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From: Shaun Deacon <sdeacon@us.socionext.com>
Newsgroups: comp.lang.tcl
Subject: Re: Operate only on the visible lines in a text window
Date: Wed, 10 Jul 2024 10:38:03 -0700
Organization: A noiseless patient Spider
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Ralf Fassel wrote:
> * Shaun Deacon <sdeacon@us.socionext.com>
> | The obvious solution to me is to just highlight the currently visible
> | lines (or a range of lines spanning the current view - say 1000 or so)
> | and when the user scrolls the window, highlight the new set of lines.
>>
> | Suggestions on the best way to find the indexes for the currently
> | visible lines when the widget has been scrolled would be great.
>>
> | Can someone please point me in the right direction ?
> 
> You could try 'text yview' and derive the displayed lines from that:
> 
>      grid [text .t -yscrollcommand {recalc {.s set}}] -sticky ewns -row 0 -column 0
>      grid [scrollbar .s -command {recalc {.t yview}} -orient vertical] -sticky ns -row 0 -column 1
>      grid [label .l] - -sticky ewns
>      grid rowconfigure . 0 -weight 1
>      grid columnconfigure . 0 -weight 1
>      for {set l 1} {$l < 1000} {incr l} {
>          .t insert end "Line $l\n"
>      }
> 
>      proc recalc {cmd args} {
>          # scroll if required
>          if {$cmd ne ""} {
>              {*}$cmd {*}$args
>          }
>          # Determine the visible range
>          lassign [.t yview] start end
>          # Get line count
>          set lines [lindex [split [.t index end] "."] 0]
>          # Get first and last line displayed.
>          # Here you need to determine what to do with the partially displayed lines
>          set first [format %.1f [expr {$start*$lines+1}]]
>          set last [format %.1f [expr {$end*$lines}]]
>          
>          .l configure -text "visible Lines $first ... $last"
>      }
>      bind . <Configure> [list recalc ""]
> 
> HTH
> R'
> 

Thanks Ralf, this looks promising to me.
Shaun