CS349 - Implementing User Interfaces - Spring 2010

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Lecture 2 - Graphics Abstractions for User Interfaces

Hardware

An operating system provides, among other things, an abstraction of the hardware in the form of

• an API (application programming interface), which is
• a set of function calls that exercise the hardware, as if the abstraction is a complete description of the hardware.

When there is a user interface, this includes the hardware that provides the interface

• output
  • visual display for graphics and text
  • speakers for audio output
  • force-generating devices for tactile output
  • mechanical devices for proprioceptive output
  • etc
• input
  • keyboard, which is shared between different interfaces in many systems that provide a programming affordance,
  • mouse for pointing,
  • buttons,
    
    • including mouse buttons,

    which are just a specialized keyboard,

  • touch screen, which is just a specialized pointing device.

The base window system (BWS) provides the lowest level abstraction, which consists of

1. a display screen divided into a rectangular array of pixels, which includes
   • a coordinate system for referring to individual pixels
   • a colour model to colour each pixel
   • API

     writepixel( x, y, colour )

2. an audio device, which includes
   • a pipe into which sampled audio is poured

     writeaudio( sound-buffer )

3. a collection of button devices
   • each is either pressed or not pressed
   • API

     down = button-pressed( )

4. a collection of pointing devices, each of which includes
   • a coordinate system
   • a location in the coordinate system
   • API

     x, y = where-is( )

The input devices, the last two, have an interesting feature

• time is taken for granted
• using the OS abstraction of time
  • time = get-time-of-day( )
  sequences of samples can be organized into trajectories

And one problematic feature

• It is hard to write application software without polling.

Events

To avoid the necessity for polling

• sometimes polling is desirable

most BWSs abstract input into a time-ordered sequence of events, usually called the event queue.

Data structure

typedef struct{
     int time;
     Type type;
     Whatever parameters;
} Event;

API

evt = get-next-event( );

• This is blocking, like getch( )

The Event Loop

Basic code structure

evt = get-next-event( ) {
    switch( evt.type ) {
    case KEYSTROKE:
        if ( evt.parameters.byte == 'q' ) exit( 0 );
    ...
}

This is made for an object-oriented programming style, which is why we like Java.

X

X is a BWS based on a network. That is,

• The interface hardware may be connected to the application over the internet.
• Thus, X is a protocol for transmitting I/O over a network
  • which means, between two well-separated components
    • the BWS, and
    • the application program.
• On the BWS side
  • the X-server creates and encodes events that are sent to the application;
  • the X-server receives encoded drawing commands and draws on the display.
• On the application side
  • the X-client uses an API that provides events and accepts drawing commands
  • Xlib provides the implementation of the API, maintaining local state, and transmitting/receiving X-protocol messages with the X-server.

Many properties of X are the side effects of reducing communication bandwidth between the X-server and its clients.

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