[[Property:title|Containers]]
[[Property:weight|1]]
[[Property:uuid|aa71e29d-f0e0-9eb2-a289-675d24aac927]]
All EiffelVision 2 containers inherit [[ref:libraries/vision2/reference/ev_container_chart|EV_CONTAINER]] 

==What is a container?==

A container is a EiffelVision 2 widget that may contain other widgets. 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. As a container is of type [[ref:libraries/vision2/reference/ev_widget_chart|EV_WIDGET]] , this means that a container may be placed within a container. Windows inherit from [[ref:libraries/vision2/reference/ev_cell_chart|EV_CELL]]  and are therefore classed as containers also. 

{{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 be contained in an [[ref:libraries/vision2/reference/ev_list_chart| EV_LIST]] . }}

==Inheritance from Base==

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 adapt to using EiffelVision 2 containers. 
# The underlying structures used have been tried and tested for many years and their design has been proved to be robust and effective.
# It provides powerful methods of querying the contents. In the original EiffelVision library, you needed to keep reference to widgets as you could not query the contents of a container.

For example, [[ref:libraries/vision2/reference/ev_container_chart|EV_CONTAINER]]  inherits <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_BOX]]  may also inherit other structures from [[EiffelBase]] . 

==Usage of 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 along the horizontal axis. 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 is
            -- Add 3 buttons to a hbox and then show in window.
        local 
            a_window: EV_TITLED_WINDOW
            a_hbox: EV_HORIZONTAL_BOX
            a_button: EV_BUTTON
        do
            create a_window 
            create a_hbox 
            create a_button.make_with_text ("Button1")
            a_hbox.extend (a_button)
            a_hbox.extend (create {EV_BUTTON}.make_with_text ("Button2"))
            a_hbox.extend (create {EV_BUTTON}.make_with_text ("Button3"))
            a_window.extend (a_hbox)
            a_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 `extend'.
* To remove a widget from the container call `prune'.
* To empty a container call `wipe_out'.
* To traverse the container, you can use features such as start, forth, item and off.

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

==Size of 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]]  contains the convenient call disable_item_resize. 

==Sizing of containers==
As [[ref:libraries/vision2/reference/ev_container_chart|EV_CONTAINER]]  is of type [[ref:libraries/vision2/reference/ev_widget_chart|EV_WIDGET]] , the following facilities are provided for setting the minimum size: set_minimum_width, set_minimum_height and set_minimum_size. 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. <br/>
}}



