What is Business logic in Clean Architecture (domain)

Question

People say business logic should handled in domain layer.

But I don't exactly understand what is business logic in Android.

In my project I cache data when API request is successful.
And if API Request got failed, get local data.

In my understand It can be a kind of business rule.
1. API request
2. If API request is failed load data from local.

I think this logic should be done in the repository.
But business logic should handle in domain layer.

Am I misunderstanding the business logic?

Answer 1

The business logic is refereed to ifs and elses you have after you receive the data from the API. Let's say that you want to load an image using Picasso. The url is:

https://.../imageId.jpg

But first you will need to fetch this object which has an id attached to it's JSON:

{
 "id" : "1234123",
 "name" : "SomeName"
}

Well, first you would need to fetch this:

val request = repository.fetchTheObject()
//let's assume the result is successful
//start of the business logic
val imageId = request.id
if(id == WHATEVER){
val url = "someUrl/${id}.jpg"
someMutableLiveData.postValue(url)
}else{
 showSomeError()    
}
//end of business logic

And then in your fragment you just receive the final result:

viewModel.someMutableLiveData.observe(this, Observer{
  Picasso.get().load(it).into(imageView)
})

In other words:

Business logic is all the actions you need to perform to the data you receive from the source of truth (database, files, network API etc) in order to achieve a desirable result

In case of Android:

• Fragment/Activity for interacting with the user
• ViewModel for business logic (sometimes when you have tones of ifs/else, a lot of for-loops etc, Google suggests to use a Presenter attached to a ViewModel, never done it)
• Repository, just a source to receive the data (even though sometimes it's redundant, like when you have only one source of truth)

In your case the business logic is exactly what you describe: If your request fails then get local data, this is a business logic of the app.

I think this logic should be done in the repository.

Well you can do that, and I have seen many examples like that and it really depends, sometimes could be correct. But in case when you are using a ViewModel in order to avoid all Transformations you can just handle it in the ViewModel instead of the Repository.