c# listview取消选择项目

Question

我正在开发一个 Windows 应用程序，该应用程序的 ListView 包含一堆项目。 当用户单击一个项目时，应用程序会显示该项目的详细信息。这 然后用户有机会编辑这些详细信息。用户应该点击 每次更改后的“保存”按钮，当然这并不总是发生。

如果用户进行了更改但没有点击保存，应用会显示一条消息 框询问他们是否要保存更改。此框包括取消 按钮，如果他们点击取消，我想短路选择 另一个项目并将用户保持在他们正在编辑的项目上。

我找不到执行此操作的方法，如果项目更改但未保存，我会从 itemselectedchanged 事件中显示对话框，如果用户单击取消，我会从事件中删除我的函数并手动更改所选项目，然后我将函数返回给事件，但在此之后事件调用和我手动选择的项目没有被选中。

    private bool EnsureSelected()
    {
        bool continue_ = true;
        if (_objectChange)
        {
            var res = MessageBox.Show("Do you want to save changes?", "Warning", MessageBoxButtons.YesNoCancel, MessageBoxIcon.Warning);
            switch (res)
            {
                case DialogResult.Cancel:
                    if (!string.IsNullOrEmpty(_selectedKey))
                    {
                        listView_Keys.ItemSelectionChanged -= listView_Keys_ItemSelectionChanged;
                        listView_Keys.Focus();
                        listView_Keys.Items[_selectedKey].Selected = true;
                        listView_Keys.ItemSelectionChanged += listView_Keys_ItemSelectionChanged;
                    }
                    continue_ = false;
                    break;
                case DialogResult.Yes:
                    button_Save.PerformClick();
                    _objectChange = false;
                    break;
                case DialogResult.No:
                    _objectChange = false;
                    break;
                default:
                    throw new ArgumentOutOfRangeException();
            }              
        }
        return continue_;
    }

更新::

我试过这个解决方案：

        public partial class Form1 : Form
{
    public Form1()
    {
        InitializeComponent();
    }
    private ListViewItem currentSelection = null;
    private bool pending_changes = false;
    private void listView1_ItemSelectionChanged(object sender, ListViewItemSelectionChangedEventArgs e)
    {
        if (e.Item == currentSelection)
        {
            // if the current Item gets unselected but there are pending changes
            if (!e.IsSelected && pending_changes)
            {
                var res = MessageBox.Show("Do you want to save changes?", "Warning", MessageBoxButtons.YesNoCancel, MessageBoxIcon.Warning);
                switch (res)
                {
                    case DialogResult.Cancel:
                        // we dont want to change the selected item, so keep it selected
                        e.Item.Selected = true;
                        break;
                    case DialogResult.Yes:
                        //button_Save.PerformClick();
                        pending_changes = false;
                        break;
                    case DialogResult.No:
                        pending_changes = false;
                        break;
                    default:
                        throw new ArgumentOutOfRangeException();
                }
            }

        }
        else // not the selected button
        {
            if (!pending_changes && e.IsSelected)
            {
                // Item may be selected and we save it as the new current selection
                currentSelection = e.Item;
            }
            else if (pending_changes && e.IsSelected)
            {
                // Item may not be enabled, because there are pending changes
                e.Item.Selected = false;
            }
        }
    }

    private void Form1_Load(object sender, EventArgs e)
    {
        listView1.Items[0].Selected = true;
    }

    private void button1_Click(object sender, EventArgs e)
    {
        pending_changes = true;
    }
}

但这不起作用，第一次挂起的更改为真，消息框调用了两次，第二次没有发生任何事情。

Answer 1

首先，当您选择另一个项目时，该事件应该触发两次。

首先是被取消选择的项目（其中e.IsSelected 是false）

被选中的项目的秒数（e.IsSelected 是 true）

假设您设置了一个标志pending_changes，只要有未保存的更改，则以下代码应取消项目选择。

不幸的是，每当您显示 MessageBox 时，listView 就会再次失去焦点。当您单击 MessageBox 时，焦点又回到 listView 上，这会导致控件再次触发其事件。这就是为什么需要一个肮脏的解决方法，需要记住我们点击了消息框上的“取消”并再次对下一个事件执行操作。

这是包含解决方法的代码：

private ListViewItem currentSelection = null;
private bool pending_changes = false;
private bool cancel_flag = false;
private void listView1_ItemSelectionChanged(object sender, ListViewItemSelectionChangedEventArgs e)
{
    Console.WriteLine("Item " + e.ItemIndex + " is now " + e.IsSelected);
    if (e.Item != currentSelection)
    {
        // if another item gets selected but there are pending changes
        if (e.IsSelected && pending_changes)
        {
            if (cancel_flag)
            {
                // this handles the second mysterious event
                cancel_flag = false;
                currentSelection.Selected = true;
                e.Item.Selected = false;
                return;
            }
            Console.WriteLine("uh oh. pending changes.");
            var res = MessageBox.Show("Do you want to save changes?", "Warning", MessageBoxButtons.YesNoCancel, MessageBoxIcon.Warning);
            switch (res)
            {
                case DialogResult.Cancel:
                    // we dont want to change the selected item, so keep it selected
                    currentSelection.Selected = true;
                    e.Item.Selected = false;
                    // for some reason, we will get the same event with the same argments again, 
                    // after we click the cancel button, so remember our decision
                    cancel_flag = true;
                    break;
                case DialogResult.Yes:
                    // saving here. if possible without clicking the button, but by calling the method that is called to save
                    pending_changes = false;
                    currentSelection = e.Item;
                    break;
                case DialogResult.No:
                    pending_changes = false;
                    currentSelection = e.Item;
                    break;
                default:
                    throw new ArgumentOutOfRangeException();
            }
        }
        else if (e.IsSelected && !pending_changes)
        {
            currentSelection = e.Item;
        }
    }
}

