Thursday, September 24, 2009

Building a Dial Control

A friend posed a question a few weeks back about implementing a dial control. I found a few but they seemed problematic so I decided it can't possibly be that hard. So the idea is a control with a custom event for dial position change that I can add templates etc to change the look and feel of the control to any kind of dial I want and bind an event handler to so that in can control say volumn. I posted how to implement a custom event last week so I'll focus specifically on the rest of the dial control. Lets start with the template in Generic.xaml.

<Style TargetType="DialTest:Dial">
<Setter Property="Template">
<ControlTemplate TargetType="DialTest:Dial">
<Grid x:Name="Knob" Background="Transparent" >
<ContentPresenter x:Name="Background" Content="{TemplateBinding KnobBackground}"/>
<ContentPresenter x:Name="DialKnob" Content="{TemplateBinding KnobFace}" RenderTransformOrigin="0.5,0.5" >
<RotateTransform x:Name="DialAngle" Angle="0" />

So from this template we can see that the control will have a wrapper grid to force content to be resized as set and two content presenters one for the background and one for the dial and further a rotate transform on the dial so we can make it turn. As noted these content presenters imply that we have two dependency properties for the dial face and background but we will also have DP's for setting the start or minimum or max angle setting of the dial (0 to 360 degrees).

In our constructor then we set our template and then OnApplyTemplate() we wire things together like so:

Knob = ((Grid)GetTemplateChild("Knob"));
DialAngle = ((RotateTransform)GetTemplateChild("DialAngle"));

Knob.MouseLeftButtonUp += new MouseButtonEventHandler(Knob_MouseLeftButtonUp);
Knob.MouseLeave += new MouseEventHandler(Knob_MouseLeave);
Knob.MouseLeftButtonDown += new MouseButtonEventHandler(Knob_MouseLeftButtonDown);
Knob.MouseMove += new MouseEventHandler(Knob_MouseMove);


if (Minimum > 0 && Minimum < 360)

this wires our internal events we use to make the wiring work. What we are doing todo is on mouse down we will calculate the angle of that related to the control and then turn the dial to that position. we then fire off the angle changed event or custom event. The hard part turned out to the just figuring out the math and it turns out that this not the best method 'mathmatically' speaking but this works well. Once get a point from the mouse down we do this to get the relative angle:

double TheDiameter = Knob.ActualWidth;
double CurrentHeight = Knob.ActualHeight;

double Radius = TheDiameter / 2;

double AngleOfRotation = AngleOfRotationQuadrant(TheDiameter, CurrentHeight, NewPoint);

//int DialQuadrant = 1;
if ((NewPoint.X > Radius) && (NewPoint.Y <= Radius))
//DialQuadrant = 2;
AngleOfRotation = 90.0 + (90.0 - AngleOfRotation);
else if ((NewPoint.X > Radius) && (NewPoint.Y > Radius))
//DialQuadrant = 4;
AngleOfRotation = 180.0 + AngleOfRotation;
else if ((NewPoint.X < Radius) && (NewPoint.Y > Radius))
//DialQuadrant = 3;
AngleOfRotation = 270.0 + (90.0 - AngleOfRotation);
return AngleOfRotation;

Basically we calculate 90 degress of position for he dial face quadrant that the event was in and then add the relative additional quadrants as needed to get the correct angle. getting the quadrant goes like this:

double DialRadius = CurrentWidth / 2;

Point CenterPoint = new Point(DialRadius, CurrentHeight / 2);
Point StartPoint = new Point(0, CurrentHeight / 2);

double TriangleTop = Math.Sqrt(ToThePowerOfTwo(NewPoint.X - CenterPoint.X) + ToThePowerOfTwo(CenterPoint.Y - NewPoint.Y));

double TriangleHeight = (NewPoint.Y > CenterPoint.Y) ? NewPoint.Y - CenterPoint.Y : CenterPoint.Y - NewPoint.Y;

return ((TriangleHeight * Math.Sin(90.0)) / TriangleTop) * 100;

now we can set the angle in 'SetPosition(); like this:
if (Minimum > 0 && Max > 0 && Minimum < 360 && Max <= 360 )
if (AngleOfRotation < Minimum)
AngleOfRotation = Minimum;
if (AngleOfRotation > Max)
AngleOfRotation = Max;

DialAngle.Angle = AngleOfRotation;

OnPositionChanged(new DialEventArgs(AngleOfRotation));

the last line being our custom event. and poof it works... So in xaml using the control might look like this:

<cc:Dial x:Name="NewKnobControl" Height="100" Width="100" PositionChangedEvent="NewKnobControl_PositionChangedEvent" Minimum="45.0" Max="135" >
<Grid >
<Ellipse Fill="Aquamarine" />
<Rectangle x:Name="Indicator" Height="10" Width="49" Fill="Blue" Margin="1,45,50,45" />

nice and simple, needs some design love but you get the point. The dial control code will be up on some time this week.


  1. yes, waiting on Ariel for a better design. hopefully get it posted on codeplex over the weekend.

  2. the control has been added to the collection here with Ariels ( design.

  3. added source to the HSL library at