awesome/docs/03-declarative-layout.md

# The declarative layout system

This system provide an alternative to the system used in Awesome 3.5 and is
inspired by the one once used by Awesome 3.2-3.4 and Qt QML system.

## A simple layout

* Display `my_first_widget` only on screen one
* Add a background color to `my_third_widget`
* Dispose in a `wibox.layout.fixed.horizontal` layout

Code:

    mywibox[s] : setup {
        s == 1 and my_first_widget, -- Only display on screen 1
        my_second_widget,
        { -- Add a background color/pattern for my_third_widget
              my_third_widget,
              bg     = beautiful.bg_focus,
              widget = wibox.container.background,
        },
        layout = wibox.layout.fixed.horizontal,
    }


In this example `s == 1` is an inline expression. In the default `rc.lua`,
there is an `s` variable represent to define the current screen. Any lua
logic expression can be used as long as it return a valid widget, or a
declarative layout, or `nil`.


## Define widgets inline and place them

* Create a `wibox.widget.textbox` with various properties
* Force the textbox size using `wibox.layout.constraint`
* Add a margin around another textbox
* Add a `wibox.container.background` (for visualization)

Code:

    mywibox[s] : setup {
        {
            -- Force the textbox to always be 300 pixel long
            {
                {
                    markup = "<b>Hello World!</b>",
                    align  = "center",
                    widget = wibox.widget.textbox
                },
                bg     = "#ff0000",
                widget = wibox.container.background,
            },
            width    = 300,
            strategy = "min",
            layout   = wibox.layout.constraint
        },
        {
            -- Add a border around the background
            {
                {
                    markup = "Foobar",
                    widget = wibox.widget.textbox
                },
                bg     = "#0000ff",
                widget = wibox.container.background
            },
            left   = 10,
            right  = 10,
            top    = 1,
            bottom = 2,
            layout = wibox.container.margin
        },
        layout = wibox.layout.fixed.horizontal,
    }


Result:
![Example2 screenshot](../images/widgetlayout1.png)


## Use an `wibox.layout.align` layout
The `wibox.layout.align` is a little different. While most layouts will
ignore any `nil` lines, the `align` layout rely on them so `left`, `middle`
and `right` can be defined

Code:

    mywibox[s] : setup {
        my_textbox1, -- Left
        nil,         -- Nothing in the middle
        my_textbox2, -- Right
        layout = wibox.layout.fixed.horizontal,
    }



## Define new widgets

New trivial widgets can be created directly in the layout declaration. Here
is a simple circle widget:

Code:

    mywibox[s] : setup {
        fit    = function(self, context, width, height)
            return height, height -- A square taking the full height
        end,
        draw   = function(self, context, cr, width, height)
            cr:set_source_rgb(1, 0, 0) -- Red
            cr:arc(height/2, height/2, height/2, 0, math.pi*2)
            cr:fill()
        end,
        layout = wibox.widget.base.make_widget,
    }

Result:
![Example4 screenshot](../images/widgetlayout2.png)

For more information about how to draw widgets, refer to the `Cairo` api:

* [Path](http://cairographics.org/manual/cairo-Paths.html)
* [Context](http://cairographics.org/manual/cairo-cairo-t.html)
* [Pattern](http://cairographics.org/manual/cairo-cairo-pattern-t.html)
* [transformation](http://cairographics.org/manual/cairo-Transformations.html)
* [Operator](http://cairographics.org/operators/)
* [Pango text](https://developer.gnome.org/pango/stable/)


## Externally defined widgets and layouts

This is useful when the widget is provided by an external module or when it
requires complex manipulations which would make the declaration unreadable.

Code:

    local tb = wibox.widget.textbox()
    tb:set_markup("Hello world! ")
    
    -- Repeat "tb" 3 times
    mywibox[s] : setup {
        tb,
        tb,
        tb,
        layout = wibox.layout.fixed.horizontal,
    }



## Accessing widgets

For each widget or container, it is possible to add an `identifier` attribute
so the widget can be accessed later.

Widgets defined using `setup` can be access by 3 means:

* Avoid the issue by using externally created widgets
* Use `my_wibox.my_first_widget.my_second_widget` style access
* Use JavaScript like `my_wibox:get_children_by_id("my_second_widget")[1]`

The first method mixes the imperative and declarative syntax, but makes the code
less readable. The second is a little verbose and only works if every node in
the chain have a valid identifier. The last one doesn't require long paths,
but it is not easy to get a specific instance if multiple widgets have the
same identifier.

WARNING: The widget identifier must not use reseved name. This include all
method names, existing widget attributes, `layout` and `widget`. Names should
also respect the lua variable name policies (case sensitive, alphanumeric and
underscore characters and non-numeric first character)

Code:

    mywibox[s] : setup {
        {
            id     = "second",
            widget = wibox.widget.textbox
        },
        {
            id     = "third",
            widget = wibox.widget.textbox
        },
        id     = "first",
        layout = wibox.layout.fixed.horizontal,
    }
    
    mywibox[s].first.second:set_markup("changed!")
    mywibox[s]:get_children_by_id("third")[1]:set_markup("Also changed!")



## Extending the system

This system is very flexible. Each section attribute (the entries with string
keys) is directly linked to the layout or widget API. When setting the
imaginary `myproperty`, it will first check if `set_myproperty` exist. If it
doesn't, it will check if there is a `myproperty` method. Finally, it will
just set the `mywidget.myproperty` directly in case it is used later or
catched by a lua `metatable` (operator overload).

Code:

    -- "Monkeypatch" a new function to wibox.widget.textbox to add vicious
    -- extension support
    function wibox.widget.textbox:vicious(args)
        local f = unpack or table.unpack -- Lua 5.1 compat
        vicious.register(w, f(args))
    end

    mywibox[s] : setup {
        {
            vicious = {vicious.widgets.cpu, "CPU: $1%", 3},
            widget  = wibox.widget.textbox
        },
        layout = wibox.layout.fixed.horizontal,
    }


In this example, the system is extended to that the popular
[Vicious](http://awesome.naquadah.org/wiki/Vicious) extension module can be
used directly in the layout declaration. This example will update the textbox
every 3 seconds to show the CPU usage.


## Handling sections

The system allows sections to be defined externally, then composed into
the final layout declaration. Here is an example re-using one of the above
example:

Code:

    local circle = {
        fit    = function(self, context, width, height)
            return height, height -- A square taking the full height
        end,
        draw   = function(self, context, cr, width, height)
            cr:set_source_rgb(1, 0, 0) -- Red
            cr:arc(height/2, height/2, height/2, 0, math.pi*2)
            cr:fill()
        end,
        layout = wibox.widget.base.make_widget,
    }
    
    -- Define a layout with the imperative syntax
    local l = wibox.widget.align()
    
    -- 3 circle
    mywibox[s] : setup {
        circle,
        circle,
        circle,
        l,
        layout = wibox.layout.align.horizontal
    }
    
    -- This can be done instead
    local three_circle = {layout = wibox.layout.align.horizontal}
    for i=1, 3 do
        table.insert(three_circle, circle)
    end
    
    mywibox[s] : setup (three_circle)