- Multiple input devices at the same time
- Multiple interaction techniques
- Events: any action from the user or program
- input events: leftdown
- synthetic to control the program: modeDrawingLine
- usually have parameters, such as (X,Y) of event
- Event handlers: procedures to process events
- have an event that starts them and code that is executed.
- Code can compute, interface with the application, generate new events, change internal state, etc.
- Computation is arbitrary, so event languages have full power.
- Distinction: Where events are qualified with the object performed over.
- Cardelli's Squeak (C)
- textual language for communicating with mice
- Flecchia & Bergeron's ALGAE (Pascal) 1987
- Hill's SASSAFRAS & ERL (lisp) 1986
- HyperCard's HyperTalk
- Martin Frank's EET
[ACM Trans. Graphics, July 86]
- list of rules: condition -> action
- conditions: name of an event + list of flags or just a list of flags.
- action: flags to raise | events to send ! assignments <-
- example rules:
1. MouseDown waitingForLine ->
StartLine.X <- MouseDown.X
StartLine.Y <- MouseDown.Y
StartLine!
lineDragging |
2. MouseDown waitingForCircle ->
CircleCenter.X <- MouseDown.X
CircleCenter.Y <- MouseDown.Y
CircleCenter!
circleDragging |
3. MouseUp circleDragging ->
EnterCircle.X <- MouseUp.X
EnterCircle.Y <- MouseUp.Y
EnterCircle!
waitingForCircle |
- Note that CircleCenter! stores the center for EnterCenter
- Another example (unordered inputs):
Command getCmd -> cmd |
Argument getArg -> arg |
- - - cmd arg -> Process!
waitProcessing |
DoneProcessing waitProcessing ->
getCmd |
getArg | - Note that more than one rule may fire
- Define scripts to be executed on events
- Attempts to be "English-like" but is a real programming
language
on mouseUp
global PreviousName
put field "Name" into PreviousName
PurgeName
end mouseUp
on PurgeName
put "???" into field "Name"
end PurgeName - "real" and synthetic events like Sassafras
- Always interpreted, so slow
- Scripts can be associated with buttons, fields (text editing), cards,
backgrounds, stacks, and global. Events given to the most specific
first:
- PhD, 1996, Georgia Tech
- http://www.isi.edu/~frank/
- Define scripts to be executed on events
- "Transitions" happen on events:
Transition PropertiesButton.pressed()
{
PropertiesWindow.mapped := 1;
PropertiesButton.enabled := false;
}
Transition PropertiesCancelButton.pressed()
{
PropertiesWindow.mapped := 0;
PropertiesButton.enabled := true;
} - Can be conditional on values of events:
Transition (*.isSelectable=="true").pressed()
{
self.color := "red";
}- this transition invoked whenever a "press" event occurs on an element which has an "isSelectable" attribute with value equal "true"
- operates on all items of set that it applies to
- "Implicit" events generated for parameter values changing, lists
change size, etc.
- for objects created, deleted or attributes
changed
Transition bb.changed(string attr == "selectedPins", int NewLength != 2)
{
connectButton.enabled := false;
}
Transition bb.changed(string attr == "selectedPins", int NewLength == 2)
{
connectButton.enabled := true;
} - button is enabled whenever the list in the bb element's selectedPins attribute is have length = 2
- like "active variables"
- can implement a form of constraints
- for objects created, deleted or attributes
changed
- Programmer can "throw" events explicitly
Transition ((*.isAButton=="true") &&
(*.status =="enabled")).released()
{
throw self.invoke();
} - + Advantage over ERL: events on objects, like HyperTalk
- - Difficult to figure out meaning of the transitions (when they will fire)
- - Flow of control hard to program and read