Decorator Pattern

This is an example of the Decorator pattern, in this case a decorator for ObservableCollection.  Working with WPF and ObservableCollection using threads, one will run into the problem whereby the ObservableCollection cannot be updated from outside the owning thread; Delegates and Invoke will not work. A solution is to contain and expand the class, as is done in this example on michIG's Blog.

This linked file contains the original code in C#, as well as the same code converted to VB.NET.

Salient Characteristic(s)
  • Sets an internal pointer to the decorated object, sending method calls and property actions to the internal object
  • Extends the object by wrapping it and adding some aspect handled by the decorator

Code

using System;
using System.Collections.ObjectModel;
using System.Windows.Threading;
using System.Collections.Specialized;
using System.ComponentModel;

namespace DesignPatterns
{
    /// <summary>  
    /// This class is an observablecollection which invokes automatically.  
    /// This means that any change will be done in the right thread.  
    /// </summary>  
    /// <typeparam name="T">The type of the elements</typeparam>  
    public class DispatchingObservableCollection<T> : ObservableCollection<T>
    {
        /// <summary>  
        /// The default constructor of the ObservableCollection  
        /// </summary>  
        public DispatchingObservableCollection()
        {
            //Assign the current Dispatcher (owner of the collection)  
            _currentDispatcher = Dispatcher.CurrentDispatcher;
        }

        private readonly Dispatcher _currentDispatcher;


        /// <summary>  

        /// Executes this action in the right thread  
        /// </summary>  
        ///<param name="action">The action which should be executed</param>  
        private void DoDispatchedAction(Action action)
        {
            if (_currentDispatcher.CheckAccess())
                action.Invoke();
            else
                _currentDispatcher.Invoke(DispatcherPriority.DataBind, action);
        }

        /// <summary>  

        /// Clears all items  
        /// </summary>  
        protected override void ClearItems()
        {
            DoDispatchedAction(BaseClearItems);
        }

        private void BaseClearItems()

        {
            base.ClearItems();
        }

        /// <summary>  

        /// Inserts a item at the specified index  
        /// </summary>  
        ///<param name="index">The index where the item should be inserted</param>  
        ///<param name="item">The item which should be inserted</param>  
        protected override void InsertItem(int index, T item)
        {
            DoDispatchedAction(() => BaseInsertItem(index, item));
        }

        private void BaseInsertItem(int index, T item)

        {
            base.InsertItem(index, item);
        }

        /// <summary>  

        /// Moves an item from one index to another  
        /// </summary>  
        ///<param name="oldIndex">The index of the item which should be moved</param>  
        ///<param name="newIndex">The index where the item should be moved</param>  
        protected override void MoveItem(int oldIndex, int newIndex)
        {
            DoDispatchedAction(() => BaseMoveItem(oldIndex, newIndex));
        }

        private void BaseMoveItem(int oldIndex, int newIndex)

        {
            base.MoveItem(oldIndex, newIndex);
        }

        /// <summary>  

        /// Removes the item at the specified index  
        /// </summary>  
        ///<param name="index">The index of the item which should be removed</param>  
        protected override void RemoveItem(int index)
        {
            DoDispatchedAction(() => BaseRemoveItem(index));
        }

        private void BaseRemoveItem(int index)

        {
            base.RemoveItem(index);
        }

        /// <summary>  

        /// Sets the item at the specified index  
        /// </summary>  
        ///<param name="index">The index which should be set</param>  
        ///<param name="item">The new item</param>  
        protected override void SetItem(int index, T item)
        {
            DoDispatchedAction(() => BaseSetItem(index, item));
        }

        private void BaseSetItem(int index, T item)

        {
            base.SetItem(index, item);
        }
        
        /// <summary>  
        /// Fires the CollectionChanged Event  
        /// </summary>  
        ///<param name="e">The additional arguments of the event</param>  
        protected override void OnCollectionChanged(NotifyCollectionChangedEventArgs e)
        {
            DoDispatchedAction(() => BaseOnCollectionChanged(e));
        }

        private void BaseOnCollectionChanged(NotifyCollectionChangedEventArgs e)

        {
            base.OnCollectionChanged(e);
        }

        /// <summary>  

        /// Fires the PropertyChanged Event  
        /// </summary>  
        ///<param name="e">The additional arguments of the event</param>  
        protected override void OnPropertyChanged(PropertyChangedEventArgs e)
        {
            DoDispatchedAction(() => BaseOnPropertyChanged(e));
        }

        private void BaseOnPropertyChanged(PropertyChangedEventArgs e)

        {
            base.OnPropertyChanged(e);
        }
    }
}


Comments

  1. Hi,
    I really found your article informative. I would like to know more about design patterns, could you help me know what are elite sites for acquiring knowledge? Thanking in advance.

    ReplyDelete
  2. No sites, mostly reading books, as well as using the patterns in code:

    - Professional Enterprise .NET
    - Microsoft .NET: Architecting Applications for the Enterprise: Architecting Applications for the Enterprise
    - Framework Design Guidelines: Conventions, Idioms, and Patterns for Reusable .NET Libraries
    - Design Patterns Explained: A New Perspective on Object-Oriented Design
    - Design Patterns: Elements of Reusable Object-Oriented Software

    James

    ReplyDelete

Post a Comment

Popular posts from this blog

Multiton

How do you deal with making sure your use of new technology is correct and free from code-smells, security issues, etc.? - Hashnode