使用ProcessCmdKey保持向下箭头按下事件

Question

I need to perform a certain method as long as an arrow key is being held down. Now, for regular arrow key presses in Visual studio 2012, using OnKeyDown did not work, so I needed to use ProcessCmdKey , which works like a charm. Is there a way to use ProcessCmdKey to detect when an arrow has been released / being held down?

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I've taken both Markus' and Hans' advice, and combined the two ideas. I used the ProcessKeyPreview method, but I am still having some issues. When I hold down any arrow key, this method will not detect that WM_KEYDOWN has happened... but as soon as I release my finger from the key, it actually DOES notice that WM_KEYUP has happened.

The interesting this is that when I hold down any other key (ie letter 'S'), it properly recognizes when it has been pressed and released. I've posted a fragment of my code below:

const int WM_KEYUP = 0x0101;
const int WM_KEYDOWN = 0x0100;

    protected override bool ProcessKeyPreview(ref Message m)
            {
                int msgVal = m.WParam.ToInt32();            
                if (m.Msg == WM_KEYDOWN)
                {
                    switch ((Keys)msgVal) {
                        case Keys.Down:
                            Console.WriteLine("down pressed"); //not detected
                            break;
                        case Keys.S:
                            Console.WriteLine("S pressed!"); //detected
                            break;
                    }
                }
                if (m.Msg == WM_KEYUP)
                {
                    switch ((Keys)msgVal)
                    {
                        case Keys.Down:
                            Console.WriteLine("down released"); //detected
                            break;
                        case Keys.S:
                            Console.WriteLine("s released!"); //detected
                            break;
                    }
                }
                return base.ProcessKeyPreview(ref m);
            }        

Answer 1

You cannot see the KeyUp event with ProcessCmdKey(), it was made to only handle KeyDown events. You'll need to tackle this at a much lower level if the form contains controls. The trick is to intercept the message before Winforms sends it through the normal key-handling and WndProc chain. That requires implementing the IMessageFilter interface. Like this:

public partial class Form1 : Form, IMessageFilter {  // NOTE: added IMessageFilter
    public Form1() {
        InitializeComponent();
        Application.AddMessageFilter(this);
    }
    protected override void OnFormClosed(FormClosedEventArgs e) {
        Application.RemoveMessageFilter(this);
        base.OnFormClosed(e);
    }

    bool IMessageFilter.PreFilterMessage(ref Message m) {
        // Trap WM_KEYUP/DOWN for Keys.Down key
        if ((m.Msg == 0x100 || m.Msg == 0x101) && (Keys)m.WParam.ToInt32() == Keys.Down) {
            bool repeat = (m.LParam.ToInt32() & (1 << 30)) != 0;
            bool down = m.Msg == 0x100;
            // But only for this form
            Form form = null;
            var ctl = Control.FromHandle(m.HWnd);
            if (ctl != null) form = ctl.FindForm();
            if (form == this) {
                OnCursorDown(down, repeat & down);
                return true;
            }
        }
        return false;
    }
    private void OnCursorDown(bool pressed, bool repeat) {
        // etc..
    }
}

Answer 2

You can overload ProcessKeyPreview instead, which will let you see the WM_KEYDOWN message as well as the WM_KEYUP message. Be sure to turn on KeyPreview for the form.

WM_KEYDOWN:
http://msdn.microsoft.com/en-us/library/windows/desktop/ms646280(v=vs.85).aspx

WM_KEYUP:
http://msdn.microsoft.com/en-us/library/windows/desktop/ms646281(v=vs.85).aspx

Example:

public partial class TestForm : Form
{
    public TestForm()
    {
        InitializeComponent();
        this.KeyPreview = true;
    }

    const int WM_KEYUP = 0x0101;
    const int WM_KEYDOWN = 0x0100;

    protected override bool ProcessKeyPreview(ref Message m)
    {
        switch (m.Msg)
        {
            case WM_KEYDOWN:
                Console.WriteLine(m.LParam);
                break;

            case WM_KEYUP:
                Console.WriteLine(m.LParam);
                break;
        }
        return base.ProcessKeyPreview(ref m);
    }
}