Catch Textbox Scroll Event?

Question

Textbox or richtextbox, only thing i want is triggering a function when scrollbar moves.

I already found GetScrollPos and SetScrollPos. I thought of checking scrollbar position periodically but there must be a better way. So, what is the better way?

Update: Using WinForms

Answer 1

Assuming WinForms, you can try pinvoking:

public class MyRTF: RichTextBox {

  private const int WM_HSCROLL = 0x114;
  private const int WM_VSCROLL = 0x115;
  private const int WM_MOUSEWHEEL = 0x20A;

  protected override void WndProc(ref Message m) {
    base.WndProc(ref m);

    if (m.Msg == WM_VSCROLL || m.Msg == WM_HSCROLL || m.Msg == WM_MOUSEWHEEL) {
      // scrolling...
    }
  }

}

Answer 2

Even better -- the hidden MouseWheel event: https://stackoverflow.com/a/26672097/1037948

public Form1()
{
    InitializeComponent();
    textBox1.MouseWheel += textBox1_MouseWheel;
}

void textBox1_MouseWheel(object sender, MouseEventArgs e)
{
    throw new NotImplementedException();
}

Answer 3

Adapted the accepted answer to include HorizontalScrollPosition and VerticalScrollPosition properties (to get and set the scroll positions) and ScrollChanged event (which uses .NET's ScrollEventArgs). ScrollChanged will also fire on key navigation and text modification as well as during selection drag, control resize, and font resize.

  /// <summary>
  /// TextBox with support for getting and setting the vertical scroll bar
  /// position, as well as listening to vertical scroll events.
  /// </summary>
  public class ScrollTextBox : System.Windows.Forms.TextBox
  {

     public ScrollTextBox()
     {  _components = new System.ComponentModel.Container();
        // Calculate width of "W" to set as the small horizontal increment
        OnFontChanged(null);
     }

     [  System.ComponentModel.DefaultValue(0)
     ,  System.ComponentModel.Category("Appearance")
     ,  System.ComponentModel.Description
        (  "Gets or sets the vertical scroll bar's position"
        )
     ]
     public int VerticalScrollPosition
     {  set { SetScroll(value, Win32.WM_VSCROLL, Win32.SB_VERT); }
        get { return GetScroll(Win32.SB_VERT); }
     } 

     [  System.ComponentModel.DefaultValue(0)
     ,  System.ComponentModel.Category("Appearance")
     ,  System.ComponentModel.Description
        (  "Gets or sets the horizontal scroll bar's position"
        )
     ]
     public int HorizontalScrollPosition
     {  set { SetScroll(value, Win32.WM_HSCROLL, Win32.SB_HORZ); }
        get { return GetScroll(Win32.SB_HORZ); }
     } 

     [  System.ComponentModel.Description
        (  "Fired when the scroll bar's vertical position changes"
        )
     ,  System.ComponentModel.Category("Property Changed")
     ]
     public event System.Windows.Forms.ScrollEventHandler ScrollChanged;

     // Fire scroll event if the scroll-bars are moved
     protected override void WndProc
     (  ref System.Windows.Forms.Message message
     ){ base.WndProc(ref message);
        if(message.Msg == Win32.WM_VSCROLL
        || message.Msg == Win32.WM_HSCROLL
        || message.Msg == Win32.WM_MOUSEWHEEL
        )  TryFireScrollEvent();
     }

     // Key-down includes navigation keys, but also typing and pasting can
     // cause a scroll event
     protected override void OnKeyDown
     (  System.Windows.Forms.KeyEventArgs e
     ){ base.OnKeyDown(e);
        TryFireScrollEvent();
     }

     protected override void OnKeyUp
     (  System.Windows.Forms.KeyEventArgs e
     ){ base.OnKeyUp(e);
        TryFireScrollEvent();
     }

     // Resizing can alter the word-wrap or fit the content causing the text
     // to scroll
     protected override void OnResize
     (  System.EventArgs e
     ){ base.OnResize(e);
        TryFireScrollEvent();
     }

     // Clicking an empty space can move the carot to the beginning of the
     // line causing a scroll event
     protected override void OnMouseDown
     (  System.Windows.Forms.MouseEventArgs e
     ){ base.OnMouseDown(e);
        TryFireScrollEvent();
     }

     protected override void OnMouseUp
     (  System.Windows.Forms.MouseEventArgs e
     ){ base.OnMouseUp(e);
        TryFireScrollEvent();
     }

     // If the mouse button is down, a text selection is probably being done
     // where dragging the selection can caues scroll events
     protected override void OnMouseMove
     (  System.Windows.Forms.MouseEventArgs e
     ){ base.OnMouseMove(e);
        if(e.Button != System.Windows.Forms.MouseButtons.None)
           TryFireScrollEvent();
     }

