Thread.Sleep永不返回

Question

I have an application that draws somethong on a form (just a test application). I faced a problem with Thread.Sleep. I use thread that initiates DrawinArea repaint every 100 milliseconds. It works fine, but sometimes this thread cannot return from This.Sleep(100) call. I've checked in the debugger, and I'm sure that problem is in Thread.Sleep().

Attention: it requires Gtk# to run.

using System;
using Gtk;
using Cairo;
using System.Threading;
using System.Timers;

public partial class MainWindow: Gtk.Window
{

    Thread redrawThread = new Thread(Redrawer);

    //System.Timers.Timer timer = new System.Timers.Timer();

    static int calls = 0;

    public MainWindow()
        : base(Gtk.WindowType.Toplevel)
    {
        Build();
        this.drawingarea2.ExposeEvent += OnExpose;

        redrawThread.Name = "Redraw Thread";

        redrawThread.Start(this);
        /*
        timer.Interval = 100;
        timer.Elapsed += (sender, e) =>
        {
            drawingarea2.QueueDraw();
        };
        timer.AutoReset = true;
        timer.Start();*/

    }

    protected void OnDeleteEvent(object sender, DeleteEventArgs a)
    {
        redrawThread.Abort();
        Application.Quit();
        a.RetVal = true;
    }


    static void Redrawer(object wnd) {
        var area = (MainWindow)wnd;
        while (true)
        {
            Thread.Sleep(100);
            area.QueueDraw();
        }
    }

    int x = 200;
    int x_mod = 10;
    int y = 150;
    int y_mod = 10;



    void OnExpose(object sender, ExposeEventArgs args) {

        var area = (DrawingArea)sender;

        if (x + 10 >= drawingarea2.Allocation.Width || x - 10 < 0)
            x_mod = -x_mod;

        if (y + 10 >= drawingarea2.Allocation.Height || y - 10 < 0)
            y_mod = -y_mod;

        x += x_mod;
        y += y_mod;

        var ny = Math.Abs(y - drawingarea2.Allocation.Height);

        using (var ctx = Gdk.CairoHelper.Create(area.GdkWindow))
        {


            ctx.LineWidth = 9;
            ctx.SetSourceRGB(0.7, 0.2, 0.0);

            ctx.Arc(x, ny, 10, 0, 2*Math.PI);
            ctx.StrokePreserve();

            ctx.SetSourceRGB(0.3, 0.4, 0.6);
            ctx.Fill();

            ctx.GetTarget().Dispose();
        }


    }

}

Once faced this problem I decided to switch to System.Timers.Timer, but I've also faced a problem with it. Timer stops firing event after some time. I searched for this problem and found that GC can destroy timer if there are no references to it. My timer is a class member, so reference always exists, but in any case, it stops. What is the problem?

Answer 1

From the documentation of QueueDraw() it stated

Equivalent to calling Widget.QueueDrawArea for the entire area of a widget.

Reading the documentation for QueueDrawArea it states

Invalidates the rectangular area of widget [...]. Once the main loop becomes idle (after the current batch of events has been processed, roughly), the window will receive Widget.ExposeEvent events for the union of all regions that have been invalidated.

What is happening is every 100ms you are putting another message on the queue for the main window and the main window is taking longer than 100ms to process this request. Because you are producing redraw requests faster than it can fulfill redraw requests this is flooding the queue and once it reaches its max size no new messages can be received and it "locks up".

When you stated you tested this by updating the window's title that update would also need to go on to the queue to get done, however because the queue is full those title changes never showed up. Sleep was not hanging, it was just your debugging method was broken due to the same underlying problem.

The solution is call QueueDraw() less often.