Styjun

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.

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