lundi 17 août 2015

Writing a custom spinner

What are spinners

A spinner is a widget in charge of displaying some indefinite amount of activity via a spinning circle-ish image. They use de "spinner" style class and are visible when the "active" state flag is set.

Turning a widget into a spinner

Turning a widget into a spinner is pretty simple, just add the "spinner" style class to the widget's style context! But now your widget is probably invisible, and that's normal: as said just before, spinners are drawn only if they have the "active" state flag on, just set your widget's state flags and you're done!

Now, depending on the widget you make spin you may have very different results. If your widget is a GtkImage, it will start spinning as if it was a spinner itself, funky results (and vomit) guaranteed! All the other widgets I tested (GtkLabel, GtkBox, …) don't change (they are visible and they don't spin).

If you want to render a spinner on an existing widget, you'll have to subclass it and to override its gtk_widget_draw() method to draw it and then render the spinner with gtk_render_activity().

If you want to render a spinner on an image, you'll unfortunately have to write a custom widget drawing the image and then the spinner on top of it.

Writing a custom spinner widget

Now that we know how to turn a widget into a spinner, we can build our very own custom spinner widgets!

I wrote a widget rendering a thumbnail for Boxes' list view. To do so:

  • I inherited GtkDrawingArea,
  • I overrode its gtk_widget_draw() method to draw it as I wanted,
  • I added the "spinner" style class to the widget so it can render the spinner,
  • I set the widget as active so it can be visible despite the "spinner" style class,
  • I drew the base of my widget as I wanted it to be,
  • I drew the spinner with gtk_render_activity() on top when I wanted it to be drawn.

Unfortunately you can't use the "active" state as a trigger without removing the "spinner" style class, hence it is simpler to just let them always on and add a custom trigger to draw the spinner itself or not.

Customizing the spinner

You can customize a spinner with CSS! As an exemple, if you want to slow down the spinner a bit, you can write the following "slow-spinner" class and add it to any spinner's style context:

.slow-spinner {
    animation-duration: 2s;
}

If you know or learn anything interesting about spinners, don't hesitate to share you knowledge in the comments! =)

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