Does USB-C charge your device (laptop/phone) while connected to a plugged-in display? Given that the new Macbook has only one USB-C port, can you indefinitely run your laptop while connected to a TV?

Let's say a TV run on electricity from a wall power outlet. You connect your macbook to it and play a video from your macbook to be displayed on the TV. 

Does USB-C keep your macbook charged while connected to a device that has its own power source? Or will I need to disconnect the macbook from the TV, charge for a while, and then connect it back to the TV once the macbook gets charged enough?

What about if the macbook is connected to a display that run on battery power? Will the macbook charge the display (e.g. tablet),  the tablet charge the macbook, or be power-neutral?
If you want to run a TV or external display from a MacBook, you need an adaptor. And the Apple USB-C Digital AV Multiport Adaptor has HDMI, USB and USB-C ports, so you can connect the MacBook's power adaptor to the USB-C port while you use the HDMI port to drive the external display. The external display won't power the MacBook itself.