Calling a method in another object

Enrique

Hi,
·
Say you have three objects: one we will call Top, which will instantiate objects A and B. Can the top object call a method in B and then can this method in B call a method in A?
·
Example:
·

{{
File Top.spin
}}
obj
  OA:"A"
  OB:"B"
pub main
  OB.MethodB
  

{{
File A.spin
}}
pub MethodA
' does something

{{
File: B.spin
}}
pub MethodB
' does something
' somehow calls MethodA in object OA

Thanks,

Enrique

Mike Green

No

The only way to call a method in object A from object B is to have a declaration of the object in B (like OBJ OA:"A"). If object A has its own VAR variables, these will be duplicated in each copy (instance) of object A. There are some objects in the Object Exchange that only have variables in a DAT section and this is shared (common) among all copies of the object. Here's one example: obex.parallax.com/objects/189/.

Enrique

I thought so, I was wondering whether there was a way to pass the reference of one object to another one.

Mike Green

No, not really. There are ways to fiddle with the internal representation of objects at run-time to substitute one method or set of methods for another, but it's not something to use for anything but experimentation by someone who really wants to learn how Spin works internally.

Enrique

Thanks

Ken Peterson

Enrique:

If your A object is a "singleton", then you can instantiate the A object within the B object as well as the Top object and they will both refer to the same object. Then you can call methods on A from both Top and B.

To make an object a "singleton", put all of the variables in that object in the DAT section instead of the VAR section. When an object is instantiated at more than one place in a program, only one copy of the code and the DAT section are used, but the compiler will make a copy of the VAR section for each instance of the object. If the variables are all in DAT, then all instances of the object are the same.

I have done this with the FullDuplexSerial object because I wanted to share the same serial interface at different levels within the program. I've also done it with the TV text object so I could share the display with several different objects.

Keep in mind that the object has to be coded to allow this. If you are using someone else's object, you may have to make a modifications to it to make it a singleton. Also, if the object has a start() or init() routine, that should only be called once, i.e., you wouldn't call start() from both Top and B.

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·"I have always wished that my computer would be as easy to use as my telephone.· My wish has come true.· I no longer know how to use my telephone."

- Bjarne Stroustrup

Enrique

In my case I instantiate OA 4 times.

Phil Pilgrim (PhiPi)

Ken Peterson said...
Also, if the object has a start() or init() routine, that should only be called once, i.e., you wouldn't call start() from both Top and B.

A well-written object should be able to tell whether it's already been started and inhibit multiple starts, assuming that that's the behavior you want. This relieves the programmer who uses the object from having to apply knowledge across object boundaries about what other objects are doing. By letting each object call start(), even if some of thoses calls are redundant, you improve object encapsulation.

-Phil

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'Just a few PropSTICK Kit bare PCBs left!

mpark

My Homespun compiler has object-passing capability of a sort, that sort being rudimentary and experimental.

Ken Peterson

Good advice, Phil. I have used Singleton objects in other languages (particularly Java) and keeping track of whether an instance already exists is key to defining class with the Singleton pattern (as well as making the constructor protected). I was just trying to cover the basic idea here in the forum.

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·"I have always wished that my computer would be as easy to use as my telephone.· My wish has come true.· I no longer know how to use my telephone."

- Bjarne Stroustrup

Post Edited (Ken Peterson) : 12/1/2008 6:43:54 PM GMT