table, which won't be the case for you, so it will just break at runtime.
Just experienced an error due to using duplicated GUID for a new interface that's copied from another.
The consequence was that, since I use Supports, although I was intended to call InterfaceB.Method, but InterfaceA.Method1 got called wrongly even it has a different method signature...I found that with the IDE debugger.
The compiler really should report duplicated interface GUIDs as an error.
I am putting together a built-in script capability using the excellent Pascal DWScript. I have also add my own Delphi-side class definition (TDemo) to DWScript using:
dwsUnit.ExposeRTTI( TDemo.ClassInfo )
This just works and is a great way of quickly adding properties and methods.
I also wish to add an existing instance in a similar way, so I have created my instance FDemo of type TDemo and then performed:
dwsUnit.ExposeInstanceToUnit( 'Demo', 'TDemo', FDemo );
This looks a promising routine to call but I get an AV from an uninitialised unit table. I've also looked in the unit test code of the SVN source to see the use of this function but to no avail. Can anyone point me at what I should add / change?
ExposeInstanceToUnit has to be used from within the TdwsUnit table initialization, see RTTIExposeTests/ExposeInstancesAfterInitTable for some sample code. It allows directly exposing dynamic instances.
The other approach is to use the Instances collection of a TdwsUnit component, you get design-time support, and more controls over your instances and their lifetime.
Also keep in mind you have to make sure the instances you expose will properly behave even if the script misbehaves, f.i. when the user attempts to manually destroys an instance you exposed, and that instance shouldn't be destroyed. By default ExposeRTTI will map the destructors, so you may want to restrict that by specifying eoNoFreeOnCleanup.
edit: a last approach recently added is to use the TdwsRttiConnector, which basically allows exposing and connection to anything that's reachable through RTTI. That's very lightweight in terms of code to setup, but the downside is you don't get any form of compile-time checks.