Hi.
it is simple, first assume that u want to find simple barcode patterns consists of 1's and 0's. to do this you should make a suitable and fast enough timing mechanism, such as timer overflow interrupt. then wait for first pulse from input. measure the width of this pulse (start bit) and add 3/2 of this time to skip 2nd start bit:

1 2 3
V V V
__
__| |__..... other bits

@1: presense of barcode data found
@2: the duration of one bit determined, so the movement speed could be found
@3: at this time you can sample other bits from input up to end of barcode bits, or a long period which no changes in input data exists.

no decode the entire bit pattern on your own.

for standard barcode pattern formats, this algorithm must be overwritten.

good luck.