How to set error on EditText using DataBinding Framework MVVM

Question

I am using Android Data Binding framework I have suppose an EditText for login form with username as below

Answer 1

If you want to do something like EditText.setError() function with databinding, here is two method.

Method 1

Used the final EditText view generated from the data binding (https://developer.android.com/topic/libraries/data-binding/index.html#views_with_ids)

You can call the EditText directly without creating it manually since it is automatically generated after you set the id for the view (also true for the included layout) .

MainActivityBinding.etext_uname.setError("Wrong email format");

Or

MainActivityBinding.etext_uname.addTextChangedListener(new MyOwnTextWatcher());

Method 2

If you want to use the binding method with xml as George mentioned (https://medium.com/google-developers/android-data-binding-custom-setters-55a25a7aea47#.su88ujqrn)

First you have to set your own binding method. Suggest to create another class for all the binding method.

Method must be static, with @BindingAdapter annotation and the corresponding binding method name (Namespace and the method name can be customized)

1. Set the Custom TextWatcher

public class MyOwnBindingUtil {
    public interface StringRule {
        public boolean validate(Editable s);
    }
    @BindingAdapter("android:watcher")
    public static void bindTextWatcher(EditText pEditText, TextWatcher pTextWatcher) {
        pEditText.addTextChangedListener(pTextWatcher);
    }
    @BindingAdapter(value = {"email:rule", "email:errorMsg"}, requireAll = true)
    public static void bindTextChange(final EditText pEditText, final StringRule pStringRule, final String msg) {
        pEditText.addTextChangedListener(new TextWatcher() {
            @Override
            public void beforeTextChanged(CharSequence s, int start, int count, int after) {
                }
                @Override
                public void onTextChanged(CharSequence s, int start, int before, int count) {
                }
                @Override
                public void afterTextChanged(Editable s) {
                    if (!pStringRule.validate(s)) {
                        pEditText.setError(msg);
                }
            }
        });
    }
    /*
    Your other custom binding method
     */
}

If you want to setup your own TextWatcher with custom action, like Toast shown, Dialog shown. You should use "android:watcher" method

mBinding.setWatcher(new TextWatcher() {
    @Override
    public void beforeTextChanged(CharSequence s, int start, int count, int after) {
    }
    @Override
    public void onTextChanged(CharSequence s, int start, int before, int count) {
    }
    @Override
    public void afterTextChanged(Editable s) {
    }
});

In xml,

Activity code

mBinding.setErrorMsg("Wrong type");
mBinding.setEmailRule(new MyOwnBindingUtil.StringRule() {
    @Override
    public boolean validate(Editable s) {
        // check if the length of string is larger than 18  
        return s.toString().length() > 18;
    }
});

Please feel free to edit my code to make the binding be more generic for the developer use.