Creating a Model Base – Part II: Calling an Action when the property changes

In this article, we are going to look at calling an Action when our property changes.

Note: Please read Part I before continuing here.

Alright, so let’s look at our ViewModelBase from Part I:

    /// <summary>
    /// Extends the INotifyPropertyChanged interface to the class properties.
    /// </summary>
    public abstract class ViewModelBase : INotifyPropertyChanged
    {
        #region Methods

        /// <summary>
        /// To be used within the "set" accessor in each property.
        /// This invokes the OnPropertyChanged method.
        /// </summary>
        /// <typeparam name="T"></typeparam>
        /// <param name="name"></param>
        /// <param name="value"></param>
        /// <param name="newValue"></param>
        protected void SetValue<T>(string name, ref T value, ref  T newValue)
        {
            if (newValue != null)
            {
                if (!newValue.Equals(value))
                {
                    value = newValue;
                    OnPropertyChanged(name);
                }
            }
            else
            {
                value = default(T);
            }
        }

        #endregion

        #region INotifyPropertyChanged

        /// <summary>
        /// The PropertyChanged event handler.
        /// </summary>
        public event PropertyChangedEventHandler PropertyChanged;
        
        /// <summary>
        /// Calls the PropertyChanged event
        /// </summary>
        /// <param name="propertyName"></param>
        protected void OnPropertyChanged(string propertyName)
        {
            PropertyChangedEventHandler handler = PropertyChanged;
            if (handler != null)
            {
                handler(this, new PropertyChangedEventArgs(propertyName));
            }
        }

        #endregion
    }

Q: Okay, so what now?
A: Glad you asked. Let’s augment the above class:

    /// <summary>
    /// Extends the INotifyPropertyChanged interface to the class properties.
    /// </summary>
    public abstract class ViewModelBase : INotifyPropertyChanged
    {
        #region Methods

        /// <summary>
        /// To be used within the "set" accessor in each property.
        /// This invokes the OnPropertyChanged method.
        /// </summary>
        /// <typeparam name="T"></typeparam>
        /// <param name="name"></param>
        /// <param name="value"></param>
        /// <param name="newValue"></param>
        protected void SetValue<T>(string name, ref T value, ref  T newValue, Action onChanged = null)
        {
            if (newValue != null)
            {
                if (!newValue.Equals(value))
                {
                    value = newValue;
                    OnPropertyChanged(name, onChanged);
                }
            }
            else
            {
                value = default(T);
            }
        }

        #endregion

        #region INotifyPropertyChanged

        /// <summary>
        /// The PropertyChanged event handler.
        /// </summary>
        public event PropertyChangedEventHandler PropertyChanged;
        
        /// <summary>
        /// Calls the PropertyChanged event
        /// </summary>
        /// <param name="propertyName"></param>
        protected void OnPropertyChanged(string propertyName, Action onChanged = null)
        {
            PropertyChangedEventHandler handler = PropertyChanged;
            if (handler != null)
            {
                handler(this, new PropertyChangedEventArgs(propertyName));

                // On changed
                if (onChanged != null)
                {
                    onChanged();
                }
            }
        }

        #endregion
    }

Essentially, we added an Action as a parameter to the SetValue method. This is then passed whenever the OnPropertyChanged method is called and executed after the event fires (if it is not null).

Using it is as easy as this:

    private string phoneNumberValue = String.Empty;

    public string PhoneNumber
    {
        get
        {
            return this.phoneNumberValue;
        }

        set
        {
            SetValue("PhoneNumber", ref this.phoneNumberValue, ref value, () => { UpdateSomeOtherUISection(); });
        }
    }

It’s not a common need, but sometimes something like this can really get you out of a bind when you need to update other areas of your application when this property changes.

Try it sometime.