Why does ; suppress Dynamic output? [duplicate]

Question

This question already has an answer here:

• Question about MapThread and Dynamic 2 answers

Dynamic[x=7]; does not make x dynamic! But Dynamic[x=7;] makes x dynamic with initial value 7. The problem with the later is the presence of Null in the output. My question is: how can I assign a (big) value (or a huge list, etc.) to a dynamic variable AND suppress any possible output (including Null)?

Answer 1

Dynamic doesn't work the way you think it does. See this answer for a full explanation.

In short, Dynamic doesn't do anything until it is actually displayed on screen.

Answer 2

Dynamic is only updated if it is displayed. Dynamic[]; is never displayed so it is never updated. Dynamic[...;] returns a dynamic that always returns Null, but is never the less updated.

Here is an example that demonstrates the problem:

x = .5;
Dynamic[Print["will not print:", x]];
Dynamic[Print["Should print:", x];]
Slider@Dynamic@x