Instantiating an object with a Classname?

Amando_Blasco · February 12, 2015, 11:53pm

I’m currently developing a REST app. Have a NXCommand class and it subclasses that handles different commands on the server side got from a JSON command received. Each JSON request has a “command” attribute that the server needs to be created.

Try

Select Case Command
  
Case "NXWelcome"
  Dim comando As New NXWelcome(Me, JData)
  
Case "NXQuery"
  Dim comando As New NXQuery(Me, JData)
  
Case "NXVersion"
  Dim comando As New NXVersion(Me, JData)
  
Case "NXAuthenticate"
  Dim comando as new NXAuthenticate(me, JData)
  
End Select

Catch
Stdout.WriteLine “** “+CurrentMethodName+” Invalid command in NXRequest”
Me.Disconnect

End Try

I am trying to instantiate these subclasses via introspection, but I can’t find a way to pass a string and instantiate an object. Will appreciate anything to avoid the select case.

P.S: Each NXCommand handles instance references until the process is done and finally deallocates the object.

Sam_Rowlands · February 13, 2015, 1:27am

The only other way I can think of is to roll them into a single object that has a type selector, doesn’t really avoid the select case (as you’d have to use it within the object.

Kem_Tekinay · February 13, 2015, 1:42am

I have a suggestion but have to type it out on my computer, not my phone. But it is doable in a fashion.

Amando_Blasco · February 13, 2015, 1:53am

@Kem Tekinay Awaiting your magic!

Jason_King · February 13, 2015, 2:00am

Rough outline but you should get the idea: You could use GetTypeInfo in App.Open to populate a dictionary with the key of the class name and value of the TypeInfo. Then you lookup the TypeInfo from the dictionary, use GetContructors on the returned TypeInfo, and finally invoke the Constructor with the two parameters (Me, JData). Since everything descends from the same class, you can call the necessary methods on the object created to do whatever is necessary.

Amando_Blasco · February 13, 2015, 2:22am

Thanks @Jason King . Seems Select case in needed to fill the dictionary. Sigh!

My intention were to dynamically add each NXCommand subclass or even better to create an instance via introspection to create an Object like Dim ty = Introspection.TypeInfo = ClassName(" ClassName")

No way to do this directly?

Jason_King · February 13, 2015, 2:33am

I don’t think so. You would do something like this in App.Open:

//classesDict is a dictionary property added to the app classesDict = new xojo.core.dictionary classesDict.value("NXCommand") = GetTypeInfo(NXCommand) classesDict.value("NSWelcome") = GetTypeInfo(NXWelcome) //continued for all of your classes
Then when you receive your JSON command you can do something like:

dim info as introspection.TypeInfo = App.classesDict.lookup(Command,nil) //Command is the same variable you did select case on if info = nil then Break //shouldn't happen unless the JSON is messed up Dim constructors() As Introspection.ConstructorInfo = info.GetConstructors dim newInstance as NXCommand = constructors(0).Invoke(Me,JData) //do something with the instance
Hopefully that makes more sense?

Amando_Blasco · February 13, 2015, 2:37am

Thanks @Jason King. Your suggestion is really good for encapsulating my class.

Do you think the GetType(String ClassName) is worth a feedback request?

Jason_King · February 13, 2015, 2:43am

This came up in another thread a while ago and basically it said that adding such a feature would mean dead code stripping couldn’t occur since all classes would have to be included in the executable for such a feature to work. So although I like the idea, I definitely prefer to do this manually so that unused classes can be stripped out by the compiler.

Amando_Blasco · February 13, 2015, 2:53am

Makes sense, didn’t thought about code stripping. I’m going to adopt your Constructor approach as seems more elegant than the select case in the DataAvailable Event.

Thanks!

Tim_Hare · February 13, 2015, 4:31am

I personally prefer the select case approach as it’s much clearer what is going on. You still have to enumerate each class explicitly with introspection, but that code is far away from where you actually use them. I think it’s better to have everything all in one spot. Easier to understand and maintain.