How to detect if window is flashing

Question

I'm using FlashWindowEx() to flash an application window when it needs to attract a user's attention. The window caption and taskbar button flashes continuously until the application receives focus. How can I check whether the application is currently flashing (i.e. has not received focus since it was instructed to flash).

Answer 1

Here are two possible solutions. One uses WH_SHELL, and one uses a NativeWindow. You will have to provide your own extension method (FlashWindow()) to start the flashing.

// base class.  Two different forms subclass this form to illustrate two
// different solutions.
public class FormFlash : Form {

    protected Label lb = new Label { Text = "Not flashing", Dock = DockStyle.Top };

    public FormFlash() {
        Controls.Add(lb);

        Thread t = new Thread(() => {
            Thread.Sleep(3000);

            if (Form.ActiveForm == this)
                SetForegroundWindow(GetDesktopWindow()); // deactivate the current form by setting the desktop as the foreground window

            this.FlashWindow(); // call extension method to flash window
            lb.BeginInvoke((Action) delegate {
                lb.Text = "Flashing";
            });
        });
        t.IsBackground = true;
        t.Start();
    }


    [DllImport("user32.dll")]
    private static extern bool SetForegroundWindow(IntPtr hWnd);

    [DllImport("user32.dll")]
    private static extern IntPtr GetDesktopWindow();
}

// this solution is a bit simpler.  Relies on the programmer knowing when the
// flashing started.  Uses a NativeWindow to detect when a WM_ACTIVATEAPP
// message happens, that signals the end of the flashing.
class FormFlashNW : FormFlash {

    NW nw = null;
    public FormFlashNW() {
    }

    protected override void OnHandleCreated(EventArgs e) {
        base.OnHandleCreated(e);
        nw = new NW(this.Handle, lb);
    }

    protected override void OnHandleDestroyed(EventArgs e) {
        base.OnHandleDestroyed(e);
        nw.ReleaseHandle();
    }

    class NW : NativeWindow {
        Label lb = null;
        public NW(IntPtr handle, Label lb) {
            AssignHandle(handle);
            this.lb = lb;
        }

        protected override void WndProc(ref Message m) {
            base.WndProc(ref m);
            const int WM_ACTIVATEAPP = 0x1C;
            if (m.Msg == WM_ACTIVATEAPP) {
                lb.BeginInvoke((Action) delegate {
                    lb.Text = "Not flashing";
                });
            }
        }
    }
}

// this solution is more complicated.  Relies on setting up the hook proc.
// The 'isFlashing' bool fires true and false alternating while the flashing
// is active.
public class FormShellHook : FormFlash {

    public FormShellHook() {
        FlashWindowExListener.Register(this);
        FlashWindowExListener.FlashEvent += FlashExListener_FlashEvent;
    }

    void FlashExListener_FlashEvent(Form f, bool isFlashing) {
        if (f == this) {
            lb.Text = DateTime.Now.ToLongTimeString() + " is flashing: " + isFlashing;
        }
    }
}

public class FlashWindowExListener {

    private delegate IntPtr CallShellProc(int nCode, IntPtr wParam, IntPtr lParam);
    private static CallShellProc procShell = new CallShellProc(ShellProc);
    private static Dictionary<IntPtr,Form> forms = new Dictionary<IntPtr,Form>();
    private static IntPtr hHook = IntPtr.Zero;

    public static event FlashWindowExEventHandler FlashEvent = delegate {};
    public delegate void FlashWindowExEventHandler(Form f, bool isFlashing);

    static FlashWindowExListener() {
        int processID = GetCurrentThreadId();

        // we are interested in listening to WH_SHELL events, mainly the HSHELL_REDRAW event.
        hHook = SetWindowsHookEx(WH_SHELL, procShell, IntPtr.Zero, processID);

        System.Windows.Forms.Application.ApplicationExit += delegate {
            UnhookWindowsHookEx(hHook);
        };
    }

    public static void Register(Form f) {
        if (f.IsDisposed)
            throw new ArgumentException("Cannot use disposed form.");

        if (f.Handle == IntPtr.Zero) {
            f.HandleCreated += delegate {
                forms[f.Handle] = f;                
            };
        }
        else
            forms[f.Handle] = f;

        f.HandleDestroyed += delegate {
            Unregister(f);
        };
    }

    public static void Unregister(Form f) {
        forms.Remove(f.Handle);
    }

    private static IntPtr ShellProc(int nCode, IntPtr wParam, IntPtr lParam) {

        if (nCode == HSHELL_REDRAW) {
            Form f = null;
            // seems OK not having to call f.BeginInvoke
            if (forms.TryGetValue(wParam, out f))
                FlashEvent(f, (int) lParam == 1);
        }

        return CallNextHookEx(hHook, nCode, wParam, lParam);
    }

    private const int WH_SHELL = 10;
    private const int HSHELL_REDRAW = 6;

    [DllImport("user32.dll")]
    private static extern int UnhookWindowsHookEx(IntPtr idHook);

    [DllImport("user32.dll")]
    private static extern IntPtr SetWindowsHookEx(int idHook, CallShellProc lpfn, IntPtr hInstance, int threadId);

    [DllImport("kernel32.dll")]
    private static extern int GetCurrentThreadId();

    [DllImport("user32.dll")]
    private static extern IntPtr CallNextHookEx(IntPtr idHook, int nCode, IntPtr wParam, IntPtr lParam);
}