Add observer pattern.

[zoziha]

• Add a observer pattern example.

Hope this PR gets your approval, thank you! 😘

[DanielEIlie]

Hello zoziha,

Here are my comments:

1. More comments needed describing the behavior of the observer and that of the subject.
2. The code is breaking the Liskov substitution principle: The functions should accept arguments from the base classes.
3. remove_from_slice and append_slice are utility functions and not necessarily part of the pattern.
4. The only functions that you need in the abstract interface to illustrate the pattern are: register, deregister, notify and update. Get_Id is a utility function
5. Put the base classes in one file and the concrete classes in a second file.

Best regards,
Daniel.

[zoziha]

Hello Daniel, thanks for your comments!

1. More comments needed describing the behavior of the observer and that of the subject.

For detailed information, see the reference in the header of every file.

2. The code is breaking the Liskov substitution principle: The functions should accept arguments from the base classes.

In fact, yes, I can't find a better way to replace the existing writing.
Fortran has many obstacles to the assignment of abstract base classes.
I use type(node) to store class(observer), can you help me try to solve this problem? It may be that I don't have enough programming experience.

3. remove_from_slice and append_slice are utility functions and not necessarily part of the pattern.

This may also be an obstacle. It is not possible to allocate memory for the same variable multiple times in the same Fortran variable domain. I tried to use these two functions (remove_from_slice and append_slice) to achieve this process.

4. The only functions that you need in the abstract interface to illustrate the pattern are: register, deregister, notify and update. Get_Id is a utility function

I refer to the Go language code here. It uses this method (get_id) in remove_from_slice like my writing in Fortran. It seems that it needs to be included in the parent class observer, even if it looks like a utility function.

5. Put the base classes in one file and the concrete classes in a second file.

I continued the writing style of FortranPatterns. Both abstract and concrete classes are placed in a file and a module. This may be a problem, or it may be okay only as an example.

[zoziha]

I want to use class(observer), allocatable :: o_list(:) here, but this writing method faces some incompatibility issues with the existing grammar of Fortran. I need help! Who can try to solve this problem?

Maybe here (?) my code is breaking the Liskov substitution principle: The functions should accept arguments from the base classes.