How to implement validation using ViewModel and Databinding?

There can be many ways to implement this. I am telling you two solutions, both works well, you can use which you find suitable for you.

I use extends BaseObservable because I find that easy than converting all fields to Observers. You can use ObservableFields too.

Solution 1 (Using custom BindingAdapter)

In xml

<variable    name="model"    type="sample.data.Model"/><EditText    passwordValidator="@{model.password}"    android:layout_width="match_parent"    android:layout_height="wrap_content"    android:text="@={model.password}"/>

Model.java

public class Model extends BaseObservable {    private String password;    @Bindable    public String getPassword() {        return password;    }    public void setPassword(String password) {        this.password = password;        notifyPropertyChanged(BR.password);    }}

DataBindingAdapter.java

public class DataBindingAdapter {    @BindingAdapter("passwordValidator")    public static void passwordValidator(EditText editText, String password) {        // ignore infinite loops        int minimumLength = 5;        if (TextUtils.isEmpty(password)) {            editText.setError(null);            return;        }        if (editText.getText().toString().length() < minimumLength) {            editText.setError("Password must be minimum " + minimumLength + " length");        } else editText.setError(null);    }}

Solution 2 (Using custom afterTextChanged)

In xml

<variable    name="model"    type="com.innovanathinklabs.sample.data.Model"/><variable    name="handler"    type="sample.activities.MainActivityHandler"/><EditText    android:id="@+id/etPassword"    android:layout_width="match_parent"    android:layout_height="wrap_content"    android:afterTextChanged="@{(edible)->handler.passwordValidator(edible)}"    android:text="@={model.password}"/>

MainActivityHandler.java

public class MainActivityHandler {    ActivityMainBinding binding;    public void setBinding(ActivityMainBinding binding) {        this.binding = binding;    }    public void passwordValidator(Editable editable) {        if (binding.etPassword == null) return;        int minimumLength = 5;        if (!TextUtils.isEmpty(editable.toString()) && editable.length() < minimumLength) {            binding.etPassword.setError("Password must be minimum " + minimumLength + " length");        } else {            binding.etPassword.setError(null);        }    }}

MainActivity.java

public class MainActivity extends AppCompatActivity {    ActivityMainBinding binding;    @Override    protected void onCreate(@Nullable Bundle savedInstanceState) {        super.onCreate(savedInstanceState);        binding = DataBindingUtil.setContentView(this, R.layout.activity_main);        binding.setModel(new Model());        MainActivityHandler handler = new MainActivityHandler();        handler.setBinding(binding);        binding.setHandler(handler);    }}

Update

You can also replace

android:afterTextChanged="@{(edible)->handler.passwordValidator(edible)}"

with

android:afterTextChanged="@{handler::passwordValidator}"

Because parameter are same of android:afterTextChanged and passwordValidator.

This approach uses TextInputLayouts, a custom binding adapter, and creates an enum for form errors. The result I think reads nicely in the xml, and keeps all validation logic inside the ViewModel.

The ViewModel:

class SignUpViewModel() : ViewModel() {   val name: MutableLiveData<String> = MutableLiveData()   // the rest of your fields as normal   val formErrors = ObservableArrayList<FormErrors>()   fun isFormValid(): Boolean {      formErrors.clear()      if (name.value?.isNullOrEmpty()) {          formErrors.add(FormErrors.MISSING_NAME)      }      // all the other validation you require      return formErrors.isEmpty()   }   fun signUp() {      auth.createUser(email.value!!, password.value!!)   }   enum class FormErrors {      MISSING_NAME,      INVALID_EMAIL,      INVALID_PASSWORD,      PASSWORDS_NOT_MATCHING,   }}

The BindingAdapter:

@BindingAdapter("app:errorText")fun setErrorMessage(view: TextInputLayout, errorMessage: String) {    view.error = errorMessage}

The XML:

<layout>  <data>        <import type="com.example.SignUpViewModel.FormErrors" />        <variable            name="viewModel"            type="com.example.SignUpViewModel" />  </data><!-- The rest of your layout file etc. -->       <com.google.android.material.textfield.TextInputLayout            android:id="@+id/text_input_name"            android:layout_width="match_parent"            android:layout_height="wrap_content"            app:errorText='@{viewModel.formErrors.contains(FormErrors.MISSING_NAME) ? "Required" : ""}'>            <com.google.android.material.textfield.TextInputEditText                android:layout_width="match_parent"                android:layout_height="wrap_content"                android:hint="Name"                android:text="@={viewModel.name}"/>        </com.google.android.material.textfield.TextInputLayout><!-- Any other fields as above format -->

And then, the ViewModel can be called from activity/fragment as below:

class YourActivity: AppCompatActivity() {   val viewModel: SignUpViewModel  // rest of class  fun onFormSubmit() {     if (viewModel.isFormValid()) {        viewModel.signUp()        // the rest of your logic to proceed to next screen etc.     }     // no need for else block if form invalid, as ViewModel, Observables     // and databinding will take care of the UI  }}

I've written a library for validating bindable fields of an Observable object.

Setup your Observable model:

class RegisterUser:BaseObservable(){@Bindablevar name:String?=""    set(value) {        field = value        notifyPropertyChanged(BR.name)    }@Bindablevar email:String?=""    set(value) {        field = value        notifyPropertyChanged(BR.email)    }

}

Instantiate and add rules

class RegisterViewModel : ViewModel() {var user:LiveData<RegisterUser> = MutableLiveData<RegisterUser>().also {    it.value = RegisterUser()}var validator = ObservableValidator(user.value!!, BR::class.java).apply {    addRule("name", ValidationFlags.FIELD_REQUIRED, "Enter your name")    addRule("email", ValidationFlags.FIELD_REQUIRED, "Enter your email")    addRule("email", ValidationFlags.FIELD_EMAIL, "Enter a valid email")    addRule("age", ValidationFlags.FIELD_REQUIRED, "Enter your age (Underage or too old?)")    addRule("age", ValidationFlags.FIELD_MIN, "You can't be underage!", limit = 18)    addRule("age", ValidationFlags.FIELD_MAX, "You sure you're still alive?", limit = 100)    addRule("password", ValidationFlags.FIELD_REQUIRED, "Enter your password")    addRule("passwordConfirmation", ValidationFlags.FIELD_REQUIRED, "Enter password confirmation")    addRule("passwordConfirmation", ValidationFlags.FIELD_MATCH, "Passwords don't match", "password")}

}

And setup your xml file:

<com.google.android.material.textfield.TextInputLayoutstyle="@style/textFieldOutlined"error='@{viewModel.validator.getValidation("email")}'android:layout_width="match_parent"android:layout_height="wrap_content"><com.google.android.material.textfield.TextInputEditText    android:id="@+id/email"    style="@style/myEditText"    android:layout_width="match_parent"    android:layout_height="wrap_content"    android:hint="Your email"    android:imeOptions="actionNext"    android:inputType="textEmailAddress"    android:text="@={viewModel.user.email}" />