CS320n – Elements of Visual Programming

CS320n – Elements of Visual Programming

Sending Parameters to Event Handler Methods

(Slides 5-2)Thanks to Wanda Dann, Steve Cooper, and Susan Rodger for slide ideas.

Visual Programming Sending Parameters to Event Handler Methods

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What We Will Do Today• Learn how to use parameters in methods

that respond to events. (Event handler methods)

• Do a programming example in the lab


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Mouse clicks• Interactive programs often allow the user

to mouse click an object in the display.– buttons in a windows-based interface– targets in a game– checklist of items on a form

• In this session, we look at how to pass information about a mouse clicked object to an event handler method.


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Example• People are trapped in

a burning building • The user selects

which person will be rescued next.


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Storyboard• Three people are to be rescued. So, we could

write three different methods.

Event 1: click on guy1

Responding Method: Save guy on first floor

Event 1: click on girl2

Responding Method: Save guy on second floor

Event 1: click on girl3

Responding Method: Save guy on third floor


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A Better Solution• Write one event handler method • send in the information needed to perform

the action.firetruck.savePerson:

parameters: whichFloor, whichPerson, howFarDo in order point ladder at whichFloor extend the ladder howFar meters whichPerson slide down the ladder to the fire truck pull the ladder back howFar meters

whichFloor, whichPerson and howFar parameters

What should their types be?


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Demo• Demonstration of the

code for firetruck.savePerson


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Three events• The argument sent to the parameters depends on which

person is mouse clicked.

• Note: the fire truck is positioned so distance between floor X is X meters (distance to floor 3 is 3 meters)


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Example 2

• Zeus is a powerful god in Greek mythology.

• When Zeus is angry, he shoots a thunderbolt out of the heavens to strike anyone is bothering him

• The user chooses the philosopher who will be the next target of Zeus’s anger.


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Storyboard• A possible design is a method with an Object parameter,

named who, for the object that was clicked.

– The actions in this storyboard are complex. – We can break the actions down into simpler steps

using stepwise refinement.

Event: an object is mouse-clicked

Event handler: shootBolt

Parameter: who – object clickedDo in order

prepare to strike object that was clickedthunder plays and lightning strikes object clickedlighting is repositioned for next strike


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Event: An object is mouse-clicked

Event handler: shootBolt

Parameter: who — the object that was clicked

Do in ordercall prepareToShoot method — send who as the targetcall lightningAndThunder method — send who as the targetlightning move to cloud’s position

prepareToShoot:

Parameter: targetDo together turn Zeus to face the target make the lightning bolt visible

lightningAndThunder:

Parameter: target Do together play sound call specialEffects method — send target


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specialEffects:

Parameter: target

Do in order Do together lightning bolt move to target smoke move to target Do together set smoke to visible set lightning to invisible call smoke cycle — built-in method set target color to black move target up and down


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A driver• The shootBolt method is at the top level of our design. • It calls other methods (prepareToShoot and

lightningAndThunder) and controls the overall action of the program – we call this a driver.


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One link

• In the fire rescue example, we used three links – one for each person in the burning building.

• In this example, we use only one link by selecting “object under mouse cursor” as the argument.


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prepareToShoot• In setting up the initial scene, we made the lightning bolt

invisible by setting its opacity to 0 (0%). (Review Tips & Techniques 4 for more details about opacity.)

• To prepare to shoot the lightning bolt, it must be made visible – set the opacity back to 1 (100%).


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lightningAndThunder• Coordinate the sound of thunder with lightning

and other special effects.


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specialEffects• The smoke.cycleSmoke is a built-in instruction with a

duration of about 2 1/2 seconds.


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move to• Several statements in the shootBolt and specialEffects

methods use a move to instruction

• The move to instruction moves an object to a particular position in the world.

• In the example above, where is the lighting bolt moved to? (The move to instruction is described in detail in Tips &

Techniques 2.)


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move to with an object parameter

• If an object parameter is used to specify a target position, the process of creating a move to statement involves two steps:

1) Drag in an arbitrary object as the parameter

2) Substitute the object parameter name

This is necessary because an Object parameter is only a placeholder

for an object – not an actual object.


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Demo• Test run of Zeus world.• When parameters are used in interactive

programming, it is especially important to test that all possible parameter values work as expected.– What happens if you click on each philosopher, one

at a time?• Also try things that shouldn’t work.

– What happens if you click on a column?– What happens if you click on a philosopher twice?– What happens if you click on Zeus?


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Classwork Monday in Lab• Create 2 worlds (or combine into 1 world)

– Problem 14, page 140. Penguin slide: Create a world with a lake (environments) and at least 3 penguins on the slope. Make the program event driven. When a penguin is clicked it slides on its belly and spins as it slides. Each penguin should spin some number of times. When the penguin reaches the pond make it disappear into the pool. Write only one event handler method.


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Penguin Slide


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Exercise 2• Problem 15, Page 141.• Create an interactive hockey game.

Create a scene with a person on a lake. Set up a hockey net and give the person a hockey stick (both in the sports folder). Create 3 buttons (bumps from the shape folder) that determine how hard the player swings the stick and how fast the puck (another bump)travels towards the net


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Hockey Game

CS320n – Elements of Visual Programming

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