# class `ViewBase`

ViewBase is an abstract class providing common functionality for user-defined views. It provides facilities for associating the view with one or more Models, and some default behaviour for responding to changes in a model.
ViewBase is not intended to be used directly as a base class. Usable subclasses of ViewBase include View and GLView.

## Properties

> `models`
>   List of models being observed. Do not modify this list directly. Use the `add_model` and `remove_model` methods to attach and detach models.
> `model`
>   Convenience property for when only one model is being observed. Reading this property returns `models[0]` when models is not empty, otherwise it returns `None`. Assigning a model to this property removes all other models and adds it as the sole model. Assigning `None` to this property removes all models.
> cursor
>   The Cursor to display when the mouse is over the view, or None to use the default cursor.
>   To prevent any cursor from being shown over this view, set the cursor property to StdCursors.invisible.

## Abstract methods

> `model_changed()`
>   Called when an attached model's `notify_view` method is called without specifying a message name. The default action is to call the view's `invalidate` method.
> `model_destroyed(`model`)`
>   Called when an attached model is destroyed. The default action is to destroy the window containing this view, if any.
>   Note: The model parameter should only be used to identify which model is being destroyed, if the view is observing more than one model. Do not rely on the state of the model object that is being destroyed.
> `targeted()`
>   Called whenever this view becomes the current message target in a visible window.
> untargeted()
>   Called whenever this view ceases to be the current message target in a visible window.

## Methods

> `add_model(`model`)`
>   Attaches a model to this view, so that the view will be notified of changes to the model. This is an alternative to calling the model's `add_view` method.
> `remove_model``(`model`)`
>   Detaches a model from this view. This is an alternative to calling the model's `remove_view` method.

> `track_mouse()`
>   Called following the receipt of a mouse-down event, returns an iterator which yields a series of mouse-drag events as long as the mouse button is held down, followed by a mouse-up event. The positions of all events are reported in the view's local coordinate system. While the iterator is active, all mouse movement events are reported, whether they occur within the view's bounds or not, and any other user input events are ignored.