OnPaint not getting called if form doesn't have focus

Question

I have a user control with custom painting. Constructor sets styles correctly, from what I can tell. Basic code:

public partial class LineChart2 : UserControl
{
    public LineChart2()
    {
        InitializeComponent();

        //Set control styles to eliminate flicker on redraw and to redraw on resize
        this.SetStyle(
            ControlStyles.ResizeRedraw |
            ControlStyles.UserPaint |
            ControlStyles.AllPaintingInWmPaint |
            ControlStyles.DoubleBuffer,
            true);

        SetDefaultValues();
    }

    protected override void OnPaint(PaintEventArgs e)
    {
    // breakpoint set here for verification
        Paint~misc stuff(e.Graphics);

        base.OnPaint(e);
    }

    private void UpdateGraph()
    {
    // this is called when the data that the control depends on changes
        ~update stuff();

        this.Invalidate();
        //this.Refresh();
    }
}

The control is contained within a Panel on a standard WinForm.

I've tried both Invalidate and Refresh.

When using Invalidate(), the control will redraw properly as long as the form it is contained in has focus. Drawing is smooth. When I switch focus to another form, drawing ceases even though the events are still firing, and this.Invalidate() is still being called. The form is still fully visible on screen.

When using Refresh(), the control will redraw regardless of whether the form has focus, but the drawing constantly flickers, as if bypassing the double-buffering mechanism.

So how do I get the Invalidate message to properly invoke the OnPaint method regardless of focus?

Answer 1

You should not force the control do redraw (Update or Refresh) so often. The UI may get not responsive, others controls may not update, because you are giving all UI attention to the forced sync Refresh. The right way is to draw only when UI is ready to do it. For that you need a render loop. The ApplicationLoopDoWork will be fired every time the UI is ready to draw something. The period depends on the machine speed and what is being redrawn.

The class is based on this post on Tom Miller's Blog.

Here is the class that I use to control that. Make updates only on the ApplicationLoopDoWork call.

    using System;
    using System.Runtime.InteropServices;
    using System.Threading;
    using System.Windows.Forms;

    namespace Utilities.UI
    {
    /// <summary>
    /// WinFormsAppIdleHandler implements a WinForms Render Loop (max FPS possible).
    /// Reference: http://blogs.msdn.com/b/tmiller/archive/2005/05/05/415008.aspx
    /// </summary>
    public sealed class WinFormsAppIdleHandler
    {
        private readonly object _completedEventLock = new object();
        private event EventHandler _applicationLoopDoWork;

        //PRIVATE Constructor
        private WinFormsAppIdleHandler()
        {
            Enabled = false;
            SleepTime = 10;

        }

        /// <summary>
        /// Singleton from:
        /// http://csharpindepth.com/Articles/General/Singleton.aspx
        /// </summary>
        private static readonly Lazy<WinFormsAppIdleHandler> lazy = new Lazy<WinFormsAppIdleHandler>(() => new WinFormsAppIdleHandler());
        public static WinFormsAppIdleHandler Instance { get { return lazy.Value; } }

        private bool _enabled = false;

        /// <summary>
        /// Gets or sets if must fire ApplicationLoopDoWork event.
        /// </summary>
        public bool Enabled
        {
            get { return _enabled; }
            set {
                if (value)
                    Application.Idle += Application_Idle;
                else
                    Application.Idle -= Application_Idle;

                _enabled = value;
            }
        }

        /// <summary>
        /// Gets or sets the minimum time betwen ApplicationLoopDoWork fires.
        /// </summary>
        public int SleepTime { get; set; }

        /// <summary>
        /// Fires while the UI is free to work. Sleeps for "SleepTime" ms.
        /// </summary>
        public event EventHandler ApplicationLoopDoWork
        {
            //Reason of using locks:
            //http://stackoverflow.com/questions/1037811/c-thread-safe-events
            add
            {
                lock (_completedEventLock)
                    _applicationLoopDoWork += value;
            }

            remove
            {
                lock (_completedEventLock)
                    _applicationLoopDoWork -= value;
            }
        }

        /// <summary>
        ///Application idle loop.
        /// </summary>
        /// <param name="sender"></param>
        /// <param name="e"></param>
        private void Application_Idle(object sender, EventArgs e)
        {
            //Try to update interface
            while (Enabled && IsAppIdle())
            {
                OnApplicationIdleDoWork(EventArgs.Empty);
                //Give a break to the processor... :)
                //8 ms -> 125 Hz
                //10 ms -> 100 Hz
                Thread.Sleep(SleepTime);
            }
        }

        private void OnApplicationIdleDoWork(EventArgs e)
        {
            var handler = _applicationLoopDoWork;
            if (handler != null)
            {
                handler(this, e);
            }
        }

        /// <summary>
        /// Gets if the app is idle.
        /// </summary>
        /// <returns></returns>
        public static bool IsAppIdle()
        {
            bool isIdle = false;
            try
            {
                Message msg;
                isIdle = !PeekMessage(out msg, IntPtr.Zero, 0, 0, 0);
            }
            catch (Exception e)
            {
                //Should never get here... I hope...
                MessageBox.Show("IsAppStillIdle() Exception. Message: " + e.Message);
            }
            return isIdle;
        }

        #region  Unmanaged Get PeekMessage
        // http://blogs.msdn.com/b/tmiller/archive/2005/05/05/415008.aspx
        [System.Security.SuppressUnmanagedCodeSecurity] // We won't use this maliciously
        [DllImport("User32.dll", CharSet = CharSet.Auto)]
        public static extern bool PeekMessage(out Message msg, IntPtr hWnd, uint messageFilterMin, uint messageFilterMax, uint flags);

        #endregion
    }
}

Answer 2

Documentation says:

Calling the Invalidate method does not force a synchronous paint; to force a synchronous paint, call the Update method after calling the Invalidate method.

Have you tried calling Update after Invalidate?

Answer 3

You also might try Invalidate(true) to trigger child controls to repaint as well.