     // Changing the size of the font can cause a scroll event. Also, when
     // scrolling horizontally, the event will notify whether the scroll
     // was a large or small change. For vertical, small increments are 1
     // line, but for horizontal, it is several pixels. To guess what a
     // small increment is, get the width of the W character and anything
     // smaller than that will be represented as a small increment
     protected override void OnFontChanged
     (  System.EventArgs e
     ){ base.OnFontChanged(e);
        using(System.Drawing.Graphics graphics = this.CreateGraphics())
           _fontWidth = (int)graphics.MeasureString("W", this.Font).Width;
        TryFireScrollEvent();
     }

     protected override void Dispose
     (  bool disposing
     ){ if(disposing && (_components != null))
           _components.Dispose();
        base.Dispose(disposing);
     }

     private void SetScroll
     (  int value
     ,  uint windowsMessage
     ,  int scrollBarMessage
     ){ Win32.SetScrollPos
        (  (System.IntPtr)this.Handle
        ,  scrollBarMessage
        ,  value
        ,  true
        );
        Win32.PostMessage
        (  (System.IntPtr)this.Handle
        ,  windowsMessage
        ,  4 + 0x10000 * value
        ,  0
        );
     }

     private int GetScroll
     (  int scrollBarMessage
     ){ return Win32.GetScrollPos
        (  (System.IntPtr)this.Handle
        ,  scrollBarMessage
        );
     }

     // Fire both horizontal and vertical scroll events seperately, one
     // after the other. These first test if a scroll actually occurred
     // and won't fire if there was no actual movement
     private void TryFireScrollEvent()
     {  // Don't do anything if there is no event handler
        if(ScrollChanged == null)
           return;
        TryFireHorizontalScrollEvent();
        TryFireVerticalScrollEvent();
     }

     private void TryFireHorizontalScrollEvent()
     {

        // Don't do anything if there is no event handler
        if(ScrollChanged == null)
           return;

        int lastScrollPosition = _lastHorizontalScrollPosition;
        int scrollPosition = HorizontalScrollPosition;

        // Don't do anything if there was no change in position
        if(scrollPosition == lastScrollPosition)
           return;

        _lastHorizontalScrollPosition = scrollPosition;

        ScrollChanged
        (  this
        ,  new System.Windows.Forms.ScrollEventArgs
           (  scrollPosition < lastScrollPosition - _fontWidth
              ?  System.Windows.Forms.ScrollEventType.LargeDecrement
              :  scrollPosition > lastScrollPosition + _fontWidth
              ?  System.Windows.Forms.ScrollEventType.LargeIncrement
              :  scrollPosition < lastScrollPosition
              ?  System.Windows.Forms.ScrollEventType.SmallDecrement
              :  System.Windows.Forms.ScrollEventType.SmallIncrement
           ,  lastScrollPosition
           ,  scrollPosition
           ,  System.Windows.Forms.ScrollOrientation.HorizontalScroll
           )
        );

     }

     private void TryFireVerticalScrollEvent()
     {

        // Don't do anything if there is no event handler
        if(ScrollChanged == null)
           return;


        int lastScrollPosition = _lastVerticalScrollPosition;
        int scrollPosition = VerticalScrollPosition;

        // Don't do anything if there was no change in position
        if(scrollPosition == lastScrollPosition)
           return;

        _lastVerticalScrollPosition = scrollPosition;

        ScrollChanged
        (  this
        ,  new System.Windows.Forms.ScrollEventArgs
           (  scrollPosition < lastScrollPosition - 1
              ?  System.Windows.Forms.ScrollEventType.LargeDecrement
              :  scrollPosition > lastScrollPosition + 1
              ?  System.Windows.Forms.ScrollEventType.LargeIncrement
              :  scrollPosition < lastScrollPosition
              ?  System.Windows.Forms.ScrollEventType.SmallDecrement
              :  System.Windows.Forms.ScrollEventType.SmallIncrement
           ,  lastScrollPosition
           ,  scrollPosition
           ,  System.Windows.Forms.ScrollOrientation.VerticalScroll
           )
        );

     }

     private int _lastVerticalScrollPosition;
     private int _lastHorizontalScrollPosition;
     private int _fontWidth;
     private System.ComponentModel.IContainer _components = null;

     private static class Win32
     {

        public const uint WM_HSCROLL    = 0x114;
        public const uint WM_VSCROLL    = 0x115;
        public const uint WM_MOUSEWHEEL = 0x20A;
        public const int  SB_VERT       = 0x1;
        public const int  SB_HORZ       = 0x0;

        [System.Runtime.InteropServices.DllImport("user32.dll")]
        public static extern int SetScrollPos
        (  System.IntPtr hWnd
        ,  int nBar
        ,  int nPos
        ,  bool bRedraw
        );
        [System.Runtime.InteropServices.DllImport("user32.dll")]
        public static extern int GetScrollPos
        (  System.IntPtr hWnd
        ,  int nBar
        );

        [  System.Runtime.InteropServices.DllImport
           (  "User32.Dll"
           ,  EntryPoint = "PostMessageA")
        ]
        public static extern bool PostMessage
        (  System.IntPtr hWnd
        ,  uint msg
        ,  int wParam
        ,  int lParam
        );

     }

  }