Callbacks

• APIs: using objects through specs
• Callbacks: functions that an object calls
• Registration methods

A Traffic light API

• To use an object, you just need specs of methods: the API
• API for traffic light:
  • red: switch to red light
  • yellow: switch to yellow light
  • green: switch to green light
  • next: switch to next light in sequence
• Can you turn the light to green?
• Can you cycle through the colors on each click of run?

Answer: A Traffic light API

• Easy: just use the green method, and click run
• Then replace all the code by t.next() and click run

Don't call me, I'll call you

• We've seen that functions can be passed as arguments
• Callback or listener: function passed as argument, called later
• Example: account that lets you choose how success is reported

Traffic Light in the Bank

• Let's hook the account up to the traffic light
• Recall that t.green if a function that turns t green
• Can you change the withdrawal amount so that a red light shows?
• Can you make it show yellow for a failed account operation?

Answer: Traffic Light in the Bank

• To show yellow for failed account operation, just pass t.yellow instead

On the Button

• Often want to choose the callback dynamically
• Common idiom: provide a registration method
• Our button object has method onclick(f) to register f for clicks
• Run the code, and click the button to see what happens

Controlling the Lights

• How does this work?

Answer: Controlling the Lights

• The call to onclick binds the function t.next to the button
• Now every time the button is clicked, the traffic light switches to next color

Exercise: Action & Reaction

• Build a reaction tester
• Should go green after random delay
• Then you click button, and it reports your reaction time
• Hint: now() returns current time in ms

Answer: Action & Reaction

• Here's a solution

Some Functions

• Functions for the next exercise

Exercise: Action & Reaction Trace

• This time, compute average over multiple tries
• Complete the code inside the function report
• Hint: use some of map, reduce, zip, filter and pos, plus, max

Solution: Action & Reaction Trace