Prevent refresh of screen until function completes

Question

I have a function that does a lot of moving and outputting of text into the current vim buffer, and when I run it, seeing all that happening at blinding speed is a bit disconcerting.

How can I freeze the screen until the function's done?

Here's the function in question:

function! MakeChoices()
    :let save_view = winsaveview()
    let start = line('.')

    "Locate previous *choice. (b=backwards, W=nowrap, n=doNot move cursor)
    let choiceStartLine = search('^*choice', 'bW')

    if !choiceStartLine
        echo "No *choice found. (*choice must not be indented. This is to avoid finding *choice blocks nested in another *choice block.)"
        return -1
    endif
    "return getline(target_line_num, target_line_num+4)
    "Locate end of *choice block
    "echo getline(choiceStartLine, choiceStartLine+2)
    let choiceEndLine = search('^\S.*', 'W') "End is first line that starts with non-whitespace

    "If above search fails, might be at bottom of buffer
    if choiceEndLine == 0
        let choiceEndLine = search('^$', 'W') "End is first empty line
    endif

    "Now go back up to the last *goto
    let choiceEndLine = search('*goto', 'bW')

    "Get the entire *choice block and put it in gotoBlock
    let gotoBlock = getline(choiceStartLine, choiceEndLine)

    "Make labelArray (contains all labels to goto)
    let labelArray = []

    for cur in gotoBlock
        if match(cur, '*goto') != -1
            "echo 'cur: '.cur
            let curParsed = matchlist(cur, '*goto \(\S\+\)')
            "echo curParsed
            if len(curParsed) > 1
                let curLabel = curParsed[1]
            else
                echo 'ERROR: Bad *goto ('.cur.')'
                return -1
            endif
            call add(labelArray, curLabel)  
        endif
    endfor

    "Restore window to what it looked like (in case the searches scrolled
    "it)
    call winrestview(save_view)

    "Make newline after choice block if needed
    if strlen(getline(choiceEndLine+1)) > 0
        echo 'big line: '.getline(choiceEndLine+1)
        call cursor(choiceEndLine, 1)
        put=''
    endif

    call cursor(choiceEndLine+1, 1)

    "Put the new label blocks
    let skippedLabels = ''
    let numNewLabels = 0
    for cur in labelArray
        if !search('*label '.cur, 'wn')
            let numNewLabels += 1
            put='*label '.cur
            put='[This option is yet to be written.]'
            put=''
        else
            let skippedLabels .= cur.' '
        endif
    endfor

    "Remove trailing blank lines (Up to a point)
    let nextlines = getline(line('.')+1, line('.')+3)
    if len(nextlines) == 3
        if nextlines[0] == '' && nextlines[1] == '' && nextlines[2] == ''
            normal "_3dd
        elseif nextlines[0] == '' && nextlines[1] == ''
            normal "_2dd
        elseif nextlines[0] == ''
            normal "_dd
        endif
    endif

    "Move to first label's text (use ctrl-v ctrl-m to input the <CR> at
    "end)
    if numNewLabels != 0
        call cursor(choiceEndLine, 1)
        normal /\[This option is yet to be written.\]
        let @/='\[This option is yet to be written\.\]'
    endif

    "Print status message
    if len(skippedLabels) > 0
        echo 'Skipped: '.skippedLabels
    else
        echo 'New labels created: '.numNewLabels
    endif
endfunction

Answer 1

I think the problem is not :lazyredraw which, as I understand from docs, should work for functions (see :help :redraw, it says "Useful to update the screen halfway executing a script or function").

The problem is that you use normal to update buffer and it works like if you actually type something and here :lazyredraw doesn't have an effect.

Instead of normal you need to use text manipulation functions (like setline()) and ex commands (like :delete).

Compare these two functions, first one, MakeChangesNorm() will do some crazy screen updates, while the second one, MakeChangesFunctions() will do the update instantly:

function! MakeChangesNorm()
    let lastline = line('$')
    norm gg
    let linenum = line('.')
    let lastline = line('$')
    while linenum < lastline
        norm ^
        norm s/choice/test/
        norm j
        normal "_3dd
        let linenum = line('.')
        let lastline = line('$')
    endwhile
endfunction


function! MakeChangesFunctions()
    norm gg
    let linenum = line('.')
    let lastline = line('$')
    while linenum < lastline
        let line = getline(linenum)
        " Substitute using substitute() and setline()
        let line = substitute(line, 'choice', 'test', '')
        call setline(linenum, line)
        " Delete lines using :delete
        execute '.,.+2delete _'
        let linenum = line('.')
        let lastline = line('$')
    endwhile
endfunction

The file I tested it on looks like this:

*choice test2 test3 super
*choice test2 test3 super
*choice test2 test3 super
*choice test2 test3 super
*choice test2 test3 super
*choice test2 test3 super
*choice test2 test3 super
*choice test2 test3 super
*choice test2 test3 super
*choice test2 test3 super
... 60 lines like this ...

Answer 2

I don't have enough points yet to add comments to other questions, so I'll write my experience down as another answer.

I also have some vimscript functions in my ~/.vimrc that scroll the window, and they used to cause screen flickering that was very annoying to me. I spent some time on it, and I was able to eliminate the flickering by employing the following pattern in my functions:

function! MyFunction()
  set lazyredraw

  " do the stuff, using e.g.
  "   execute "normal! " .. n .. "\<C-Y>"
  " to scroll the window

  redraw
  set nolazyredraw
endfunction

See also this answer for a more detailed example.