How can I fold YAML items in VIM?

Question

I have a YAML formatted text file, and would like to define custom folding for VIM, but I'm not sure how to go about it (despite reading the VIM documentation for folding). The file consists of YAML "documents", like so:

---
title: My Title
attr1: value1
attr2: value2
---
title: Next Item
attr1: value3
---
title: One More Item
...

I would like the resulting folded text to look something like this:

+---- 2 lines: My Title ----
+---- ? lines: Next Item ---

Any suggestions are appreciated! Thanks!

Answer 1

If you want to use a foldmethod other than "manual" all the time, add this line to your ~/.vimrc:
set foldmethod=foldoption

I would recommend using foldmethod=indent. This will fold based on any indent. Then if you change your input to include the indents where you want folds to happen. For instance if you change your input to

---
title: My Title
    other attrs: other values
---
title: Next Item
---
title: One More Item
...

It will fold as you described

Answer 2

Do

%s/---\(.*\)\(\_.\{-}title: \)\(.*\)/---\1 #{{{1 \3\2\3/g
set foldmethod=marker

or

%s/\(---\_.\{-}title: \)\(.*\)/#{{{1 \2\r\1\2/g
set foldmethod=marker

That will add comment with title to the beginning of every YAML document and leave document still valid. foldmarker option must be left untouched.

Result:

1.

--- #{{{1 My Title
title: My Title
attr1: value1
attr2: value2
--- #{{{1 Next Item
title: Next Item
attr1: value3
--- #{{{1 One More Item
title: One More Item
...

Folded:

+--  4 строк: --- My Title-----------------------------
+--  3 строк: --- Next Item----------------------------
+--  3 строк: --- One More Item------------------------

2.

#{{{1 My Title
---
title: My Title
attr1: value1
attr2: value2
#{{{1 Next Item
---
title: Next Item
attr1: value3
#{{{1 One More Item
---
title: One More Item
...

Folded:

+--  5 строк: My Title--------------------------------
+--  4 строк: Next Item-------------------------------
+--  4 строк: One More Item---------------------------