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Flag enums

EVA makes use of flagged enums to ensure values are sent and stored as efficiently as possible. If you would like to know more about how exactly EVA makes use of enums - or if could use a general refresher - we've got you covered.

Problem​

Imagine you have a bunch of yes/no properties on a model, and you want to communicate about them in a JSON format between different systems.

At first hand, you might suggest a list of properties in the format of "IsSomething" and giving them either a true or false value;

{
  "ID": 123,
  "IsStock": true,
  "IsInsurance": false,
  "IsMarketing": false,
  "IsGiftCard": false
  ...
}

This list is easy to understand and perfectly readable for humans. But, this list will continue to grow and grow as new boolean properties emerge, and the size of the model keeps increasing over time. For larger models, this can take up precious memory space and consume traffic. And remember: this communication is not intended for humans - it is intended for computers.

Solution​

So we chose a way to communicate such properties by means of "flag enums", which brings communication to a binary level, which is something computers excel at. For humans, though, it's a little less intuitive, especially if binary is new for you.

Imagine the above properties are given a value - the first number is the decimal value, and the second number is the same value in binary:

Values in binary
  IsStock       1    0001
  IsInsurance   2    0010
  IsMarketing   4    0100
  IsGiftCard    8    1000

These definitions are exposed through GetEnumValues and usually directly available from your SDK. With these definitions, we can now communicate the same information as above, but in a much more efficient way:

Enums in practice
{
  "ID": 123,
  "Type": 1
}
Matching the property name

When you have a property with an enum value in your service call, that property isn't necessarily the right name to look for in the GetEnumValues call.

To take an example: GetOrder has the Characteristics property, which has an enum value that defines what kind of order it is. To read the order types behind the enum values however, you'd have to enter OrderProperties in GetEnumValues.

To check this specific example for yourself, see the GetOrder call in the API Reference section. The last part behind your property shows the right property to use: "Characteristics": "EVA.Core.DataModels.OrderProperties".

If you want to combine IsStock (1) and IsMarketing (4), you can simply take the sum of both values, so Type would equal 5.

In decimals, summing is done using ADD (+), and in binary you'd use an OR statement (|):

Example of OR statement in binary
  0001      1
  0100      4
  ---- |   -- +
  0101      5

Things get a little confusing when humans want to compare properties, let's say you want to know if Type with value 5 'contains' the flag IsStock.

In decimals, this wouldn't really make sense, but in binary it's a simple AND statement (&):

Example of AND statement in binary 
  0101
  0001
  ---- &
  0001

If the outcome value is > 0, then apparently this IsStock flag is present in the Type value!

In your common programming language, assuming the flags are in an enum called ProductTypes and your model as product, a generic if-statement to check if a product has IsStock would look like this:

Example of IF statement in common programming language
  if(product.Type & ProductTypes.IsStock > 0)
  {
    ...
  }

Practical examples​

It's important to use binary operators, such as the & and |. This opens up so many possibilities!

product.Type & (ProductTypes.IsStock | ProductTypes.IsMarketing) > 0