minetest-flow/README.md

# flow

An experimental layout manager and formspec API replacement for Minetest.
Vaguely inspired by Flutter and GTK.

## Features

 - No manual positioning of elements.
 - Some elements have an automatic size.
 - The size of elements can optionally expand to fit larger spaces
 - No form names. Form names are still used internally, however they are hidden from the API.
 - No having to worry about state.
 - Values of fields, scrollbars, checkboxes, etc are remembered when redrawing
   a formspec and are automatically applied.

## Limitations

 - This mod doesn't support all of the features that regular formspecs do.
 - [FS51](https://content.minetest.net/packages/luk3yx/fs51/) is required if
   you want to have full support for Minetest 5.3 and below.

## Basic example

See `example.lua` for a more comprehensive example which demonstrates how
layouting and alignment works.

```lua
-- GUI elements are accessible with flow.widgets. Using
-- `local gui = flow.widgets` is recommended to reduce typing.
local gui = flow.widgets

-- GUIs are created with flow.make_gui(build_func).
local my_gui = flow.make_gui(function(player, ctx)
    -- The build function should return a GUI element such as gui.VBox.
    -- `ctx` can be used to store context. `ctx.form` is reserved for storing
    -- the state of elements in the form. For example, you can use
    -- `ctx.form.my_checkbox` to check whether `my_checkbox` is checked. Note
    -- that ctx.form.element may be nil instead of its default value.

    -- This function may be called at any time by flow.

    -- gui.VBox is a "container element" added by this mod.
    return gui.VBox {
        gui.Label {label = "Here is a dropdown:"},
        gui.Dropdown {
            -- The value of this dropdown will be accessible from ctx.form.my_dropdown
            name = "my_dropdown",
            items = {'First item', 'Second item', 'Third item'},
            index_event = true,
        },
        gui.Button {
            label = "Get dropdown index",
            on_event = function(player, ctx)
                -- flow should guarantee that `ctx.form.my_dropdown` exists, even if the client doesn't send my_dropdown to the server.
                local selected_idx = ctx.form.my_dropdown
                minetest.chat_send_player(player:get_player_name(), "You have selected item #" .. selected_idx .. "!")
            end,
        }
    }
end)

-- Show the GUI to player as an interactive form
-- Note that `player` is a player object and not a player name.
my_gui:show(player)

-- Close the form
my_gui:close(player)

-- Alternatively, the GUI can be shown as a non-interactive HUD (requires
-- hud_fs to be installed).
my_gui:show_hud(player)
my_gui:close_hud(player)
```

## Other formspec libraries/utilities

These utilities likely aren't compatible with flow.

 - [fs_layout](https://github.com/fluxionary/minetest-fs_layout/) is another mod library that does automatic formspec element positioning.
 - [Just_Visiting's formspec editor](https://content.minetest.net/packages/Just_Visiting/formspec_editor) is a Minetest (sub)game that lets you edit formspecs and preview them as you go
 - [kuto](https://github.com/TerraQuest-Studios/kuto/) is a formspec library that has some extra widgets/components and has a callback API. Some automatic sizing can be done for buttons.
   - It may be possible to use kuto's components with flow somehow as they both use formspec_ast internally.
 - [My web-based formspec editor](https://forum.minetest.net/viewtopic.php?f=14&t=24130) lets you add elements and drag+drop them, however it doesn't support all formspec features.

## Elements

You should do `local gui = flow.widgets` in your code.

### Layouting elements

These elements are used to lay out elements in the formspec. They don't have a
direct equivalent in Minetest formspecs.

#### `gui.VBox`

A vertical box, similar to a VBox in GTK. Elements inside a VBox are stacked
vertically.

```lua
gui.VBox{
    -- These elements are documented later on.
    gui.Label{label="I am a label!"},

    -- The second label will be positioned underneath the first one.
    gui.Label{label="I am a second label!"},
}
```

Elements inside boxes have a spacing of 0.2 between them. To change this, you
can add `spacing = <number>` to the box definition. For example, `spacing = 0`
will remove all spacing between the elements.

