0070: Statusbar Part I – Statusbar Basics

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

The Statusbar is a simple widget, but to demonstrate it, we need something else going on, something that might have a status that needs to be reported. To that end, I borrowed from two previous examples:

In this example, mouse movements over the DrawingArea are reported in the Statusbar. And the most complex part of this example isn’t the Statusbar itself, but the communication between widgets… not that it’s complicated, just more so than any other part of this code.

The MyStatusbar Class

We only need a few lines to get a working Statusbar:

class MyStatusbar : Statusbar
{
	uint contextID;
	string contextDescription = "Mouse position within the DrawingArea";
	
	this()
	{
		contextID = getContextId(contextDescription);
		
	} // this()

} // class MyStatusBar

Context

Because your application might have a bunch of things going on at the same time—for instance, a drawing application might keep the user informed of mouse position (as in this example) but it might also report on the currently-selected color—you can categorize these things by context.

Each context needs:

  • a description of the status category (a string, in other words), and
  • a context ID associated with that description.

The context ID isn’t hard to get, just throw a context description at the getContextID() function and it’ll give back a context ID. Need a second context ID? Throw it another string description and Bob’s your uncle.

This system means that a single Statusbar can deal with as many contexts as you’re willing to describe.

Although we might assume that the description needs to be elaborate, as it is in this example—I did this just to show the lack of limitations—it’s actually for internal use by your app, the only requirement is that each description be unique, but we’ll talk more about that when the time comes.

The DrawingArea being Reported On

We’ve seen most of the DrawingArea class code before in the Cairo examples. One thing of note here, though, is that the DrawingArea is passed a pointer to the Statusbar so as to allow communication between them. And to do this, the DrawingArea constructor adds a second signal:

this(MyStatusbar myStatusbar)
{
	_myStatusbar = myStatusbar;

	setSizeRequest(640, 360);

	addOnDraw(&onDraw);
	addOnMotionNotify(&onMotion); // track mouse movements
	
} // this()

The call to addOnMotionNotify() (borrowed from the mouse example mentioned above) brings a second callback into play:

public bool onMotion(Event event, Widget widget)
{
	if(event.type == EventType.MOTION_NOTIFY)
	{
		_myStatusbar.push(_myStatusbar.contextID, "Mouse position: " ~ format("%s, %s", event.motion.x, event.motion.y));
	}

	return(true);
		
} // onMotion()

This is the meat-n-taters of our example right here. A call to the Statusbar’s push() function puts the mouse movement report onto the Statusbar’s message stack and automatically displays it in the Statusbar at the same time. The format() function is here just to supply window dressing for the string we’re passing along.

Note: The format() function uses %s as a format specifier for the double values returned by event.motion.x and event.motion.y.

Conclusion

Next time, we’ll talk about how to get more out of a Statusbar by stuffing extra widgets in there and harnessing the Statusbar’s onTextPushed signal.

Until then.

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© Copyright 2019 Ron Tarrant