Where is the best place to validate the data?

[risayew]

In order to load the model in a view i have to check the data for some preconditions. For example, for some string color value("#ffffff"), which is coming from backend, we have to check it if its not null, then parse it with Color.parseColor(). Where is the best place to put such code like this?
Is it a good idea, if i define the presentation layer model's backgroundColor propery as int and perform checks/transformation from String (domain layer object) to color int (presentation layer) in the model data mapper in the presentation layer?

---

DOMAIN LAYER
Object {
private String mBackgroundColor;
}

-------------------tranformation ---

PRESENTATION LAYER
Model {
private int mBackgroundColor;
}

Or maybe validation code have to be a part of the buisiness logic but for transformation in the presnetation layer from String to int i just call "trusted" Color.parseColor().

[Trikke]

Well, i'll just give my opinion on the matter, other people can chime in. Basically your Api is sending you a String and you'd want to have an Integer. (You'd only make a Color of it in the Presentation Layer, of course). First and foremost, i'd check if it is possible to have the Api accommodate your needs. But going by your question, i'm guessing this is not an option.

So i think it is best to validate any data at the point and layer where it enters the application. If a user enters data in a TextField, we validate the data in the Presentation Layer. Any data that is passed into the Domain Layer is validated there and then, before applying business rules. Any data that is passed on to the Data Layer is also validated before it is stored or passed on to internal or external sources.

The data you are talking about is first accessible in the Data Layer, when your Api sends it to you. Now imagine your entire Application hinges on it being an Integer. It makes perfect sense to parse/validate it there and then, in the Data Layer, before storing it, or sending the data on its way. You can think of it a bit like a Contract. Your application wants an Integer because it's about a color, so you make sure the Api sends you the correct data, or fail. Depending on the importance, this could range from throwing an exception (and even crashing), to parsing it as a default color to your liking.

I hope this helps you a bit.