Xamarin MvvmCross ViewModel Validation

I'm building my first Xamarin MvvmCross application at the moment and I'm currently looking at validating user input to the view models.

Doing a lot of searching around everything (including the MvvmCross team) link to this plugin:

MVVMCross.Plugins.Validation

This plugin makes use of a very old version of MvvmCross v3. I have tried taking the code from this plugin and building it directly into my application Core project until I came across the Bindings breaking change. I then came to the conclusion that this plugin would actually require a complete re-write due to this in order to use the latest version of MvvmCross.

So I'm now a little stuck.

What is the currently recommended best approach for performing input validation in a view model?

Solution

EDIT: Add sample project on GitHub https://github.com/kiliman/mvx-samples/tree/master/MvxSamples.Validation

I use MVVM Validation Helpers http://www.nuget.org/packages/MvvmValidation/

It's a simple validation library that's easy to use. It's not tied to MvvmCross.

Here's how I use it, for example, in my SigninViewModel:

private async void DoSignin()
{
    try
    {
        if (!Validate())
        {
            return;
        }

        IsBusy = true;
        Result = "";
        var success = await SigninService.SigninAsync(Email, Password);

        if (success)
        {
            Result = "";
            ShowViewModel<HomeViewModel>();
            Close();
            return;
        }

        Result = "Invalid email/password. Please try again.";
    }
    catch (Exception ex)
    {
        Result = "Error occured during sign in.";
        Mvx.Error(ex.ToString());
    }
    finally
    {
        IsBusy = false;
    }
}

private bool Validate()
{
    var validator = new ValidationHelper();
    validator.AddRequiredRule(() => Email, "Email is required.");
    validator.AddRequiredRule(() => Password, "Password is required.");

    var result = validator.ValidateAll();

    Errors = result.AsObservableDictionary();

    return result.IsValid;
}

The nice part of it is that you can get Errors as a collection and bind them in your view. For Android, I set the Error property to the keyed Error message.

<EditText
    android:minHeight="40dp"
    android:layout_margin="4dp"
    android:layout_width="fill_parent"
    android:layout_height="wrap_content"
    android:inputType="textEmailAddress"
    android:hint="Email"
    local:MvxBind="Text Email; Error Errors['Email']"
    android:id="@+id/EmailEditText" />
<EditText
    android:minHeight="40dp"
    android:layout_margin="4dp"
    android:inputType="textPassword"
    android:layout_width="fill_parent"
    android:layout_height="wrap_content"
    android:hint="Password"
    local:MvxBind="Text Password; Error Errors['Password']"
    android:id="@+id/PasswordEditText" />

And here's what the validation looks like:

Validation message

EDIT: show helper code

public static class ValidationResultExtension
{
    public static ObservableDictionary<string, string> AsObservableDictionary(this ValidationResult result)
    {
        var dictionary = new ObservableDictionary<string, string>();
        foreach (var item in result.ErrorList)
        {
            var key = item.Target.ToString();
            var text = item.ErrorText;
            if (dictionary.ContainsKey(key))
            {
                dictionary[key] = dictionary.Keys + Environment.NewLine + text;
            }
            else
            {
                dictionary[key] = text;
            }
        }
        return dictionary;
    }
}

public class ObservableDictionary<TKey, TValue> : IDictionary<TKey, TValue>, INotifyCollectionChanged, INotifyPropertyChanged
{
    private const string CountString = "Count";
    private const string IndexerName = "Item[]";
    private const string KeysName = "Keys";
    private const string ValuesName = "Values";

    private IDictionary<TKey, TValue> _dictionary;

    protected IDictionary<TKey, TValue> Dictionary
    {
        get { return _dictionary; }
    }

    public ObservableDictionary()
    {
        _dictionary = new Dictionary<TKey, TValue>();
    }

    public ObservableDictionary(IDictionary<TKey, TValue> dictionary)
    {
        _dictionary = new Dictionary<TKey, TValue>(dictionary);
    }

    public ObservableDictionary(IEqualityComparer<TKey> comparer)
    {
        _dictionary = new Dictionary<TKey, TValue>(comparer);
    }

    public ObservableDictionary(int capacity)
    {
        _dictionary = new Dictionary<TKey, TValue>(capacity);
    }

    public ObservableDictionary(IDictionary<TKey, TValue> dictionary, IEqualityComparer<TKey> comparer)
    {
        _dictionary = new Dictionary<TKey, TValue>(dictionary, comparer);
    }

    public ObservableDictionary(int capacity, IEqualityComparer<TKey> comparer)
    {
        _dictionary = new Dictionary<TKey, TValue>(capacity, comparer);
    }

    #region IDictionary<TKey,TValue> Members

    public void Add(TKey key, TValue value)
    {
        Insert(key, value, true);
    }

    public bool ContainsKey(TKey key)
    {
        return Dictionary.ContainsKey(key);
    }

    public ICollection<TKey> Keys
    {
        get { return Dictionary.Keys; }
    }

    public bool Remove(TKey key)
    {
        if (key == null)
        {
            throw new ArgumentNullException("key");
        }

