VHDL - Register for Push Button - vhdl

Question

I'm trying to create a simple push button in VHDL that turns on after an input switch or pb goes from 0 to 1 to 0 using a clock and a process. However, my code seems to be giving me undefined output. Here's what I have so far.
use ieee.std_logic_1164.all;
use ieee.numeric_std.all;
Entity captureInput is port
(
CLK : in std_logic := '0';
RESET_n : in std_logic := '0';
buttonState : in std_logic := '0';
buttonOut : out std_logic := '0'
);
end Entity;
ARCHITECTURE one of captureInput is
signal lastButtonState: std_logic := '0';
signal btnState : std_logic := '0';
BEGIN
process (CLK, RESET_n) is
begin
if (RESET_n = '0') then
lastButtonState <= '0';
elsif (rising_edge(CLK)) then
if (buttonState ='0' and lastButtonState = '1') then
btnState <= '1';
end if;
lastButtonState <= buttonState;
end if;
end process;
buttonOut <= btnState
end;

Answer 1

Try to initialize your btnState in the reset branch of your register and also have an else statement where you set your btnState back to 0, under some condition. I would bet that your undefined output comes from the fact that you do not define your btnState anywhere else outside your if conditions. It's good practice to not rely on the initial value of your declaration: Synthesis tools ignore it and some simulators will as well. Also, remember that the clocked body of the if will generate a register for every signal that gets assigned a value inside it, and that signals will keep the last value assigned to them inside a process.
You are also missing the Library ieee; statement at the top and a semicolon after buttonOut <= btnState.

Answer 2

Reading a button do need a debouncer.
Please take a look at:
VHDLWhiz generate statement
or
VHDLWhiz How to read a button in VHDL

Answer 3

Even though I see that you have already accepted Dimitris' answer, I can add that your code is almost right, you just need to toggle on the falling edge of of the latch instead of setting it to '1' as you do.
Try
if(rising_edge(CLK)) then
lastButtonState <= buttonState;
if(buttonState='0' and lastButtonState='1') then
btnState <= not btnState;
end if;
end if;
buttonOut <= btnState -- etc...
You don't need to initialize anything to '0' but you DEFINITELY need a switch debouncer as lukipedio said otherwise your toggle will not be consistent.
If you think about it, what you're doing is putting a "clock divider" on your lastButtonState register by toggling in order to set the btnState register at half the "frequency" of lastButtonState, which is what you want.
BTW, if you switch your toggling condition to
(buttonState='1' and lastButtonState='0')
then it will toggle on the rising edge of lastButtonState, in other words it will be toggle-on-press instead of toggle-on-release.