Answer 2

您可以只重新选择项目并将当前状态保留在布尔标志中，以避免导致不必要的代码运行（例如，如果所选项目实际上没有更改，则重新加载所选项目的值）。

另一种方法是处理LVN_ITEMCHANGING 事件，不幸的是，它没有在WinForms 中实现。您可以在ListView item changing event thread on MSDN forums 中找到支持此事件并允许防止更改选择的扩展列表视图类。

这是来自该线程的代码：

using System;
using System.ComponentModel;
using System.Diagnostics;
using System.Runtime.InteropServices;
using System.Windows.Forms;

public partial class Form1 : Form
{
    [STAThread]
    static void Main()
    {
        Application.EnableVisualStyles();
        Application.SetCompatibleTextRenderingDefault(false);
        Application.Run(new Form1());
    }

    public Form1()
    {
        ListViewEx listView;
        Controls.Add(listView = new ListViewEx { Dock = DockStyle.Fill, Items = { "One", "Two", "Three" } });
        listView.ItemSelectionChanging += (s, e) =>
            {
                if (e.Index == 1)
                    e.Cancel = true;
                Debug.WriteLine(e);
            };
    }
}

public class ItemSelectionChangingEventArgs : CancelEventArgs
{
    public int Index { get; private set; }
    public bool NewValue { get; private set; }
    public bool CurrentValue { get; private set; }

    public ItemSelectionChangingEventArgs(int index, bool newValue, bool currentValue)
    {
        Index = index;
        NewValue = newValue;
        CurrentValue = currentValue;
    }

    public override string ToString()
    {
        return String.Format("Index={0}, NewValue={1}, CurrentValue={2}", Index, NewValue, CurrentValue);
    }
}

public class ListViewEx : ListView
{
    private static readonly Object ItemSelectionChangingEvent = new Object();
    public event EventHandler<ItemSelectionChangingEventArgs> ItemSelectionChanging
    {
        add { Events.AddHandler(ItemSelectionChangingEvent, value); }
        remove { Events.RemoveHandler(ItemSelectionChangingEvent, value); }
    }

    protected virtual void OnItemSelectionChanging(ItemSelectionChangingEventArgs e)
    {
        EventHandler<ItemSelectionChangingEventArgs> handler =
            (EventHandler<ItemSelectionChangingEventArgs>)Events[ItemSelectionChangingEvent];
        if (handler != null)
            handler(this, e);
    }

    protected override void WndProc(ref Message m)
    {
        if (m.Msg == 0x2000 + 0x004E) // [reflected] WM_NOTIFY
        {
            uint nmhdrCode = (uint)Marshal.ReadInt32(m.LParam, NmHdrCodeOffset); 
            if (nmhdrCode == LVN_ITEMCHANGING)
            {
                NMLISTVIEW nmlv = (NMLISTVIEW)Marshal.PtrToStructure(m.LParam, typeof(NMLISTVIEW));
                if ((nmlv.uChanged & LVIF_STATE) != 0)
                {
                    bool currentSel = (nmlv.uOldState & LVIS_SELECTED) == LVIS_SELECTED;
                    bool newSel = (nmlv.uNewState & LVIS_SELECTED) == LVIS_SELECTED;

                    if (newSel != currentSel)
                    {
                        ItemSelectionChangingEventArgs e = new ItemSelectionChangingEventArgs(nmlv.iItem, newSel, currentSel);
                        OnItemSelectionChanging(e);
                        m.Result = e.Cancel ? (IntPtr)1 : IntPtr.Zero;
                        return;
                    }
                }
            }
        }

        base.WndProc(ref m);
    }

    const int LVIF_STATE = 8;

    const int LVIS_FOCUSED = 1;
    const int LVIS_SELECTED = 2;

    const uint LVN_FIRST = unchecked(0U - 100U);
    const uint LVN_ITEMCHANGING = unchecked(LVN_FIRST - 0);
    const uint LVN_ITEMCHANGED = unchecked(LVN_FIRST - 1);

    static readonly int NmHdrCodeOffset = IntPtr.Size * 2;

    [StructLayout(LayoutKind.Sequential)]
    struct NMHDR
    {
        public IntPtr hwndFrom;
        public IntPtr idFrom;
        public uint code;
    }

    [StructLayout(LayoutKind.Sequential)]
    struct NMLISTVIEW
    {
        public NMHDR hdr;
        public int iItem;
        public int iSubItem;
        public int uNewState;
        public int uOldState;
        public int uChanged;
        public IntPtr lParam;
    }
}