Event Models

1
Event Models

• James Landay, UCB

2
Outline

• Event overview
• Windowing systems
• Window events
• Event dispatching and handling

3
Sequential Programs

• Program takes control, prompts for input
• Examples include
• command-line prompts (DOS, UNIX)
• LISP interpreter
• The user waits on the program
• Program tells user its ready for more input
• User enters more input

4
Sequential Programs (cont.)

• Architecture
• Program reads in a line of text
• Program parses the text
• Program evaluates the result
• Maybe some output
• Loop back to beginning
• But how do you model the many actions a user can
  take?
• For example, a word processor?
• Need to do printing, editing, inserting, etc.

5
Sequential Programs (cont.)

• Usually end up with lots of modes
• Lots of state variables
• Other examples of modes
• Paint programs (line, bucket-fill, rectangle,
  etc)
• Universal remotes with TV / VCR mode
• VI edit mode and command mode
• Problems with modes?

6
Sequential Programs (cont.)

• Problems with modes?
• Gets confusing if too many modes
• Can be easy to make errors
• Need feedback as to what mode you are in
• How to switch between modes?
• Well need a more advanced model to simplify
  windows programming

7
Event-Driven Programming

• Instead of the user waiting on program, have the
  program wait on the user
• All communication from user to computer is done
  via events
• An event is something interesting that happens
  in the system
• Mouse button goes down
• Item is being dragged
• Keyboard button was hit

8
Event Example
close box
title bar
folder
scroll bar
size control
9
Major Issues

• How to decompose the UI into interactive objects?
• How to distribute inputs to the interactive
  objects
• How to partition between application system
  software?
• Models for programming interactive objects
• Models for communications between objects

10
Windowing Systems

• Partitioning to prevent chaos
• Infrastructure to support common services
• Two major aspects
• software services to applications
• create and organize windows
• implement interaction in those windows
• window manager
• UI allowing user to control size placement of
  windows

11
Interactor Tree

• Decompose interactive objects into a tree
• interactive objects also known as widgets
• based on screen geometry of objects
• nested rectangles
• Used for dispatching events
• Events are dispatched (sent) to code in widget
• The code then handles the event
• Variety of methods for dispatching events
• Return to this later

12
Interactor Tree
Display Screen
F\cs160\Public window Inner Window
title bar horizontal scroll bar contents
area CDJukebox folder Home Ent
folder size control Web Newspaper
window
13
Interactor Tree
Display Screen
Outer Win black
?????
14
Interactor Tree
Display Screen
Outer Win black
Inner Win green
Result Win tan
Result String
Keypad Teal
button
- button
button
0 button
15
Interactor Tree (Java)
Display Screen
Frame black
Panel green
Text Entry tan
Result String
Keypad Panel Teal
Button()
Button(-)
Button()
Button(0)
16
Windows

• Top level windows known as root windows
• provide UI abstraction for multiple apps
• windowing system arbitrates interactive resources
• Each root window belongs to an app.
• all descendant windows belong to same app
• violated by OLE (ActiveX) and OpenDoc (dead?)

17
Windows (cont.)

• Windows vs. widgets/controls
• X, NeXTStep, MS Windows
• everything is window
• Mac only roots are windows - controls manage
  rect. space in a window (Motif gadgets similar)

18
Networked Windowing Systems

• X Window NeWS designed to allow apps to run on
  remote machines
• Uses client-server model

19
X Window

• Note backwards terminology
• User is on server not client
• X Server
• interprets X commands and can send events
• determines which window receives events and
  forwards over network to proper client
• X Client
• software interface to X (Xlib)
• assembles the output from Xlib routines into
  packets for transmission to server

20
X Window

• Interaction Problems?

Network
X Server std system software
Client app software
User
Network Bandwidth is bits per second Network
Latency is time to transfer and process data.
Relation to Model Human Processor?
21
Network Round Trips (NRT)

• Every mouse move on thumb involves NRT
• Solutions?
• download code that knows how to scroll
• NeWS used display PostScript to do this

22
Window Events

• User interacts with input device
• action translated into software events
• must distribute events to appropriate window
• doesnt need IPC, use method/procedure call
• Events have
• type
• mouse position or character key
• the window the event is directed to

23
Input Events

• Mouse button events
• mouse up and down
• modifier (shift keys, etc.)
• double click (X doesnt have this -gt fakes it)
• Mouse movement events
• implement painting with mouse
• mouse drag
• can mask off mouse moves w/o button down
• Mouse enter and exit events
• e.g. if you entered / exited a button region

24
Implementing Buttons
Button
mouse enter
mouse exit
Button
(But using mouse move events would be overkill)
25
Events (cont.)

