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EiffelStudio

# Widgets

## What is a Widget?

A Widget is the fundamental building block of your applications GUI, components such as Windows, Buttons and Labels are examples of such. The widget set of Vision2 provides you with the flexibility to easily create powerful graphical applications. All widgets in Vision2 inherit from EV_WIDGET and the features provided in EV_WIDGET may be called on any widget.

## Variations of Widget

Within Vision2, widgets have been classified into three different groups:

- Primitives -- These are elements of the user interface that are mainly responsible for interaction with the user, such as an EV_BUTTON.
- Containers -- These are used to contain other widgets and position them in a certain way, such as an EV_VERTICAL_BOX that stacks its children one by one vertically.
- Dialogs -- These are pop up dialog windows used for interacting with the user for tasks such as opening a file (EV_FILE_OPEN_DIALOG) or displaying a message (EV_MESSAGE_DIALOG). You may construct your own dialogs by inheriting EV_DIALOG.

## How do I create a widget?

All widgets in Vision2 are based around the default_create mechanism in Eiffel. This means that all that needs to be done to create a widget is to declare a reference to a type (such as EV_BUTTON) and then call create on that reference. An example of this is as follows.

    	a_button: EV_BUTTON
    	create a_button

Along with the default creation, Vision2 also includes a few convenience creation functions. Examples of this are make_with_text for all widgets that may have text associated with them (those that inherit from EV_TEXTABLE), this saves a call to set_text upon default creation of the textable widget.

    	create a_button.make_with_text ("Click Me")

This would be in place of

    	create a_button
    	a_button.set_text ("Click Me")

Note: When a widget is created no extra initialization has to be performed. The only exception is for a window where you have to call show in order for it to be displayed on screen.

## Sizing

As EV_PRIMITIVE is of type EV_WIDGET, the following facilities are provided for setting the minimum size: set_minimum_width, set_minimum_height and set_minimum_size.

The minimum size of a widget is the smallest possible size that it can possibly be inside its parent container. If an EV_BUTTON was created and set with a minimum_size of width/height (100, 100), if this button was inserted in to an EV_HORIZONTAL_BOX, then the box's size could never be below (100, 100) or it would break the minimum size of the button. The size of a container must always be greater or equal to the minimum sizes of its children.

Note: In Vision2, there is no way to set the current size of the primitive. The current size is dependent on the size of the parent or the minimum_size.

## Now what do I do?

Now that you can create a widget, you will need to actually make it usable to your intended user. This will usually involve these three steps.

- Setting properties for the widget such as color and the minimum size that a widget can possibly be in its parent container.
- Making the widget respond to user events via the use of agents and action sequences.
- Placing the widget inside a container widget (either a window or a child of a window) so it can be shown on screen.