File Action - Modifying arguments
The first step is to create a "Angle" value that is used to compute a string variable. The string is then used as an argument to the file action rather than the argument that was specified inline. The number is set up as a slider that can be changed interactively.
The floating window - Time Floater - can be selected from the "Window" menu. You can drag the slider variable into the time floater. As the slider is changed, the argument changes which will cause the "Swirl action" to be rerun. Since the "Bitmap file" variable is not affected, that file is not recreated ond disk.
When you uncheck the "Sparsify" button the "Bitmap file" will be recreated, and then reused as you change the angle. Since the computation is not immediate the computation will lag a little bit. When the computation is done, it will be restarted using the current value for the angle. This means that some of the angle numbers that you will see flashing by as you drag the slider will not be used.The slider will be dragged in the main thread while the computation is going on in a separate thread so the user interface does not feel sluggish.