[C++][Restbed] Add handler callback methods

[muttleyxd]

PR checklist

• Read the contribution guidelines.
• Ran the shell script under ./bin/ to update Petstore sample so that CIs can verify the change. (For instance, only need to run ./bin/{LANG}-petstore.sh, ./bin/openapi3/{LANG}-petstore.sh if updating the {LANG} (e.g. php, ruby, python, etc) code generator or {LANG} client's mustache templates). Windows batch files can be found in .\bin\windows\. If contributing template-only or documentation-only changes which will change sample output, be sure to build the project first.
  Added second commit with updated samples (not 100% sure what I should do with them), are there any tests for these? I didn't find any.
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  @ravinikam @stkrwork @fvarose @etherealjoy @MartinDelille

Description of the PR

Related issue: #273
What I don't like about current generator: you need to modify generated code, there isn't a way for injecting your own handler. librestbed does not allow to replace already existing handler, which means you either modify generated code or don't use it at all. I modified it a little, to allow setting a callback, which responds with std::pair<int (HTTP status code), std::string (response)>

I'm not an expert on Mustache, so I would be really grateful for some tips where I could optimize/refactor it.

[etherealjoy]

A generic comment here, Callback is also an OpenApi 3 keyword, won't this be confusing?

[muttleyxd]

A generic comment here, Callback is also an OpenApi 3 keyword, won't this be confusing?

Definitely a wrong name then (I didn't know this feature before). Should I change 'callback' to 'handler'?

[etherealjoy]

I think handler would be a better name

[muttleyxd]

I think handler would be a better name

Done

[wing328]

@muttleyxd thanks again for your contribution, which has been included in the v4.0.1 release (https://twitter.com/oas_generator/status/1135534738658062336)

[engpedrorafael]

I appreciate and agree that this is a very important feature, though I don't see how to make use of the set_handler_ GET/POST/DELETE without changing the generated code.
The methods set_handler_* are generated as member functions of the *Resource classes which can only be called through an object instantiated from the respective class.
From the generated code, we can see that at the constructor of the API class is where the *Resources are created as local objects and passed to the Base Class using the method publish:

PetApi::PetApi() {
	std::shared_ptr<PetApiPetResource> spPetApiPetResource = std::make_shared<PetApiPetResource>();
	this->publish(spPetApiPetResource);	
	...

I don't see a way of accessing the Resource objects pubished after the creation of the object, in order to call the respective set_handler_ to set my custom handlers, and this could not have been done before since we cannot call it without an object.

Can you please explain, maybe with a small example, how to make use of this in order to set custom handlers

[muttleyxd]

Yeah, I think I'd probably need to change generated constructor or add another method for setting handlers.
Maybe virtual dispatch would be good here, what do you think about it? So one would have to do

class Derived : public PetApi
{
public:
  <type> PetApiPetResource_GET(...) override;
};

For now I usually create new PetApi class and and use the resources myself, something like that

class MyPetApi
{
public:
  MyPetApi() {
    std::shared_ptr<PetApiPetResource> spPetApiPetResource = std::make_shared<PetApiPetResource>();
    spPetApiPetResource.set_handler_PUT([&](auto const& body) { return handlePetPut(body); })
	this->publish(spPetApiPetResource);
  }

private:
  std::pair<int, std::string> handlePetPut(std::string const& body) {
    // Do something here
    return {200, "Success"};
  }
};