chapters/38.markdown @ 72f09a8646a6

Toggling
========

In one of the first chapters we talked about how to set options in Vim.  For
boolean options we can use `set someoption!` to "toggle" the option.  This is
especially nice when we create a mapping for that command.

Run the following command:

    :::vim
    :nnoremap <leader>N :setlocal number!<cr>

Try it out by pressing `<leader>N` in normal mode.  Vim will toggle the line
numbers for the current window off and on.  Creating a "toggle" mapping like
this is really handy, because we don't need to have two separate keys to turn
something off and on.

Unfortunately this only works for boolean options.  If we want to toggle
a non-boolean option we'll need to do a bit more work.

Toggling Options
----------------

Let's start by creating a function that will toggle an option for us, and
a mapping that will call it.  Put the following into your `~/.vimrc` file (or
a separate file in `~/.vim/plugin/` if you prefer):

    :::vim
    nnoremap <leader>f :call FoldColumnToggle()<cr>

    function! FoldColumnToggle()
        echom &foldcolumn
    endfunction

Write and source the file, then try it out by pressing `<leader>f`  Vim will
display the current value of the `foldcolumn` option.  Go ahead and read `:help
foldcolumn` if you're unfamiliar with this option.

Let's add in the actual toggling functionality.  Edit the code to look like
this:

    :::vim
    nnoremap <leader>f :call FoldColumnToggle()<cr>

    function! FoldColumnToggle()
        if &foldcolumn
            setlocal foldcolumn=0
        else
            setlocal foldcolumn=4
        endif
    endfunction

Write and source the file and try it out.  Each time you press it Vim will
either show or hide the fold column.

The `if` statement simply checks if `&foldcolumn` is truthy (remember that Vim
treats the integer 0 as falsy and any other number as truthy).  If so, it sets
it to zero (which hides it).  Otherwise it sets it to four.  Pretty simple.

You can use a simple function like this to toggle any option where `0` means
"off" and any other number is "on".

Toggling Other Things
---------------------

Options aren't the only thing we might want to toggle.  One particularly nice
thing to have a mapping for is the quickfix window.  Let's start with the same
skeleton as before.  Add the following code to your file:

    :::vim
    nnoremap <leader>q :call QuickfixToggle()<cr>

    function! QuickfixToggle()
        return
    endfunction

This mapping doesn't do anything yet.  Let's transform it into something
slightly more useful (but not completely finished yet).  Change the code to
look like this:

    :::vim
    nnoremap <leader>q :call QuickfixToggle()<cr>

    function! QuickfixToggle()
        copen
    endfunction

Write and source the file.  If you try out the mapping now you'll see that it
simply opens the quickfix window.

To get the "toggling" behavior we're looking for we'll use a quick, dirty
solution: a global variable.  Change the code to look like this:

    :::vim
    nnoremap <leader>q :call QuickfixToggle()<cr>

    function! QuickfixToggle()
        if g:quickfix_is_open
            cclose
            let g:quickfix_is_open = 0
        else
            copen
            let g:quickfix_is_open = 1
        endif
    endfunction

What we've done is pretty simple -- we're simply storing a global variable
describing the open/closed state of the quickfix window whenever we call the
function.

Write and source the file, and try to run the mapping.  Vim will complain that
the variable is not defined yet!  Let's fix that by initializing it once:

    :::vim
    nnoremap <leader>q :call QuickfixToggle()<cr>

    let g:quickfix_is_open = 0

    function! QuickfixToggle()
        if g:quickfix_is_open
            cclose
            let g:quickfix_is_open = 0
        else
            copen
            let g:quickfix_is_open = 1
        endif
    endfunction

Write and source the file, and try the mapping.  It works!

Improvements
------------

Our toggle function works, but has a few problems.

The first is that if the user manually opens or closes the window with `:copen`
or `:cclose` our global variable doesn't get updated.  This isn't really a huge
problem in practice because most of the time the user will probably be opening
the window with the mapping, and if not they can always just press it again.

This illustrates an important point about writing Vimscript code: if you try to
handle every single edge case you'll get bogged down in it and never get any
work done.

Getting something that works most of the time (and doesn't explode when it
doesn't work) and getting back to coding is usually better than spending hours
getting it 100% perfect.  The exception is when you're writing a plugin you
expect many people to use.  In that case it's best to spend the time and make it
bulletproof to keep your users happy and reduce bug reports.

Restoring Windows/Buffers
-------------------------

The other problem with our function is that if the user runs the mapping when
they're already in the quickfix window, Vim closes it and dumps them into the
last split instead of sending them back where they were.  This is annoying if
you just want to check the quickfix window really quick and get back to working.

To solve this we'll introduce an idiom that comes in handy a lot when writing
Vim plugins.  Edit your code to look like this:

    :::vim
    nnoremap <leader>q :call QuickfixToggle()<cr>

    let g:quickfix_is_open = 0

    function! QuickfixToggle()
        if g:quickfix_is_open
            cclose
            let g:quickfix_is_open = 0
            execute g:quickfix_return_to_window . "wincmd w"
        else
            let g:quickfix_return_to_window = winnr()
            copen
            let g:quickfix_is_open = 1
        endif
    endfunction

We've added two new lines in this mapping.  One of them (in the `else` clause)
sets another global variable which saves the current window number before we run
`:copen`.

The second line (in the `if` clause) executes `wincmd w` with that number
prepended as a count, which tells Vim to go to that window.

Once again our solution isn't bulletproof, because the user might open or close
new split between runs of the mapping.  Even so, it handles the majority of
cases so it's good enough for now.

This strategy of manually saving global state would be frowned upon in most
serious programs, but for tiny little Vimscript functions it's a quick and dirty
way of getting something mostly working and moving on with your life.

Exercises
---------

Read `:help foldcolumn`.

Read `:help winnr()`

Read `:help ctrl-w_w`.

Read `:help wincmd`.

Namespace the functions by adding `s:` and `<SID>` where necessary.