flow

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

Online tutorial/demo (source)

Features

Layouting

• No manual positioning of elements.
• Automatic layouting using HBox and VBox containers
• Some elements have an automatic size.
• The size of elements can optionally expand to fit larger spaces

Other features

• 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 form and are automatically applied.

Limitations

• This mod doesn't support all of the features that regular formspecs do.
• 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.

-- 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)

-- You can provide an initial value for `ctx` by adding a second parameter to
-- show(). In the below example, `ctx.value` will be "test".
my_gui:show(player, {value = "test"})

-- 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)

Updating forms

If some data displayed inside a form changes (for example a timer or progress indicator), you can use form:update to update the form without resetting ctx or showing the form again if the player has closed it.

Due to formspec limitations, players may lose text typed into fields that hasn't been sent to the server when form:update is called.

-- Re-shows the form for one player if they have the form open
my_gui:update(player)

-- Re-shows the form for all players that have the form open and where
-- ctx.test == 123
my_gui:update_where(function(player, ctx)
    return ctx.test == 123
end)

-- Re-shows the form for all players with the "server" privilege
my_gui:update_where(function(player, ctx)
    return minetest.check_player_privs(player, "server")
end)

-- Re-shows the form for all players with the form open
my_gui:update_where(function() return true end)

Inside an on_event handler, you can use return true instead.

gui.Button{
    label = "Update form",
    on_event = function(player, ctx)
        return true
    end,
}

Other formspec libraries/utilities

These utilities likely aren't compatible with flow.

• fs_layout is another mod library that does automatic formspec element positioning.
• fslib is a small mod library that lets you build formspec strings.
• Just_Visiting's formspec editor is a Minetest (sub)game that lets you edit formspecs and preview them as you go
• 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 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 form. They don't have a direct equivalent in formspecs.

gui.VBox

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

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.

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.

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:

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:

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):

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

You should do this instead:

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.

gui.Nil

A tool to allow for ternary-ish conditional widgets:

gui.VBox{
    gui.Label{ label = "The box below is only present if the boolean is truthy" },
    the_boolean and gui.Box{ color = "#FF0000" } or gui.Nil{},
}

Use sparingly, flow still has to process each Nil object to be able to know to remove it, and thus could still slow things down. The fastest element is one that doesn't exist, and thus doesn't need processing.

gui.Stack

This container element places its children on top of each other. All child elements are expanded in both directions.

Note that some elements (such as centred labels) won't pass clicks through to the element below them.

Example:

gui.Stack{
    min_w = 10,
    gui.Button{label = "Hello world!"},
    gui.Image{w = 1, h = 1, texture_name = "air.png", padding = 0.2, align_h = "left"},
}

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:

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"},
}

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:

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.

Hiding elements

Elements inside boxes can have visible = false set to hide them from the player. Elements hidden this way will still take up space like with visibility: hidden; in CSS.

Experimental features

These features might be broken in the future.

no_prepend[]

You can set no_prepend = true on the "root" element to disable formspec prepends.

Example:

local my_gui = flow.make_gui(function(player, ctx)
    return gui.VBox{
        no_prepend = true,

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

        -- There will be an empty space where the second button would be
        gui.Button{label = "Button 2", visible = false},

        gui.Button{label = "Button 3"},
    }
end)

Using a form as an inventory

A form can be set as the player inventory. Flow internally generates the formspec and passes it to player:set_inventory_formspec(). This will completely replace your inventory and isn't compatible with inventory mods like sfinv.

local example_inventory = flow.make_gui(function (player, context)
    return gui.Label{ label = "Inventory goes here!" }
end)
minetest.register_on_joinplayer(function(player)
    example_inventory:set_as_inventory_for(player)
end)

Like with the show_hud function, update* functions don't do anything, so to update it, call set_as_inventory_for again with the new context. If the context is not provided, it will reuse the existing context.

example_inventory:set_as_inventory_for(player, new_context)

While the form will of course be cleared when the player leaves, if you'd like to unset the inventory manually, call :unset_as_inventory_for(player), analogue to close_hud:

example_inventory:unset_as_inventory_for(player)

This will set the inventory formspec string to "" and stop flow from processing inventory formspec input.

Rendering to a formspec

This API should only be used when necessary and may have breaking changes in the future.

Some APIs in other mods, such as sfinv, expect formspec strings. You can use this API to embed flow forms inside them. To use flow with these mods, you can call form:render_to_formspec_string(player, ctx, standalone).

• By default the the formspec_version and size elements aren't included in the returned formspec and are included in a third return value. Set standalone to include them in the returned formspec string. The third return value will not be returned.
• Returns formspec, process_event[, info]
• The process_event(fields) callback will return true if the formspec should be redrawn, where render_to_formspec_string should be called and the new process_event should be used in the future. This function may return true even if fields.quit is sent.

Do not use this API with node meta formspecs, it can and will break!