Expose previous and current resource properties

[parsnips]

Issue #, if available:

https://github.com/aws-cloudformation/cloudformation-cli-go-plugin/issues/184

Description of changes:

Allows users to use their own Unmarshal strategy by exposing the making the raw resource bytes through a get method.

e.g.

       // Populate the previous model
    prevModel := &resource.Model{}
    if err := json.Unmarshal(req.GetPreviousResourcePropertiesBody(), prevModel); err != nil {
        log.Printf("Error unmarshaling prev model: %v", err)
        return handler.NewFailedEvent(err)
    }

    // Populate the current model
    currentModel := &resource.Model{}
    if err := json.Unmarshal(req.GetResourcePropertiesBody(), currentModel); err != nil {
        log.Printf("Error unmarshaling model: %v", err)
        return handler.NewFailedEvent(err)
    }

By submitting this pull request, I confirm that my contribution is made under the terms of the Apache 2.0 license.

[kddejong]

I like this idea. A lot of times when working with other packages they already have the models I need defined. They already have the logic for Marshaling and Unmarshaling as well.

The only thing that I got tripped up is when I have to send the response the stringify would fail a lot with non standard types

[parsnips]

The only thing that I got tripped up is when I have to send the response the stringify would fail a lot with non standard types

Yes I noticed that bool is sent in as a string in my case and now understand why the stringify and unstringify methods exist... In my case I just encapsulated that logic into a type... and used it in place of bools in my model.

eg:

type StringyBool bool

func (s *StringyBool) UnmarshalJSON(data []byte) error {
    asStr := string(data)
    if asStr == "true" {
        *s = true
    } else { // else if asStr == "false"
        *s = false
    }

    return nil
}

It's a matter of different approaches I guess.. but this one gives the end user a bit more flexibility.