extract phased genotypes from "the definition of field 22438 bgen file"

Hi there 

I want to extract phased genotypes/haptotypes from the UKB haplotype file eg “ukb22438_c19_b0_v2.bgen”, I can uss bgen-reader Read — bgen-reader 4.0.9 documentation to open and browser some contents. could some offer  efficient/accurate way/s to extract a subset and convert to haplotype for a given SNPs or a given range. 

thanks

YD