Simple zoom functionality for custom controls

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Introduction

Initially, I wrote a small article to describe what is necessary to add zoom functionality to a custom control (see Details below). There were many users who had problems using the described routines in their own code. That is the reason I decided to program a custom control library (UniversalControl.dll) and provide it with a demo project. I also added a vertical and horizontal scrollbar to the control, because it seemed to be an indispensable feature.

Usage (see also the demo project)

1. Add the UniversalControl.dll library to your toolbox.
2. Drag the CUniversalControl item in your application.
3. Configure your CUniversalControl (set colors, enable/disable scrollbars, ...).
4. Write your own drawing routine (DrawContents(Graphics grc)).
5. Add it to the control drawing event.

cUniversalControl1.EDraw += new DDrawContents(DrawContents);

Details

The user selects the zoom area with his left mouse button, see the screenshots below:

The Code

Three routines to handle the mouse events are added to the control. The OnMouseDown method is called when a mouse button is pressed. It sets the starting point for the new zoom area and resets its size to zero.

/// <summary>
/// event handler for mouse button pressed
/// sets zoom area start coordinates
/// </summary>
/// <param name="sender"></param>
/// <param name="e"></param>
protected void OnMouseDown(object sender, 
          System.Windows.Forms.MouseEventArgs e)
{
  // reset zoom state
  m_blnZoom = false;
  // sets the starting point of the zoomed area
  m_ZoomRect = new Rectangle(new Point(e.X, e.Y), 
                             new Size(0, 0));
}

The OnMouseMove updates the area to zoom based on the current mouse position as long as the left mouse button is pressed. It calls the Invalidate method of the control to update the chosen zoom rectangle (black rectangle). Drawback: Invalidate called without parameters updates the whole control, that may result in some flickering during the selection of the area to zoom (solution: invalidate only the necessary region).

/// <summary>
/// handles mouse move events
/// updates current zoom area
/// </summary>
/// <param name="sender"></param>
/// <param name="e"></param>
protected void OnMouseMove(object sender, 
          System.Windows.Forms.MouseEventArgs e)
{
  // updates the current zoom area
  // during mouse mouvements 
  // so it can be drawn by the OnPaint method
  if (e.Button != MouseButtons.Left) return;
  if (m_ZoomRect.X >= e.X)
      m_ZoomRect.Width += Math.Abs(e.X - m_ZoomRect.X);
  else m_ZoomRect.Width = Math.Abs(e.X - m_ZoomRect.X);
  if (m_ZoomRect.Y >= e.Y)
      m_ZoomRect.Height += Math.Abs(e.Y - m_ZoomRect.Y);
  else m_ZoomRect.Height = Math.Abs(e.Y - m_ZoomRect.Y);
  if (m_ZoomRect.X > e.X) m_ZoomRect.X = e.X;
  if (m_ZoomRect.Y > e.Y) m_ZoomRect.Y = e.Y;
  this.Invalidate();
}

The OnMouseUp method finally is called when a mouse button is released. If the left mouse button is released, the next call to the OnPaint method will draw the chosen area onto the whole control (zoom).

/// <summary>
/// event handler for mouse button released
/// </summary>
/// <param name="sender"></param>
/// <param name="e"></param>
protected void OnMouseUp(object sender, 
          System.Windows.Forms.MouseEventArgs e)
{
  // left mouse button released,
  // zoom to the chosen area
  if (e.Button == MouseButtons.Left)
      m_blnZoom = true; 
  this.Refresh();
}

Last but not least, the code that needs to be added to the OnPaint method. If the control is in “zoom-mode”, the necessary scaling and translation on the Graphics are executed. After that, all the drawing wanted for the control is done (DrawContents()). Finally the rectangle for the area to zoom (black rectangle) is drawn. That’s all.

/// <summary>
/// redraws the control
/// </summary>
/// <param name="sender"></param>
/// <param name="e"></param>
protected void OnPaint(object sender, 
          System.Windows.Forms.PaintEventArgs e)
{
  if (m_blnZoom)
  {
    // scale and translate the transform
    // that the selected area (m_ZoomRect)
    // is drawn on the whole control (zoom)
    e.Graphics.ScaleTransform((float)this.ClientSize.Width/m_ZoomRect.Width, 
      (float)this.ClientSize.Height/m_ZoomRect.Height);
    e.Graphics.TranslateTransform(-m_ZoomRect.X, -m_ZoomRect.Y);
  }
  
  // draw control specific contents (data to be zoomed)
  DrawContents(e.Graphics);

  if (!m_blnZoom)
  {
    // draws the selected zoom area during mouse movements
    e.Graphics.DrawRectangle(new Pen(new SolidBrush(Color.Black), 2), 
                                                         m_ZoomRect);
  }
}