Can we receive user input anytime?

So just like the title said.

Can we receive user input anytime when a script is running?

let's make a simple assumption
a = false/true -- let's say if I press the "1" key the value will be false, and if I press the "2" key the values will be true

while (true) do
if (a == true) then
do activity a
else if (a == false) then
do activity b
end

or maybe there is a workaround to do something similar to that I want to achieve here.

Thank you