Cuando lees un archivo, a menudo hay mucho texto irrelevante que te impide entender lo que realiza ese archivo. Para ocultar ese *ruido* innecesario, utiliza el plegado de texto que ofrece Vim (Vim Fold).
Imagina que estás doblando una hoja de papel para cubrir una parte de texto. El texto actual no ha desaparecido, todavía sigue ahí. El plegado de Vim funciona de una manera similar. Este pliega una parte del texto, evitando mostrarlo pero sin eliminarlo del archivo.
El comando para plegar texto en Vim es un operador, por lo que cumple las reglas gramaticales de Vim (`verbo + sustantivo`). Puedes pasarle al operador de plegado un movimiento o un objeto de texto. Para plegar el párrafo en el que se encuentra el cursor, ejecuta `zfip`. Para plegar desde la posición actual hasta el final del archivo, ejecuta `zfG`. Para plegar los textos entre `{` y `}`, ejecuta `zfa{`.
También puedes ejecutar un plegado de texto desde la línea de comandos con el comando `:fold`. Para plegar la línea actual y la línea posterior, ejecuta:
`,+1` es el rango. Si no le pasas parámetros al rango, el comando de manera predeterminada lo aplica a la línea actual. `+1` es el indicador de rango para la línea siguiente. Para plegar las líneas de la 5 a la 10, ejecuta `:5,10fold`. Para plegar desde la posición actual hasta el final de la línea, ejecuta `:,$fold`.
Puedes ejecutar `zR` y `zM` estando en cualquier línea, pero `za` solo funciona cuando estás en una línea que pertenece a un plegado de texto. Para aprender más sobre los comandos de los plegados de texto, echa un vistazo a `:h fold-commands`.
Para ver el método de plegado de texto que estás utilizando actualmente, ejecuta `:set foldmethod?`. De manera predeterminada, Vim utiliza el método `manual`.
Con este método de plegado de texto, Vim mira cuantos espacios tiene cada línea al comienzo y lo compara con la opción `'shiftwidth'` para determinar cómo lo puede plegar. `'shiftwidth'` devuelve el número de espacios requeridos para cada paso del sangrado de línea. Si ejecutas:
Vim's default `'shiftwidth'` value is 2. On the text above, there are two spaces between the start of the line and the text "Two" and "Two again". When Vim sees the number of spaces and that the `'shiftwidth'` value is 2, Vim considers that line to have an indent fold level of one.
Right now if you run `:set foldmethod=indent`, Vim does not fold the indented line because there isn't sufficient space on each line. One space is not considered an indentation. However, if you change the `'shiftwidth'` to 1:
Nested folds are valid. The text "Two" and "Two again" have fold level of one. The text "Three" and "Three again" have fold level of two. If you have a foldable text with a higher fold level within a foldable text, you will have multiple fold layers.
Vim sees `{{{` and `}}}` as fold indicators and folds the texts between them. With the marker fold, Vim looks for special markers, defined by `'foldmarker'` option, to mark folding areas. To see what markers Vim uses, run:
Syntax fold is determined by syntax language highlighting. If you use a language syntax plugin like [vim-polyglot](https://github.com/sheerun/vim-polyglot), the syntax fold will work right out of the box. Just change the fold method to syntax:
It will be folded with a syntax fold. When you define a syntax highlighting for a particular language (typically inside the `syntax/` directory), you can add a `fold` attribute to make it foldable. Below is a snippet from vim-polyglot JavaScript syntax file. Notice the `fold` keyword at the end.
Expression fold allows you to define an expression to match for a fold. After you define the fold expressions, Vim scans each line for the value of `'foldexpr'`. This is the variable that you have to configure to return the appropriate value. If the `'foldexpr'` returns 0, then the line is not folded. If it returns 1, then that line has a fold level of 1. If it returns 2, then that line has a fold level of 2. There are more values other than integers, but I won't go over them. If you are curious, check out `:h fold-expr`.
-`v:lnum` is Vim's special variable for the `'foldexpr'` expression. Vim scans each line and at that moment stores each line's number in `v:lnum` variable. On line 5, `v:lnum` has value of 5. On line 10, `v:lnum` has value of 10.
-`[0]` in the context of `getline(v:lnum)[0]` is the first character of each line. When Vim scans a line, `getline(v:lnum)` returns the content of each line. `getline(v:lnum)[0]` returns the first character of each line. On the first line of our list, "donut", `getline(v:lnum)[0]` returns "d". On the second line of our list, "pancake", `getline(v:lnum)[0]` returns "p".
-`==\\"p\\"` is the second half of the equality expression. It checks if the expression you just evaluated is equal to "p". If it is true, it returns 1. If it is false, it returns 0. In Vim, 1 is truthy and 0 is falsy. So on the lines that start with an "p", it returns 1. Recall if a `'foldexpr'` has a value of 1, then it has a fold level of 1.
Vim automatically folds some of the identical lines. When you are running the `vimdiff` command, Vim automatically uses `foldmethod=diff`. If you run `:set foldmethod?`, it will return `diff`.
Your folds are restored. However, you have to manually run `mkview` and `loadview`. I know that one of these days, I will forget to run `mkview` before closing the file and I will lose all the folds. How can we automate this process?
-`BufWinLeave` for when you remove a buffer from a window.
-`BufWinEnter` for when you load a buffer in a window.
Now after you fold inside a `.txt` file and exit Vim, the next time you open that file, your fold information will be restored.
By default, Vim saves the fold information when running `mkview` inside `~/.vim/view` for the Unix system. For more information, check out `:h 'viewdir'`.
When I first started Vim, I neglected ot learn fold because I didn't think it was useful. However, the longer I code, the more useful I find folding is. Strategically placed folds can give you a better overview of the text structure, like a book's table of content.
When you learn fold, start with the manual fold because that can be used on-the-go. Then gradually learn different tricks to do indent and marker folds. Finally, learn how to do syntax and expression folds. You can even use the latter two to write your own Vim plugins.
