Formspecs

Introduction

Furnace Inventory
Screenshot of furnace formspec, labelled.

In this chapter we will learn how to create a formspec and display it to the user. A formspec is the specification code for a form. In Minetest, forms are windows such as the player inventory, which can contain labels, buttons and fields to allow you to enter information.

Note that if you do not need to get user input, for example when you only need to provide information to the player, you should consider using Heads Up Display (HUD) elements instead of forms, because unexpected windows tend to disrupt gameplay.

Formspec Syntax

Formspecs have an unusual syntax. They consist of a series of tags which are in the following form:

element_type[param1;param2;...]

Firstly the element type is declared, and then the attributes are given in square brackets.

Elements are items such as text boxes or buttons, or can be metadata such as size or background.

Here are two elements, of types foo and bar.

foo[param1]bar[param1]

Size[w, h]

Nearly all forms have a size tag. This declares the size of the form window. Note that forms don’t use pixels as co-ordinates; they use a grid based on inventories. A size of (1, 1) means the form is big enough to host a 1x1 inventory. This means the size of the form is independent of screen resolution and it should work just as well on large screens as small screens. You can use decimals in sizes and co-ordinates.

size[5,2]

Co-ordinates and sizes only use one attribute. The x and y values are separated by a comma, as you can see above.

Field[x, y; w, h; name; label; default]

This is a textbox element. Most other elements have a similar style of attributes. The name attribute is used in callbacks to get the submitted information. The x and y attributes determine the position of the element, and the w and h attributes provide the size.

field[1,1;3,1;firstname;Firstname;]

It is perfectly valid to not define an attribute.

Other Elements

You should refer to lua_api.txt for a list of all possible elements. Search for “Formspec” to locate the correct part of the document. At the time of writing, formspec information begins on line 1765.

Displaying Formspecs

Here is a generalised way to show a formspec:

minetest.show_formspec(playername, formname, formspec)

Formnames should be itemnames; however, this is not enforced. There is no need to override a formspec, because formspecs are not registered like nodes and items are. The formspec code is sent to the player’s client for them to see, along with the formname. Formnames are used in callbacks to identify which form has been submitted, and to see if the callback is relevant.

Example

This example shows a formspec to a player when they use the /formspec command.

Name Formspec
The formspec generated by
the example's code
-- Show form when the /formspec command is used.
minetest.register_chatcommand("formspec", {
    func = function(name, param)
        minetest.show_formspec(name, "mymod:form",
                "size[4,3]" ..
                "label[0,0;Hello, " .. name .. "]" ..
                "field[1,1.5;3,1;name;Name;]" ..
                "button_exit[1,2;2,1;exit;Save]")
    end
})

Note: the .. is used to join two strings together. The following two lines are equivalent:

"foobar"
"foo" .. "bar"

Callbacks

It’s possible to expand the previous example with a callback:

-- Show form when the /formspec command is used.
minetest.register_chatcommand("formspec", {
    func = function(name, param)
        minetest.show_formspec(name, "mymod:form",
                "size[4,3]" ..
                "label[0,0;Hello, " .. name .. "]" ..
                "field[1,1.5;3,1;name;Name;]" ..
                "button_exit[1,2;2,1;exit;Save]")
    end
})

-- Register callback
minetest.register_on_player_receive_fields(function(player,
        formname, fields)
    if formname ~= "mymod:form" then
        -- Formname is not mymod:form,
        -- exit callback.
        return false
    end

    -- Send message to player.
    minetest.chat_send_player(player:get_player_name(),
            "You said: " .. fields.name .. "!")

    -- Return true to stop other callbacks from
    -- receiving this submission.
    return true
end)

The function given in minetest.register_on_player_receive_fields is called every time a user submits a form. Most callbacks will check the formname given to the function, and exit if it is not the right form; however, some callbacks may need to work on multiple forms, or all forms - it depends on what you want to do.

Malicious clients can submit anything at anytime

You should never trust a formspec submission. A malicious client can submit anything they like at any time - even if you never showed them the formspec. This means that you should check privileges and make sure that they should be allowed to perform the action.

Fields

The fields parameter to the function is a table, index by string, of the values submitted by the user. You can access values in the table via fields.name, where ‘name’ is the name of the element.

As well as retrieving the values of each element, you can also get which button was clicked. In this case, the button called ‘exit’ was clicked, so fields.exit will be true.

Some elements can submit the form without the user clicking a button, such as a check box. You can detect these cases by looking for a clicked button.

-- An example of what fields could contain,
--   using the above code
{
    name = "Foo Bar",
    exit = true
}

Contexts

In many cases you want minetest.show_formspec to give information to the callback which you don’t want to send to the client. This might include what a chat command was called with, or what the dialog is about.

For example, you might make a form to handle land protection information:

--
-- Step 1) set context when player requests the formspec
--

-- land_formspec_context[playername] gives the player's context.
local land_formspec_context = {}

minetest.register_chatcommand("land", {
    func = function(name, param)
        if param == "" then
            minetest.chat_send_player(name,
                    "Incorrect parameters - supply a land ID")
            return
        end

        -- Save information
        land_formspec_context[name] = {id = param}

        minetest.show_formspec(name, "mylandowner:edit",
                "size[4,4]" ..
                "field[1,1;3,1;plot;Plot Name;]" ..
                "field[1,2;3,1;owner;Owner;]" ..
                "button_exit[1,3;2,1;exit;Save]")
    end
})



--
-- Step 2) retrieve context when player submits the form
--
minetest.register_on_player_receive_fields(function(player,
        formname, fields)
    if formname ~= "mylandowner:edit" then
        return false
    end

    -- Load information
    local context = land_formspec_context[player:get_player_name()]

    if context then
        minetest.chat_send_player(player:get_player_name(), "Id " ..
                context.id .. " is now called " .. fields.plot ..
                " and owned by " .. fields.owner)

        -- Delete context if it is no longer going to be used
        land_formspec_context[player:get_player_name()] = nil

        return true
    else
        -- Fail gracefully if the context does not exist.
        minetest.chat_send_player(player:get_player_name(),
                "Something went wrong, try again.")
    end
end)

Node Meta Formspecs

minetest.show_formspec is not the only way to show a formspec; you can also add formspecs to a node’s meta data. For example, this is used with chests to allow for faster opening times - you don’t need to wait for the server to send the player the chest formspec.

minetest.register_node("mymod:rightclick", {
    description = "Rightclick me!",
    tiles = {"mymod_rightclick.png"},
    groups = {cracky = 1},
    after_place_node = function(pos, placer)
        -- This function is run    when the chest node is placed.
        -- The following code sets the formspec for chest.
        -- Meta is a way of storing data onto a node.

        local meta = minetest.get_meta(pos)
        meta:set_string("formspec",
                "size[5,5]"..
                "label[1,1;This is shown on right click]"..
                "field[1,2;2,1;x;x;]")
    end,
    on_receive_fields = function(pos, formname, fields, player)
        if(fields.quit) then return end
        print(fields.x)
    end
})

Formspecs set this way do not trigger the same callback. In order to receive form input for meta formspecs, you must include an on_receive_fields entry when registering the node.

This style of callback triggers when you press enter in a field, which is impossible with minetest.show_formspec; however, this kind of form can only be shown by right-clicking on a node. It cannot be triggered programmatically.

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