Exploring Events
Try this
• Start Alice and create a blank world using
the grass template.
• Add an instance of a BlueBallerina.
• Add an instance of a PinkBallerina.
• Move the ballerinas apart from each other
and turn them to face the camera.
• Exit Scene Editor mode.
Continuing…
• Create a world-level method called
bothSpin to make both ballerinas spin at
once.
• You’ll need a do together tile
• Turn each ballerina left 1 revolution
Setting up a recognized event
• Drag the tile for bothSpin to the event
area and replace world.my first method
in the When the world starts tile.
• We no longer need world.my first method;
throw it away.
• Play the world and you should see both
ballerinas spin together.
• Save your world; name it bothBallerinas1
Now let’s experiment with a
few more events
• Save your world as blueBallerinas2
• Click the create new event button and then
click When the world starts.
• Change the form of the newly added event.
– Right-click the newly added event tile.
– Select change to then click While the world is
running.
Events interacting
• Play the world
• Note two behaviors:
– First, the pink ballerina continues to spin while
the world is running.
– Second, when the world starts, she spins
more quickly.
• Play the world a couple of times until you
see this.
Events interacting, cont’d
• Why does she spin more quickly when the world
starts?
• This is a side effect of two event handlers
running at the same time.
• The default event handler causes pinkBallerina
to spin one time at one revolution per second.
• Combined with the spin from the new event (also
one rev./sec.) , she spins twice as fast for one
revolution.
Side effects
• Change the direction of the spin to the
right in the while the world is running
event tile.
• Play the world
Side effects
• Change the direction of the spin to the
right in the while the world is running
event tile.
• Play the world
• Now what happens?
Why?
Side effects
• pinkBallerina does not spin at all this time
• The two method calls triggered by the two
events cancel out each other.
• After the first method stops, she spins to
her right.
• Two events or methods that overlap might
sometimes cause unintended results
known as side effects.
The BDE event
• Begin, during, and end event
• Makes use of up to three event handlers
Begin
• When the trigger condition first becomes
true call the Begin method once.
During
• While the trigger condition continues to
hold, continue to call the During method
End
• When the trigger condition becomes no
longer true, call the End method.
When a key is typed event
• The event trigger is the press of a key
• The event handler can be almost anything
that can be coded in Alice.
When a key is typed event
• Add a “when a key is typed” event to your
ballerinas world.
• Click on “any key” in the event tile
• Select letters from the menu and then B
• Drag a blueBallerina turn method tile from
the Details area and drop in on “Nothing”.
Select right and 1 revolution.
When a key is typed event
• Play the world.
• Try the b key a few times.
• Note that if you hold b down blueBallerina
will not continue to spin.
• When a key is typed will call the event
handler only once per key press.
While a key is pressed event
• You can change the form of the When a
key is typed event to make an event
handler run continuously as long as the
triggering key is held down.
While a key is pressed event
• Right-click the When B is pressed tile’s blue
background.
• Select change to then While a key is
pressed.
• Drag and drop a blueBallerina turn tile into
place after “During” (right, 1 revolution).
While a key is pressed event
• Play the world.
• Note that blueBallerina continues to turn
while the b key is held down.
• She stops when the key is released, even if
in mid-turn.
When the mouse is clicked on
anything event
• In your ballerinas world, click the create a new
event button and select when the mouse is
clicked on something.
• Let’s make blueBallerina roll one revolution
whenever the mouse is clicked on her.
• Select blueBallerina and the entire
blueBallerina as the target object.
• Drag the blueBallerina roll tile from the
methods tab and drop it to replace “Nothing” in
the event tile (right, 1 rev.).
More mouse events
• Changing When the mouse is clicked on
something to While the mouse is
pressed on something by right clicking
on the event tile.
• Let the mouse move <object> is a little
beyond us at the moment. We need to
know about Lists first.
