Prepend to lines of a program as they come in - bash

Question

I'm running xinput test and trying to timestamp the data.
From another question, I'm using :
xinput test $KEYBOARD_ID | (echo -n $(date +"$date_format") && cat) > $LOGFILE_NAME
However, that dates the first line, not every line.
If I do a while loop:
while IFS= read -r line
do
echo -n $(date +"date_format") &&cat)
done < $(xinput test $KEYBOARD_ID)
The loop exits right away, since xinput test is yet to generate any text.
Process substitution fails as well, only dating the first line of the file.
while IFS= read -r line
do
(echo -n $(date +"$date_format") && cat) > $LOGFILE_NAME
done < <(xinput test $KEYBOARD_ID)
Writing to file and post-processing won't work, because I need the timestamp when each line was processed.
I feel like I'm making a small error, but I can't find it, any input?

Answer 1

The following GNU awk command is equivalent to #karakfa's answer, but launches fewer processes, so it could be faster if the device is generating a lot of events:
xinput test "$KEYBOARD_ID" | gawk '{print strftime(), $0}' > "$LOGFILE_NAME"

Answer 2

perhaps this will help...
$ seq 10 | xargs -n1 -I {} echo $(date) {}
Wed May 10 14:43:09 EDT 2017 1
Wed May 10 14:43:09 EDT 2017 2
Wed May 10 14:43:09 EDT 2017 3
Wed May 10 14:43:09 EDT 2017 4
Wed May 10 14:43:09 EDT 2017 5
Wed May 10 14:43:09 EDT 2017 6
Wed May 10 14:43:09 EDT 2017 7
Wed May 10 14:43:09 EDT 2017 8
Wed May 10 14:43:09 EDT 2017 9
Wed May 10 14:43:09 EDT 2017 10
Note that, as commented below, this time stamp won't be updated for each line, if you want to time stamp each new line the gawk solution by user000001

Answer 3

I feel like I'm making a small error, but I can't find it
Yep. It's the cat. It reads the rest of the input and puts it there. Instead, you should just write the current line, and append it to the file:
while IFS= read -r line
do
(echo "$(date +"$date_format") $line") >> $LOGFILE_NAME
done < <(xinput test $KEYBOARD_ID)
Which can more canonically be written as
while IFS= read -r line
do
echo "$(date +"$date_format") $line"
done < <(xinput test $KEYBOARD_ID) > "$LOGFILE_NAME"
I would go for #user000001's shorter and more efficient solution though.