• Keyboard events
• must translate raw scan codes into ASCII
• Windowing events on window
• creation / destruction
• opening / closing
• iconifying / deiconifying
• selection / deselection
• resize
• redraw
• redraw newly exposed portions of the window
  (rect.)

26
Main Event Loop

• Main event loop
• Initialization
• While (not time to quit)
• Get next event E
• Dispatch event E
• The meat of the program is in the code that
  handles the dispatch

27
Event Dispatch
Dispatch (event E) switch (E.window)
... case FIVE-KEY if (E.type
left-down) cur 5 10cur display
(cur) last-op NUMBER ...
Hit the 5 key
28
Event Dispatch
Dispatch (event E) switch (E.window)
... case TWO-KEY if (E.type left-down)
cur 2 10cur display (cur)
last-op NUMBER ...
Hit the 2 key
29
Event Dispatch
Dispatch (event E) switch (E.window)
... case ENTER-KEY if (E.type
left-down) push (cur) cur 0
last-op COM ...
Hit the enter key
30
Event Dispatch
Dispatch (event E) switch (E.window)
... case SIX-KEY if (E.type left-down)
cur 6 10cur display (cur)
last-op NUMBER ...
Hit the 6 key
31
Event Dispatch
Dispatch (event E) switch (E.window)
... case PLUS-WIN if (E.type
left-down) if (last-op NUMBER) push
(cur) result pop() pop() push
(result) display (result) cur
0 last-op COM
Hit the key
32
Event Dispatch
J
33
Event Queues

• Input events are placed in a queue
• Ensures events are processed in order
• Main event loop removes them from the queue
  (get_next_event) dispatches for processing

Mouse move (22, 33) Mouse move (40, 30) Mouse
down left (45, 34) Mouse up left (46, 35)
34
Event Queues (cont.)

• Can use event masks to filter unwanted events
• e.g., filter mouse moves in a forms-based program
• just get enter/exit events

35
Object-Oriented Event Handling

• Older methods prone to programmer error
• OO languages more naturally handle passing
  messages between independent objects
• Basis for NeXTStep, Mac App, Visual C, Java

36
Object-Oriented Event Loop

• Tool kit defines an application class
• provides a run method which contains event loop
• technique used by Visual C and MacApp

Application myApp Intialize windows
application data structures Set any special
event masks by sending messages to
myApp myApp.Run()
37
Dispatching Events

• If user scrolls the text, the software must
• direct the mouse events to the scroll bar
• update the scroll bar display during the drag
• notify the text editing window it needs to scroll
  itself so that the text appears to have moved

38
Dispatching Events (cont.)

• Algorithm selects the bottom-most, front-most
  region in the interactor tree
• scroll bar or contents over outerwin
  (bottom-most)
• scroll bar over contents (front-most)
• each window need only consider its own events
• difficult to impose a high level of control
• known as bottom-first event dispatch
• Top-down event dispatch
• events passed to top-most, front-most window
• it dispatches to one or more of its children...

39
Event Focus

• Where should keyboard events go?
• mouse-based
• attach mouse position to all key events and
  dispatch events in the same way as mouse events
• click-to-type (Mac)
• send all key events to last window where mouse
  down occurred
• key focus
• windows take and give away keyboard focus
• Mouse focus
• long narrow scrollbar...

40
Simple Event Handling

• Event tables (in the early days)
• indexed by event types (integer from 0 - 255)
• holds pointers to functions that handle each
  event
• one table per / window
• lots of things to maintain when attached to a
  widget that you want to make reusable
• Callbacks
• separate things like labels/colors into resources
  read from files
• each kind of widget defines a set of named
  callbacks which it will invoke

41
Callback Example

• How do we notify text window to scroll when the
  scroll bar is moved?
• create a vertical scroll bar widget
• write a callback procedure which has code to
  notify text windows of their new position
• register callback with scroll bar as callback to
  invoke when the scroll bar is moved
• also register a pointer to the text window as the
  callback data -gt knows which window to scroll

42
Simple Event Handling (cont.)

• WindowProc style (MS Windows)
• newer and better than older models
• define window classes, each of which have a
  WindowProc (similar to callback)
• whenever event dispatch algorithm identifies a
  window that should receive an event, that
  windows WindowProc is invoked
• body of WindowProc is a switch on the event type
  with a handler for each event
• 100s of events, but most is inherited/delegated

43
Summary

• Windowing systems
• special problems with networked WS
• Interactor trees
• Input events
• Main event loop
• Dispatching events
• Event focus
• Simple event handling