UFO-Scripts/ui/LUA-scripts

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Basic processing

In ufo, scripts create (among other things), the user interface. These scripts all have extension .ufo. When ufo encounters such a script it is processed. Ufo now accepts two script languages to create the user interface: a custom script language, called ufo script and a lua script. Both script languages can be used side-by-side. Ufo determines the script type by scanning the first line of the script file. If it starts with: 

--!/usr/bin/lua

the script is considered to be a lua script and is processed by the embedded lua interpreter, else the script is considered to be ufo script and is processed by ufo script parser.

For a lua script, the entire script is first parsed by the lua interpreter. If no errors are found, the lua script is then executed by the lua interpreter. This is the moment youre script actually gets executed for the first time. Scripts are loaded in the order as they appear in the folder. This is why files containing generic stuff usually start with an underscore.

UI-scripting 'The lua way...'

Before you start, you should become familiair with the lua language itself and most of all with the fact that lua is not an object oriented language. Objects in lua are represented by a data structure called a table, essentially being a container holding data and functions. This mechanism is broadly used since every user interface node becomes a table. However, since lua is not an OO-language, there is no mechanism to add the "self" parameter in a method call. To solve this, lua programmers since long have used the following trick: 

function fooTable.fooMethod(self, arg1)
    -- the code
end

Since it was so broadly use, eventually Lua added a second function call mechanism that does add implicitly a "self" parameter. It uses the ":", like this: 

function fooTable:fooMethod(arg1)
    -- the code
end

Be aware however that this calling mechanism applies only for functions calls and not for data fields. So to access a data field on a table you use the "." seperator, like this: 

local x = fooTable()
x.fooData = 123

This distinction is important since everything in the UFO lua API is exposed using functions with the exception of event handlers (note the ":" when accessing functions on a table and the "." when accessing the field holding the on_click event handler. Here is some sample code: 

-- get the window node
local x = node:root()
-- attach an event handler to respond  to a mouseclick
x.on_click = function (sender)
        -- do whatever you want to do when the user clicks
    end

Accessing the API

All parts of ufo exposed to lua is accessible from within lua using the "ufo" namespace. It contains the global functions, classes and constants. For a complete list of the API see [...].

The API follows the naming of properties and methods in the ufo script language with a few exceptions. In general this is the case when the naming in the ufo script is either ambigious or confusing. For instance, in ufo script the property "string" in general is being used to refer to the text displayed by a control, so it was renamed to "text".

The lua API uses the following naming convention: - Getters are not prefixed, so e.g. to get the name of the node you call "name()" - Getters for flags however are prefixed by "is_", e.g. "is_virtual()" - Setters are prefixed by "set_", e.g. "set_background("icon/windowplusb")"

Processing after loading

After a script is processed it is possible to run a onload event. To do so, just put a call to register_onload somewhere in the script. A sample is given below: 

function foo_onload ()
    -- do whatever you want to do after the script is processed
end

register_onload (foo_onload)

The difference by the way is subtle. The onload is executed just after the lua script is processed so in theory it is possible to achieve the same result by positioning foo_onload at the end of the script (it is then executed last). However, in future, it might be that onload will be executed after the loading/processing of all ufo lua scripts.

Creating a user interface

To create a user interface, the second step in processing of the .ufo file should yield a uiNode (single node or tree of nodes). To create nodes, the api exposes node creation functions through the common ufo namespace. Even though lua will allow it, you should never create a uiNode directly. With the uiNode object (or any of its descendents) in hand, you can start calling methods to alter the desired behaviour (e.g. set the background). 

main = ufo.create_window ("main", nil)
main::set_background("ui/main_bg")

The user interface makes a distinction between "controls", "windows" and "components". Controls are user interface elements, e.g. buttons. Controls are always located inside a container. A "window" is a top-level user interface element. A "component" is a clonable group of controls. A component typically is created around a container control (e.g. a panel). Like controls, components can be created and added to windows. 

local smallstring = ufo.create_component ("string", "smallstring", nil)
smallstring:set_font("f_verysmall")
smallstring:set_color(0.56, 0.81, 0.76, 1.0)

Creating functions in lua

It is easy to write functions in lua. Functions can be shared through the entire lua environment, just make sure that the function is known at the moment it is used. It is also possible to attach a function to a user interface node. If you declare a function on a lua node, it is reusable anywhere else even in the old script language. For example, the code below creates a panel component to be reused. It defines two functions on the node. These functions do not exist in the C++ side of ufo, only on the lua side. However they can now be used in the user interface. Even if the old style script is used. 

local common_panel = ufo.create_component ("panel", "common_panel", nil)
common_panel.enable = function (sender)
        common_panel:set_background("ui/enabled")
    end
common_panel.disable = function (sender)
        common_panel:set_background("ui/disabled")
    end

Creating and using confunc's

A confunc is a script defined node that can be called as a command either from the console or from any other script. To define a confunc, you must create a node of class "confunc". Then define an on_click event with the signature of the command. Any parameters to the confunc are accessible as additional parameters following "sender", the first parameter of any function. 

local foo_func = ufo.create_confunc(parent_node, "pop_initready_aircraft", nil)
foo_func.on_click = function (sender, arg1, arg2)
        sender:parent():child("craftname"):set_text(arg1)
        sender:parent():child("missionname"):set_text(arg2)
    end

Note that in the example above the arguments are named arg1 and arg2 but they can have any name. In this case a more logical name would be "craft" for arg1 and "mission" for arg2.

Storing data on a node

Like functions, in lua it is easy to add data fields to a table. However, using this mechanism is not advised since there is no guarantee that the lua table will not be destroyed by the garbage collector. Ufo has it's own mechanism to create persistent variables, either by creating a "cvar" or by using a data node.

Lua identifier scoping and the use of do..end

Identifiers have a scope (if you get lost here and don't know what 'scope' is, then I suggest you first start reading some books about programming before continuing). Defining the same identifier twice in a scope results in an error. Whenever you define a ui element, variables are prefixed with the keyword 'local', indicating that the scope of the variable is limited to this lua file only. If you forget the 'local' keyword the identifier becomes visible in all lua files, possibly resulting in a duplicate identifier error.

Another way of limiting the scope of identifiers is to use the 'do..end' block. 

do
    local foo_panel = ufo.create_component ("panel", "common_panel", nil)
    ...
end

In this case the identifier 'foo_panel' is visible only inside the 'do..end', thereby limiting the visibility to only a part of the file. The use of 'do..end' is strongly advised since it prevents accidental nameclash. Also it is a must in the asset files, wich define a number of generic ui components.

Utility libaries

One advantage of using lua is that it allows for libraries of support functions that can greatly enhance the process of creating the user interface. A most useful library is the ufox library. For more info, see UFO-Scripts/ui/LUA-utility-functions

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