What power sources are used for elements of smart home systems?

Question

Electronics

Tell me, please, who is in the subject, what power sources are usually used for smart home elements?
For definiteness, let's take a remote switch. It is intended to be installed in place of a conventional switch or socket without replacing existing electrical wiring. Accordingly, it is logical to take power for it from 220 V.
Ie . resulting in the following requirements:
power supply

• ~220 V -> =5 V,
  
• small (can be built inside the socket)
  
• adequate price
  
• low consumption when the load is sleeping.
  

Answer 1

If the switch is without replacing the wiring, then you will have to turn on the device in the gap between the load and the network. And it's not all that simple, because. it is quite difficult to predict what kind of load it is: today there was an incandescent lamp, tomorrow they screwed in the CFL.
In general, IMHO, usually everything comes down to either a capacitor power supply unit, or microcircuits - linear voltage regulators (like FSAR001), or transformers in one form or another.
But IMHO, if you make a smart home, it is better to immediately change the wiring for it, including taking into account low-voltage power.

Answer 2

How is it necessary?
If anything, look towards the flex.