Android Databinding with LiveData - Validating multiple fields together - android-databinding

Question

I am using two-way Databinding with LiveData inside a ViewModel to handle a sign-up form.
As fields are filled out, there are fields that need to be evaluated together for validity (as well as the overall form), and only enable the Submit button when everything is correct.
With a regular Observable object and using #Bindable this would be straightforward by just annotating a 'getter' function for each validation that is required and doing the necessary validation steps and return the appropriate result inside each function.
BUT I am working with LiveData. I came up with a solution using MediatoLiveData (see basic example below), but it seems to me like there should be another, perhaps better, approach.
class RegistrationViewModel : ViewModel() {
val email: MutableLiveData<String> by lazy { MutableLiveData<String>() }
val emailConf: MutableLiveData<String> by lazy { MutableLiveData<String>() }
val emailValid: MediatorLiveData<Boolean> = MediatorLiveData()
val emailChanged: (Any) -> Unit = cc# {
//Obviously there is more validation needed but this is just an example
if (email.value != null && emailConf.value != null) {
emailValid.value = email.value.isNotEmpty() && emailConf.value.isNotEmpty() && email.value == emailConf.value
return#cc
}
emailValid.value = false
}
init {
emailValid.addSource(email, emailChanged)
emailValid.addSource(emailConf, emailChanged)
}
}
To tie it to the UI, for example, the enabled property of a button or the visibility property of a label could be bound to the emailValid property appropriately.

Answer 1

Your approach is correct, but code has some design issues:
lazy fields is used in init section. Field become not lazy
emailValid field can be exposed as LiveData
validation can be encapsulated in inner class, but it's not required
class RegistrationViewModel : ViewModel() {
val email = MutableLiveData<String>()
val emailConf = MutableLiveData<String>()
#Suppress("UNCHECKED_CAST")
val emailValid: LiveData<Boolean> = MediatorLiveData<Boolean>().apply {
val validator = Validator(::postValue)
addSource(email, validator as Observer<String>)
addSource(emailConf, validator as Observer<String>)
}
private inner class Validator(private val validationConsumer: (Boolean) -> Unit) : Observer<Any> {
override fun onChanged(ignored: Any?) {
//Obviously there is more validation needed but this is just an example
val email = email.value
val emailConf = emailConf.value
validationConsumer(when {
email.isNullOrEmpty() -> false
emailConf.isNullOrEmpty() -> false
email == emailConf -> true
else -> false
})
}
}
}
If you require some heavy validation, you can make hidden mutable field for emailValid and manually update it when background validation completed