define-custom-feature-flags.md

Introduction

In this document the process of implementing custom feature flags is presented. In the subsequent sections we will create a feature flag that is enabled if it's number field is odd.

Functions to be implemented

Each feature flag requires 1 function to be implemented:

• is_enabled.<FEATURE_FLAG> - S3 method computing whether the feature flag is enabled

However, it is recommended to additionally include:

• create_<YOUR_FEATURE_FLAG>_feature_flag - function that creates an instance of your feature flag (note: this function can be called however you want, this is just a suggestion)

• is_<YOUR_FEATURE_FLAG_NAME> - function that returns TRUE if the passed object is an instance of the feature flag class (that we are currently defining)

Create feature flag function

Let's start with a construction method that will use create a base feature flag, add the number field to it and assign it to the proper class.

create_odd_feature_flag <- function(number) {
  flag <- create_feature_flag()
  
  flag$number <- number
  class(flag) <- c("odd_feature_flag", class(flag))
  
  flag
}

Class checking function

Next, we will create a function checking the class attribute of the object.

is_odd_feature_flag <- function(feature_flag) {
  inherits(feature_flag, "odd_feature_flag")
}

is_enabled function

Finally, we define the is_enabled function which checks whether the number field of our flag is odd.

is_enabled.odd_feature_flag <- function(feature_flag) {
  stopifnot(is_odd_feature_flag(feature_flag))
  (feature_flag$number %% 2) != 0
}