DisplayHandles - representing components.

The fundamental Haggis type is that of user interface components, called DisplayHandle. A value of type DisplayHandle represent a handle to an as-yet unrealised component.

The explicit handle representation of components is used to build and construct larger components by composing and encapsulating existing ones together, e.g.,

hbox :: [DisplayHandle] -> DisplayHandle

hbox returns a new DisplayHandle, constructed by aligning a list of DisplayHandles horisontally.

The properties of a user interface component / DisplayHandle:

• It has a current size, that can be set and queried. This size defines the bounding box of the component.
• Geometric requirements recording the natural size and how willing it is to deviate from that size.
• A clipping region that can be set. The clipping region control what part of the component's bounding box is visible.
• A local-to-global Transform that maps coordinates within the component's bounding box to the place in the window where it can be set. Can currently only be set.

When realised, a DisplayHandle is receptive to system commands such as resize and redisplay, and the notification of user input events.

To start the composition of DisplayHandles off, the Haggis core provide the following basic DisplayHandle:

nullDH :: DisplayHandle

representing the component with no appearance nor size.