#### `gui.HBox`

Like `gui.VBox` but stacks elements horizontally instead.

```lua
gui.HBox{
    -- These elements are documented later on.
    gui.Label{label="I am a label!"},

    -- The second label will be positioned to the right of first one.
    gui.Label{label="I am a second label!"},

    -- You can nest HBox and VBox elements
    gui.VBox{
        gui.Image{w=1, h=1, texture_name="default_dirt.png", align_h="centre"},
        gui.Label{label="Dirt", expand=true, align_h="centre"},
    }
}
```

#### `gui.ScrollableVBox`

Similar to `gui.VBox` but uses a scroll_container and automatically adds a
scrollbar. You must specify a width and height for the scroll container.

```lua
gui.ScrollableVBox{
    -- A name must be provided for ScrollableVBox elements. You don't
    -- have to use this name anywhere else, it just makes sure flow
    -- doesn't mix up scrollbar states if one gets removed or if the
    -- order changes.
    name = "vbox1",

    -- Specifying a height is optional but is probably a good idea.
    -- If you don't specify a height, it will default to
    -- min(height_of_content, 5).
    h = 10,

    -- These elements are documented later on.
    gui.Label{label="I am a label!"},

    -- The second label will be positioned underneath the first one.
    gui.Label{label="I am a second label!"},
}
```

#### `gui.Spacer`

A "flexible space" element that expands by default. Example usage:

```lua
gui.HBox{
    -- These buttons will be on the left-hand side of the screen
    gui.Button{label = "Cancel"},
    gui.Button{label = "< Back"},

    gui.Spacer{},

    -- These buttons will be on the right-hand side of the screen
    gui.Button{label = "Next >"},
    gui.Button{label = "Confirm"},
}
```

I advise against using spacers when `expand = true` and `align = ...` would
work just as well since spacers are implemented hackily and won't account for
some special cases.

You can replicate the above example without spacers, however the code doesn't
look as clean:

```lua
gui.HBox{
    -- These buttons will be on the left-hand side of the screen
    gui.Button{label = "Cancel"},
    gui.Button{label = "< Back", expand = true, align_h = "left"},

    -- These buttons will be on the right-hand side of the screen
    gui.Button{label = "Next >"},
    gui.Button{label = "Confirm"},
}
```

You should not use spacers to centre elements as it creates unnecessary boxes,
and labels may be slightly off-centre (because label widths depend on screen
size, DPI, etc and this code doesn't trigger the centering hack):

```lua
-- This is bad!
gui.HBox{
    gui.Spacer{},
    gui.Label{label="I am not properly centered!"},
    gui.Spacer{},
}
```

You should do this instead:

```lua
gui.Label{label="I am centered!", align_h = "centre"},
```

This applies to other elements as well, because using HBox and Spacer to centre
elements creates unnecessary containers.

### Minetest formspec elements

There is an auto-generated `elements.md` file which contains a list of elements
and parameters. Elements in this list haven't been tested and might not work.

#### Dynamic element types

If you want to generate element types from a variable, you can use
`{type = "label", label = "Hello world!"}` instead of
`gui.Label{label="Hello world!"}`. HBoxes and VBoxes can be created
this way as well (with `type = "hbox"` and `type = "vbox"`), however other
layouting elements (such as ScrollableVBox and Spacer)
won't work correctly.

An example of this is in `example.lua`.

## Padding, spacing, and backgrounds

All elements can have a `padding` value, which will add the specified amount of
padding around the element. The "root" element of the form (the one returned by
`build_func`) has a default padding of 0.3, everything else has a default
padding of 0.

`HBox` and `VBox` have a `spacing` field which specifies how much spacing there
is between elements inside the box. If unspecified, `spacing` will default to
0.2.

Container elements (HBox and VBox) can optionally have `bgimg` and `bgimg_middle`
parameters that specify a background for the container. The background will be
drawn behind any padding that the container has.

Example:

```lua
gui.VBox{
    padding = 0.5,
    spacing = 0.1,

    -- bgimg can be used without bgimg_middle
    bgimg = "air.png",
    bgimg_middle = 2,

    gui.Button{label="Button 1"},
    gui.Button{label="Button 2"},
}
```

![Screenshot](https://user-images.githubusercontent.com/3182651/198194381-4812c0fa-1909-48f8-b50d-6713c4c126ec.png)

The padding around the VBox is 0.5 and the spacing between the buttons inside
it is 0.1.

## Styling forms

To style forms, you use the `gui.Style` and `gui.StyleType` elements:

```lua
gui.Style{
    selectors = {"btn1"},
    props = {
        bgimg = "button.png",
        border = false,
    }
},

gui.Button{
    name = "btn1",
    label = "Button",
},
```

The style elements are invisible and won't affect padding.