NeWS started by implementing a PostScript interpreter that runs in a
cooperative multitasking fashion, since, unlike PostScript in a printer, NeWS would be displaying a number of PostScript programs at the same time on one screen. It also added a complete view hierarchy system, based on viewports known as canvases. Like the view system in most GUIs, NeWS included the concept of a tree of embedded views along which events were passed. For instance, a mouse click would generate an event that would be passed to the object directly under the mouse pointer, say a button. If this object did not respond to the event, the object "under" the button would then receive the message, and so on. NeWS included a complete model for these events, including timers and other automatic events, input queues for devices such as
mice and
keyboards, and other functionality required for full interaction.
To support event-based programming, NeWS expanded the original PostScript
stack-based language into a complete
object oriented (OO) programming style with
inheritance. This eliminated the need for an external OO language to build a complete application.
Since all of these additions were implemented as extensions to PostScript, it was possible to write simple PostScript code that would result in a running, onscreen, interactive program. Two popular demonstration programs were an onscreen clock, which required about two pages of code, and a program which drew a pair of eyes that followed the cursor as it moved around the screen. The eyeball program was shown at SIGGRAPH in 1988, and was the inspiration for the later well-known X application
xeyes.
NeWS included several libraries of user interface elements (
widgets), themselves written in NeWS. These widgets ran all of their behaviour in the NeWS interpreter, and only required communications to an outside program (or more NeWS code) when the widget demanded it. For example, a toggle button's display routine can query the button's state (pressed or not) and change its display accordingly. The button's PostScript code can also react to mouse clicks by changing its state from "pressed" to "not pressed" and vice versa. All this can happen in the windowing server without interaction with the client program, and only when the mouse is released on the button will an event be sent off for handling.
This was more sophisticated than the
X Window System server model, which can only report "mouse was pushed down here", "mouse is now here", "mouse was released here" events to a client, which then has to figure out if the event is in the button, switch the state, and finally instruct the server to display the new state. If client and server are not on the same machine, these interactions must travel over the network, which results in
a delay in responding.
The best example of such a library is TNT (The NeWS Toolkit) which was released by Sun in 1989. Sun also shipped a smaller toolkit intended for example purposes and making small programs.
