How to animate two interacting objects

Question

I'm not very familiar with animations but as far as I know, most of the animations are done frame by frame. For example animating a character walking etc. However, let's imagine this scene:

There's a man coming from the left towards the sign. It's easy to animate this. But when he comes to the sign, he picks it up and starts waving with it. Now this is something I don't know how to do. What is generally the recommended way to do this kind of animation (two separate objects)? Specifically we're talking about android live wallpaper here (probably made with libgdx but I can use anything if it's better for me).

Answer 1

If you use skeletal animation it is quite trivial - your just attach your signpost to your character hand bone, and your engine does the rest. Assuming you use 2d frame-per-frame animations, well, you shouldn't. This is a case where that approach simply fails. Workarounds like defining a hand position in your animation file, and then attaching your sign to that are just complicated, and essentially trying to do the same as skeletal animation.
Btw, that there are 2d skeletal animation libraries available, like Spine for libgdx(and other engines).