# The declarative layout system

The declarative layout system provides an alternative to the imperative system.
It is inspired by the one used by Awesome 3.2-3.4 and the Qt QML style.

## The default widgets

### Widgets

Awesome provides 2 collections of widgets:

 * `wibox.widget`: Generic widgets, containers and layouts
 * `awful.widget`: The Awesome specific widgets


@DOC_widget_WIDGET_LIST@

### Containers

A container is a widget that wraps another widget. It can be used to add
decorations or to modify the content of the child widget.

@DOC_container_WIDGET_LIST@

### Layouts

Layouts are collections of children widgets. They are placed according to
configurable rules.

@DOC_layout_WIDGET_LIST@

## Placing widgets

### A simple layout

* Display `my_first_widget` only on primary screen
* Display `my_second_widget` only on screen two
* Add a background color to `my_fourth_widget`
* Dispose in a `wibox.layout.fixed.horizontal` layout

Code:

    s.mywibox : setup {
        s == screen.primary and my_first_widget, -- Only display on primary screen
        s.index == 2 and my_second_widget, -- Only display on screen 2
        my_third_widget, -- Displayed on all screens
        { -- Add a background color/pattern for my_fourth_widget
              my_fourth_widget,
              bg     = beautiful.bg_focus,
              widget = wibox.container.background,
        },
        layout = wibox.layout.fixed.horizontal,
    }


In this example `s == screen.primary` 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 returns a valid widget or a
declarative layout, or `nil`.

### Composite widgets

@DOC_wibox_widget_progressbar_encapsulation_EXAMPLE@


### 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:

    s.mywibox : 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 a `wibox.layout.align` layout
The `wibox.layout.align` is a little different. While most layouts will
ignore any `nil` lines, the `align` layout relies on them so `left`, `middle`
and `right` can be defined.

Code:

    s.mywibox : 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:

    s.mywibox : 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
    s.mywibox : 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 that it can be accessed later.

Widgets defined using `setup` can be accessed using these methods:

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

The first method mixes the imperative and declarative syntax, and makes the code
less readable. The second is a little verbose and only works if every node in
the chain has 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 a reserved name. This includes all
method names, existing widget attributes, `layout` and `widget`. Names should
also respect the Lua variable conventions (case-sensitive, alphanumeric,
underscore characters and non-numeric first character).

Code:

    s.mywibox : setup {
        {
            id     = "second",
            widget = wibox.widget.textbox
        },
        {
            id     = "third",
            widget = wibox.widget.textbox
        },
        id     = "first",
        layout = wibox.layout.fixed.horizontal,
    }
    
    s.mywibox.first.second:set_markup("changed!")
    s.mywibox: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` exists. 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
caught by a Lua `metatable` (operator overload).

Code:

    -- "Monkeypatch" a new function to 3 widget classes to add vicious
    -- extension support
    for _, wdg in ipairs {
        wibox.widget.textbox , wibox.widget.progressbar, wibox.widget.graph
    } do
        function wdg:vicious(args)
            local f = unpack or table.unpack -- Lua 5.1 compat
            vicious.register(self, f(args))
        end
    end

    s.mywibox : 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](https://github.com/Mic92/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
    s.mywibox : 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
    
    s.mywibox : setup (three_circle)