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add a slider

Pump up the volume!

To make the program more useful, the DJ wants to be able to dynamically
and interactively control the volume of the track as it plays. Basically, as the Louder!
track plays, the DJ wants to fiddle with the volume.

Softer, softer,
softer...

In the physical world, most devices
that have a volume control provide
either a large, circular knob or
a slider on a scale:

The control might look like this, ...or the control
a slider that can be moved from
might look like this.
left (down) to right (up)... Turn it up to the
right, down to the
left..
Because a slider is much easier to manipulate with a click and drag of a
mouse than a knob, the slider on a scale is used in lots of modern GUIs. It’s the
classic choice for showing a volume control.

Look closely at the slider. What do you need to model?

336 Chapter 9