Examples of collection changed events on Observable Collection

Question

I have listbox in a WPF application that displays an observable collection of photo objects. When a photo is added to the collection the UI needs show the new image right away. I understand this can be handled using the CollectionChanged event. I have looked around for examples on how to use the handle collection changed events but I havent found anything that worked. Does anyone know of any good examples?

Another thing is that the images are coming from a directory on my computer and i have a file system watcher watching the directory for new photos being added or deleted. I am currently using file system event handler to update the collections when a photo is added or deleted but the problem is when i add a new photo to the directory, an exception is thrown saying that I cant modify the collection from a thread thats not the main thread. Does anyone know how to solve this problem too? Here is the code for this problem:

public class PhotoList : ObservableCollection<Photo>
{
    DirectoryInfo _directory;
    private FileSystemWatcher _watcher;

    public PhotoList()
    {
        _watcher = new FileSystemWatcher();
        MessageBox.Show("Watching..");
        _watcher.Filter = "*.jpg";
        _watcher.Path = Environment.GetFolderPath(Environment.SpecialFolder.MyPictures);
        _watcher.EnableRaisingEvents = true;

        _watcher.Created += new FileSystemEventHandler(FileSystemWatcher_Created);

        _directory = new DirectoryInfo(Environment.GetFolderPath(Environment.SpecialFolder.MyPictures));
    }

    public void Update()
    {
        foreach(FileInfo f in _directory.GetFiles("*.jpg"))
        {
            Add(new Photo(f.FullName));
        }
    }


    public string Path
    {
        set
        {
            _directory = new DirectoryInfo(Environment.GetFolderPath(Environment.SpecialFolder.MyPictures));
            Update();
        }
        get
        {
            return _directory.FullName;
        }
    }

    public void FileSystemWatcher_Created(object sender, FileSystemEventArgs e)
    {
        Add(new Photo(e.FullPath));
    }
}

Answer 1

First question: ObservableCollection<T> already implements INotifyCollectionChanged, so simply bind on that object and you are fine, the UI will get the updates happening in the collection automatically.

Second question: See this post: WPF threading: can I update a control's data context in a non-UI thread? and the accompening comments.

Answer 2

wrap the Add(new Photo(e.FullPath)) inside of Dispatcher.Invoke(). That way Add will be called on the UI thread