需要帮助在我的TreeView中实现多线程（C＃，WPF）

Question

Okay, I have a TreeView that serves as a directory tree for Windows. I have it loading all the directories and functioning correctly, but it is pausing my GUI while it loads directories with many children. I'm trying to implement multithreading, but am new to it and am having no luck.

This is what I have for my TreeView:

private readonly object _dummyNode = null;

public MainWindow()
{
    InitializeComponent();

    foreach (string drive in Directory.GetLogicalDrives())
    {
        DriveInfo Drive_Info = new DriveInfo(drive);                

        if (Drive_Info.IsReady == true)
        {
            TreeViewItem item = new TreeViewItem();
            item.Header = drive;
            item.Tag = drive;
            item.Items.Add(_dummyNode);
            item.Expanded += folder_Expanded;

            TreeViewItemProps.SetIsRootLevel(item, true);

            Dir_Tree.Items.Add(item);
        }
    }
}

private void folder_Expanded(object sender, RoutedEventArgs e)
{
    TreeViewItem item = (TreeViewItem)sender;

    if (item.Items.Count == 1 && item.Items[0] == _dummyNode)
    {
        item.Items.Clear();

        try
        {
            foreach (string dir in Directory.GetDirectories(item.Tag as string))
            {
                DirectoryInfo tempDirInfo = new DirectoryInfo(dir);

                bool isSystem = ((tempDirInfo.Attributes & FileAttributes.System) == FileAttributes.System);

                if (!isSystem)
                {
                    TreeViewItem subitem = new TreeViewItem();
                    subitem.Header = tempDirInfo.Name;
                    subitem.Tag = dir;
                    subitem.Items.Add(_dummyNode);
                    subitem.Expanded += folder_Expanded;
                    subitem.ToolTip = dir;
                    item.Items.Add(subitem);
                }
            }
        }
        catch (Exception ex)
        {
            Console.WriteLine(ex);
        }
    }
}

Whenever I expand a directory that has a large number of subdirectories, the program appears to be frozen for a few seconds. I would like to display a loading message or animation while it's processing, but I'm not sure how to begin with multithreading. I know I have to use the TreeView's Dispatcher.BeginInvoke method, but other than that I'm kinda lost.

Any help would be greatly appreciated!!!

Answer 1

One of the easiest ways to start a async process is to use an anonymous delegate with BeginInvoke. As an example you could move your code in the constructor to a separate method (say RenderTreeView) and then call it asynchronously to begin a new thread as follows:

Action action = RenderTreeView;
action.BeginInvoke(null, null);

The trick to this is that any time you interact with any UI elements from the async process you need to rejoin the main UI thread, otherwise you will get an exception about cross thread access. This is relatively straight forward as well.

In Windows Forms it's:

if (InvokeRequired)
    Invoke(new MethodInvoker({item.Items.Add(subitem)}));
else
    item.Items.Add(subitem);

In WPF it's:

if (!Dispatcher.CheckAccess())
    Dispatcher.Invoke(new Action(() => item.Items.Add(subitem)));
else
    item.Items.Add(subitem);

You really need to break up the code to make it more flexible in terms of methods. At the moment everything is bundled in one method which makes it hard to work with and re-factor for async processes.

Update Here you go :)

public partial class MainWindow : Window
{
    private readonly object dummyNode = null;

    public MainWindow()
    {
        InitializeComponent();

        Action<ItemCollection> action = RenderTreeView;
        action.BeginInvoke(treeView1.Items, null, null);
    }

    private void RenderTreeView(ItemCollection root)
    {
        foreach (string drive in Directory.GetLogicalDrives())
        {
            var driveInfo = new DriveInfo(drive);

            if (driveInfo.IsReady)
            {
                CreateAndAppendTreeViewItem(root, drive, drive, drive);
            }
        }
    }

    private void FolderExpanded(object sender, RoutedEventArgs e)
    {
        var item = (TreeViewItem) sender;

        if (item.Items.Count == 1 && item.Items[0] == dummyNode)
        {
            item.Items.Clear();
            var directory = item.Tag as string;
            if (string.IsNullOrEmpty(directory))
            {
                return;
            }
            Action<TreeViewItem, string> action = ExpandTreeViewNode;
            action.BeginInvoke(item, directory, null, null);
        }
    }

    private void ExpandTreeViewNode(TreeViewItem item, string directory)
    {
        foreach (string dir in Directory.GetDirectories(directory))
        {
            var tempDirInfo = new DirectoryInfo(dir);

            bool isSystem = ((tempDirInfo.Attributes & FileAttributes.System) == FileAttributes.System);

            if (!isSystem)
            {
                CreateAndAppendTreeViewItem(item.Items, tempDirInfo.Name, dir, dir);
            }
        }
    }

    private void AddChildNodeItem(ItemCollection collection, TreeViewItem subItem)
    {
        if (Dispatcher.CheckAccess())
        {
            collection.Add(subItem);
        }
        else
        {
            Dispatcher.Invoke(new Action(() => AddChildNodeItem(collection, subItem)));
        }
    }

    private void CreateAndAppendTreeViewItem(ItemCollection items, string header, string tag, string toolTip)
    {
        if (Dispatcher.CheckAccess())
        {
            var subitem = CreateTreeViewItem(header, tag, toolTip);
            AddChildNodeItem(items, subitem);
        }
        else
        {
            Dispatcher.Invoke(new Action(() => CreateAndAppendTreeViewItem(items, header, tag, toolTip)));
        }
    }

    private TreeViewItem CreateTreeViewItem(string header, string tag, string toolTip)
    {
        var treeViewItem = new TreeViewItem {Header = header, Tag = tag, ToolTip = toolTip};

        treeViewItem.Items.Add(dummyNode);
        treeViewItem.Expanded += FolderExpanded;

        return treeViewItem;
    }
}

Answer 2

Multithreading may not help much here because the TreeView has to be updated on it's Dispatcher thread.

TreeViews will pause when loading a large number of entries. One way to get around this is to store the contents into an object that mirrored the TreeView structure, and then programmatically load just the first level of the TreeView.

When a user clicks on a node, load the next level of child nodes and expand the node. When that node is collapsed, delete its child nodes to conserve TreeView memory. This has worked well for me. For exceedingly large structures I've used a local Sqlite database (via System.Data.Sqlite ) as my backing store and even then the TreeView loaded quickly and was responsive.

Answer 3

You can also look at using BackgroundWorker; that's easiest way of executing an operation on a separate thread(For me :) ).

BackgroundWorker Component Overview: http://msdn.microsoft.com/en-us/library/8xs8549b.aspx

BackgroundWorker Class: http://msdn.microsoft.com/en-us/library/system.componentmodel.backgroundworker.aspx

You can use it with Command pattern as explained here -

Asynchronous WPF Commands