[[Property:title|Containers]]
[[Property:weight|1]]
[[Property:uuid|aa71e29d-f0e0-9eb2-a289-675d24aac927]]
A container is a EiffelVision 2 widget that may contain other widgets. All containers inherit from [[ref:libraries/vision2/reference/ev_container_chart|EV_CONTAINER]].  Some containers such as [[ref:libraries/vision2/reference/ev_cell_chart|EV_CELL]]  may only hold one widget while containers such as [[ref:libraries/vision2/reference/ev_box_chart|EV_BOX]]  may hold multiple widgets. Since all containers inherit from type [[ref:libraries/vision2/reference/ev_widget_chart|EV_WIDGET]], this means that a container may be placed inside another container. Windows inherit from [[ref:libraries/vision2/reference/ev_cell_chart|EV_CELL]]  and are therefore also classified as containers.

{{note|Containers may only contain other widgets. Items may only be placed in certain primitives. For example, an [[ref:libraries/vision2/reference/ev_list_item_chart| EV_LIST_ITEM]]  may only be contained in an [[ref:libraries/vision2/reference/ev_list_chart| EV_LIST]]. }}


==Inheritance from EiffelBase==

Wherever possible, EiffelVision 2 containers reuse the container structures provided by [[EiffelBase]]. This provides three main advantages: 

# Familiarity for operations such as access, insertions, and removals. Anybody who already uses [[EiffelBase]]  should find it easy to use EiffelVision 2 containers. 
# The underlying structures used have been tried and tested for many years and their designs have proven to be robust and effective.
# Containers provide powerful methods of traversing and searching their contents. In the original EiffelVision library, you needed to keep references to widgets since you could not query the contents of their containers.

For example, [[ref:libraries/vision2/reference/ev_container_chart|EV_CONTAINER]]  inherits from <eiffel>BOX</eiffel> and <eiffel>COLLECTION</eiffel>. Descendents of [[ref:libraries/vision2/reference/ev_container_chart|EV_CONTAINER]]  such as [[ref:libraries/vision2/reference/ev_box_chart|EV_TABLE]]  may also inherit from other structures in [[EiffelBase]]. 


==Using Containers==

The main role of a container is to position its children in a certain way. An [[ref:libraries/vision2/reference/ev_horizontal_box_chart| EV_HORIZONTAL_BOX]]  for example positions its children side by side. To achieve vertical stacking you would use an [[ref:libraries/vision2/reference/ev_vertical_box_chart| EV_VERTICAL_BOX]] . 
A code example of adding widgets to a container is as follows:
<code>
    add_to_container
            -- Add 3 buttons to a hbox and then show in window.
        local
            my_window: EV_TITLED_WINDOW
            my_hbox: EV_HORIZONTAL_BOX
            my_button: EV_BUTTON
        do
            create my_window 
            create my_hbox 
            create my_button.make_with_text ("Button1")
            my_hbox.extend (my_button)
            my_hbox.extend (create {EV_BUTTON}.make_with_text ("Button2"))
            my_hbox.extend (create {EV_BUTTON}.make_with_text ("Button3"))
            my_window.extend (my_hbox)
            my_window.show
        end
</code>

The mapping of a EiffelVision 2 container interface is very close to that of containers in [[EiffelBase]], such as a linked list. 
* To add a widget to a container call <code>extend</code>.
* To remove a widget from the container call <code>prune</code>.
* To empty a container call <code>wipe_out</code>.
* To traverse the container, you can use features such as <code>start</code>, <code>forth</code>, <code>item</code>, and <code>off</code>.

{{note|When a widget is added to a container, the container is the parent of the widget. }}


==Size of a Widget Within a Container==

The size of a widget in a container is always at least its minimum size. By default, when a widget is added to a container it is expandable. This means that if the container gets bigger, the widget will expand to fit the extra space. This convenient functionality means that you don't have to worry about handling the size of the widget (such as when a user alters the size of a window) as this is all performed automatically. If, on the other hand, you decide you want the widget to remain at a certain size (its minimum size), [[ref:libraries/vision2/reference/ev_box_chart| EV_BOX]] (and therefore its descendants) contain the convenient call <code>disable_item_resize</code>.
 

==Sizing of Containers==

Since [[ref:libraries/vision2/reference/ev_container_chart|EV_CONTAINER]]  inherits from type [[ref:libraries/vision2/reference/ev_widget_chart|EV_WIDGET]], the following facilities are provided for setting the minimum size: <code>set_minimum_width</code>, <code>set_minimum_height</code> and <code>set_minimum_size</code>. It is important to remember that as a general rule, '''the current size of a container is always at least the minimum size of all its contents.'''

{{note|The exception to this is: [[ref:libraries/vision2/reference/ev_fixed_chart|EV_FIXED]], [[ref:libraries/vision2/reference/ev_viewport_chart|EV_VIEWPORT]], and [[ref:libraries/vision2/reference/ev_scrollable_area_chart|EV_SCROLLABLE_AREA]] which do actually allow their contents to be larger than their current size.}}