Yes, for it to be direct, it should behave like an on-off-switch each time you have new targets, since direct spells require you to have targets.
So that as long as you sustain the spell, you bath any viable target in flames, when it comes in range, but if not the effect is inactive/suppressed.
As indirect spell you could use it to show off and try to impress people with a burst of flame in front of you, but not so much with a direct spell.