        TValue value;
        Dictionary.TryGetValue(key, out value);
        var removed = Dictionary.Remove(key);
        if (removed)
        {
            OnCollectionChanged(NotifyCollectionChangedAction.Remove, new KeyValuePair<TKey, TValue>(key, value));
        }
        return removed;
    }

    public bool TryGetValue(TKey key, out TValue value)
    {
        return Dictionary.TryGetValue(key, out value);
    }

    public ICollection<TValue> Values
    {
        get { return Dictionary.Values; }
    }

    public TValue this[TKey key]
    {
        get
        {
            return Dictionary.ContainsKey(key) ? Dictionary[key] : default(TValue);
        }
        set
        {
            Insert(key, value, false);
        }
    }

    #endregion IDictionary<TKey,TValue> Members

    public void Add(KeyValuePair<TKey, TValue> item)
    {
        Insert(item.Key, item.Value, true);
    }

    public void Clear()
    {
        if (Dictionary.Count > 0)
        {
            Dictionary.Clear();
            OnCollectionChanged();
        }
    }

    public bool Contains(KeyValuePair<TKey, TValue> item)
    {
        return Dictionary.Contains(item);
    }

    public void CopyTo(KeyValuePair<TKey, TValue>[] array, int arrayIndex)
    {
        Dictionary.CopyTo(array, arrayIndex);
    }

    public int Count
    {
        get { return Dictionary.Count; }
    }

    public bool IsReadOnly
    {
        get { return Dictionary.IsReadOnly; }
    }

    public bool Remove(KeyValuePair<TKey, TValue> item)
    {
        return Remove(item.Key);
    }

    public IEnumerator<KeyValuePair<TKey, TValue>> GetEnumerator()
    {
        return Dictionary.GetEnumerator();
    }

    IEnumerator IEnumerable.GetEnumerator()
    {
        return ((IEnumerable)Dictionary).GetEnumerator();
    }

    public event NotifyCollectionChangedEventHandler CollectionChanged;

    public event PropertyChangedEventHandler PropertyChanged;

    public void AddRange(IDictionary<TKey, TValue> items)
    {
        if (items == null)
        {
            throw new ArgumentNullException("items");
        }

        if (items.Count > 0)
        {
            if (Dictionary.Count > 0)
            {
                if (items.Keys.Any((k) => Dictionary.ContainsKey(k)))
                {
                    throw new ArgumentException("An item with the same key has already been added.");
                } 
                else
                {
                    foreach (var item in items)
                    {
                        Dictionary.Add(item);
                    }
                }
            }
            else
            {
                _dictionary = new Dictionary<TKey, TValue>(items);
            }

            OnCollectionChanged(NotifyCollectionChangedAction.Add, items.ToArray());
        }
    }

    private void Insert(TKey key, TValue value, bool add)
    {
        if (key == null)
        {
            throw new ArgumentNullException("key");
        }

        TValue item;
        if (Dictionary.TryGetValue(key, out item))
        {
            if (add)
            {
                throw new ArgumentException("An item with the same key has already been added.");
            }
            if (Equals(item, value))
            {
                return;
            }
            Dictionary[key] = value;

            OnCollectionChanged(NotifyCollectionChangedAction.Replace, new KeyValuePair<TKey, TValue>(key, value), new KeyValuePair<TKey, TValue>(key, item));
        }
        else
        {
            Dictionary[key] = value;

            OnCollectionChanged(NotifyCollectionChangedAction.Add, new KeyValuePair<TKey, TValue>(key, value));
        }
    }

    private void OnPropertyChanged()
    {
        OnPropertyChanged(CountString);
        OnPropertyChanged(IndexerName);
        OnPropertyChanged(KeysName);
        OnPropertyChanged(ValuesName);
    }

    protected virtual void OnPropertyChanged(string propertyName)
    {
        if (PropertyChanged != null)
        {
            PropertyChanged(this, new PropertyChangedEventArgs(propertyName));
        }
    }

    private void OnCollectionChanged()
    {
        OnPropertyChanged();
        if (CollectionChanged != null)
        {
            CollectionChanged(this, new NotifyCollectionChangedEventArgs(NotifyCollectionChangedAction.Reset));
        }
    }

    private void OnCollectionChanged(NotifyCollectionChangedAction action, KeyValuePair<TKey, TValue> changedItem)
    {
        OnPropertyChanged();
        if (CollectionChanged != null)
        {
            CollectionChanged(this, new NotifyCollectionChangedEventArgs(action, changedItem));
        }
    }

    private void OnCollectionChanged(NotifyCollectionChangedAction action, KeyValuePair<TKey, TValue> newItem, KeyValuePair<TKey, TValue> oldItem)
    {
        OnPropertyChanged();
        if (CollectionChanged != null)
        {
            CollectionChanged(this, new NotifyCollectionChangedEventArgs(action, newItem, oldItem));
        }
    }

    private void OnCollectionChanged(NotifyCollectionChangedAction action, IList newItems)
    {
        OnPropertyChanged();
        if (CollectionChanged != null)
        {
            CollectionChanged(this, new NotifyCollectionChangedEventArgs(action, newItems));
        }
    }
}