Using events for something
more practical
Building a flying machine
• Remember the program development
cycle: design, code, test, debug?
Initial specifications
Initial specifications for our world:
1. It should contain a flying machine in a
somewhat realistic environment.
2. The flying machine should be able to fly
throughout the world.
3. There should be user controls to turn the
object up, down, left, and right while the
flying machine is in motion
4. The user should be able to find the flying
machine if it moves off camera.
Refining specifications
• Beginning with #1: It should contain a
flying machine in a somewhat realistic
environment.
• First, let’s find objects that can serve as a
flying machine.
• How about a seaplane with a water
template?
• We also need some realism.
Revised specification #1
1. Create a water world with a seaplane in it
1. Select the water template.
2. Add a seaplane to the world.
3. Add a few more items – perhaps an island
or two, and a sailboat.
Refining specification #2
2. The flying machine should be able to fly
throughout the world.
– This means it needs to be able to move
along the x, y, and z axes
– Let’s not worry about moving backward.
– Planes don’t move backward while in the air.
– We need to be able to turn left, right, up,
down
– We also need to be able to go forward.
Refining specification #2
2. The flying machine should be able to fly
throughout the world.
– How about if we just make the plane move
forward continuously so that the user simply
needs to drive it?
– Let’s make the following refinement:
– Create an event – while the world is running
do: seaplane move forward one meter
Refining specification #2
2. Create an event – while the world is running
do: seaplane move forward one meter
– The specifications don’t for the user to be able
to modify the seaplane’s speed, let’s assume
the speed to be constant.
Revised specifications
1. Create a water world with a seaplane in it
1. Select the water template.
2. Add a seaplane to the world.
3. Add a few more items – perhaps an island
or two, and a sailboat.
2. Create an event – while the world is
running do: seaplane move forward one
meter
Refining specification #3
3. There should be controls to turn the object
up, down, left, and right while the flying
machine is in motion.
– Left and right are easy
– Is turning up, turning the object backward or
forward?
– We also need to decide by how much to turn
when a turn is requested.
Refining specification #3
3. Create four turn control events:
– When the left arrow key is pressed, turn left 1/8
revolution
– When the right arrow key is pressed, turn right
1/8 revolution
– When the up arrow key is pressed, turn
backward 1/8 revolution.
– When the down arrow key is pressed, turn
forward 1/8 revolution.
Revised specifications
1.
Create a water world with a seaplane in it
1.
2.
3.
2.
3.
Select the water template.
Add a seaplane to the world.
Add a few more items – perhaps an island or two, and a
sailboat.
Create an event – while the world is running do:
seaplane move forward one meter
Create four turn control events:
1.
2.
3.
4.
When the left arrow key is pressed, turn left 1/8 revolution
When the right arrow key is pressed, turn right 1/8 revolution
When the up arrow key is pressed, turn backward 1/8
revolution.
When the down arrow key is pressed, turn forward 1/8
revolution.
Revising specification #4
4. The user should be able to find the flying
machine if it moves off camera.
– Several ways to do this.
– A simple way is to point the camera at the
seaplane.
– Should be easy to do; let’s use the
spacebar.
Revising specification #4
4. Create an event: When the spacebar is
pressed, point the camera at the seaplane.
Revised specifications
1.
Create a water world with a seaplane in it
1.
2.
3.
2.
3.
Create an event – while the world is running do: seaplane move
forward one meter
Create four turn control events:
1.
2.
3.
4.
4.
Select the water template.
Add a seaplane to the world.
Add a few more items – perhaps an island or two, and a sailboat.
When the left arrow key is pressed, turn left 1/8 revolution
When the right arrow key is pressed, turn right 1/8 revolution
When the up arrow key is pressed, turn backward 1/8 revolution.
When the down arrow key is pressed, turn forward 1/8 revolution.
Create an event: When the spacebar is pressed, point the camera
at the seaplane.
Let’s code this specification. Test and debug after each step