0111: A Scale Widget to Control Graphic Placement

Results of this example:
Current example output
Current example output
Current example terminal output
Current example terminal output (click for enlarged view)

Last time, we did the simple version of a Scale button. If you didn’t read that—depending on your skill level—it might be helpful in understanding what’s going on in today’s post. So, no mucky about, let’s get down to it…

The Scale-v-DrawingArea Demo

Okay, because we’re doing a bit of graphic work this time, we’ll need to import the usual graphical culprits:

import cairo.Context;
import gtk.DrawingArea;

And in the AppBox class, we’ll be setting up two widgets, the Scale and a DrawingArea:

class AppBox : Box
	MyScale myScale;
	int localPadding = 0, globalPadding = 10;
	bool expand = false, fill = false;
	MyDrawingArea myDrawingArea;
		super(Orientation.VERTICAL, globalPadding);
		myDrawingArea = new MyDrawingArea();
		myScale = new MyScale(myDrawingArea);

		packStart(myScale, expand, fill, localPadding);
		packStart(myDrawingArea, true, true, 0); // LEFT justify
	} // this()

} // class AppBox

We went over all this stuff in the Cairo drawing series (which started in blog post 0057 and continued to blog post 0064), so let’s move on to the more relevant details…

The MyScale Callback Function

This is where the action is:

void valueChanged(Range range)
	double scaleValue = getValue();
} // valueChanged()

The callback hook-up is the same as the previous example, so no need to go over it again here. If you need to refresh your memory on this point, take a look at the previous post.

Breaking it down, we:

  • call the inherited function Scale.getValue(),
  • so we can set the function-local variable scaleValue,
  • which is then passed to ballDisplay’s setBallPosition() function.

What’s ballDisplay? That’s how the MyScale class refers to the DrawingArea we stuffed into the AppBox. Here’s what I mean…

MyScale’s Constructor

Take a look at the constructor’s argument:

this(MyDrawingArea myDrawingArea)
	super(Orientation.HORIZONTAL, minimum, maximum, step);
	ballDisplay = myDrawingArea;
} // this()

The AppBox passes in a pointer to the DrawingArea which is then renamed as ballDisplay to put it into context for the coming operations. Not Context as in Cairo Context, but context as in circumstances… specifically, the circumstances of accessing the DrawingArea from the Scale button.

The MyDrawingArea Class

Let’s break this class down a bit…

The Class Preamble

class MyDrawingArea : DrawingArea
	int ballX;

This variable keeps track of where the ball’s position… where it is at any given moment at runtime. We’ll see how that works momentarily.

The Constructor

	ballX = 30;
} // this()

We set an initial position for the graphic—a ball—and hook up the callback function which we’ll talk about now…

The Callback

As is usual with a DrawingArea callback, all we do is draw:

bool onDraw(Scoped!Context context, Widget w)
	context.setLineWidth(3); // prepare the context
	context.arc(ballX, 50, 10, 0, 2 * 3.1415); // define the circle as an arc
	context.stroke(); // and draw

} // onDraw()

We set a line width, define the ball shape, draw it, and we’re out of there. Responding to changes in the Scale’s slider happens in a different function:

The setBallPosition() Function

And this function is called directly from MyScale’s callback (the callback being the function which reacts to movement of the Scale slider):

void setBallPosition(int x)
	x *= 15; // move the ball a noticeable distance
	ballX = x + 30;
} // setBallPosition()

Now, because the Scale’s Range is from 0 to 10, using a one-to-one ratio for moving the ball isn’t going to be that noticeable, so the first thing we do is take the value passed in from MyScale’s callback and multiply it by 15. That way we don’t have to squint to see the ball move.

Next, we update the value of ballX, the ball’s position, and finally, force the DrawingArea to do a redraw.

And that, ladies and gents, is that.


It’s fairly straightforward to control the position of a graphic in a DrawingArea using a Scale button (or, in fact, a ScaleButton, but that I’ll leave as an exercise for you if you’re so inclined).

Until next time, be brave, code well, and don’t let the bugs bite.

If you'd like to leave a comment...

You can also subscribe via RSS so you won't miss anything. Thank you very much for dropping by.

© Copyright 2020 Ron Tarrant