PropertyObserver.cs

using System;
using System.Collections.Generic;
using System.ComponentModel;
using System.Linq.Expressions;
using System.Reflection;

#nullable enable
namespace Prism.Commands
{
    /// <summary>
    /// Provide a way to observe property changes of INotifyPropertyChanged objects and invokes a 
    /// custom action when the PropertyChanged event is fired.
    /// </summary>
    internal class PropertyObserver
    {
        private readonly Action _action;

        private PropertyObserver(Expression propertyExpression, Action action)
        {
            _action = action;
            SubscribeListeners(propertyExpression);
        }

        private void SubscribeListeners(Expression? propertyExpression)
        {
            var propNameStack = new Stack<PropertyInfo>();
            while (propertyExpression is MemberExpression temp) // Gets the root of the property chain.
            {
                propertyExpression = temp.Expression;

                if (temp.Member is PropertyInfo propertyInfo)
                {
                    propNameStack.Push(propertyInfo); // Records the member info as property info
                }
            }

            if (propertyExpression is not ConstantExpression constantExpression)
                throw new NotSupportedException("Operation not supported for the given expression type. " +
                                                "Only MemberExpression and ConstantExpression are currently supported.");

            var propObserverNodeRoot = new PropertyObserverNode(propNameStack.Pop(), _action);
            PropertyObserverNode previousNode = propObserverNodeRoot;
            foreach (var propName in propNameStack) // Create a node chain that corresponds to the property chain.
            {
                var currentNode = new PropertyObserverNode(propName, _action);
                previousNode.Next = currentNode;
                previousNode = currentNode;
            }

            object? propOwnerObject = constantExpression.Value;

            if (propOwnerObject is not INotifyPropertyChanged inpcObject)
                throw new InvalidOperationException("Tried to subscribe to PropertyChanged in the object that " +
                                                    $"defines the '{propObserverNodeRoot.PropertyInfo.Name}' property, but the object does not implement INotifyPropertyChanged.");

            propObserverNodeRoot.SubscribeListenerFor(inpcObject);
        }

        /// <summary>
        /// Observes a property that implements INotifyPropertyChanged, and automatically calls a custom action on 
        /// property changed notifications. The given expression must be in this form: "() => Prop.NestedProp.PropToObserve".
        /// </summary>
        /// <param name="propertyExpression">Expression representing property to be observed. Ex.: "() => Prop.NestedProp.PropToObserve".</param>
        /// <param name="action">Action to be invoked when PropertyChanged event occurs.</param>
        internal static PropertyObserver Observes<T>(Expression<Func<T>> propertyExpression, Action action)
        {
            return new PropertyObserver(propertyExpression.Body, action);
        }
    }
}