Using Timers

Using Timers

Timers are a key part of creating animations. As such, they are often used to create fun and interesting graphical programs. Let's take a look at one such program, called "Random Circles."

/**
 * The Random Circles Program
 */

var MAX_RADIUS = 100;
var MAX_CIRCLES = 1000;
var counter = 0;

function start(){
    setTimer(draw, 50);
}

function draw(){
    drawCircle(Randomizer.nextInt(0, MAX_RADIUS),
               Randomizer.nextColor(),
               Randomizer.nextInt(0, getWidth()),
               Randomizer.nextInt(0, getHeight()));

    counter++;

    if(counter == MAX_CIRCLES){
        stopTimer(draw);
    }
}

function drawCircle(radius, color, x, y){
    var circle = new Circle(radius);
    circle.setColor(color);
    circle.setPosition(x, y);
    add(circle);
}

Using Timers

Let's start by examining the timer. The timer is created at the very beginning of the program in the start function.

Stopping a Timer

Timers will continue to run until they are given the command to stop. A timer can be stopped by issuing the stopTimer(functionName) command. The functionName should be the name of the function that was originally passed to the timer when creating it with startTimer().

For example, Karel could create a timer that called a giveTreat function every 1000 milliseconds:

startTimer(giveTreat, 1000);

Karel could then stop the timer by issuing the following command:

stopTimer(giveTreat);

Using Multiple Functions with a Timer

Programs often need to call multiple functions every tick of the timer. However, the call to startTimer can only take one single function. This code would not work for Karel:

startTimer(giveTreat, giveToy, takeKarelForWalk, 1000);

Instead of passing multiple functions when calling startTimer, Karel will need to create one function that then makes calls to the other functions:

startTimer(makeKarelHappy, 1000);

...

function makeKarelHappy(){
    giveTreat();
    giveToy();
    takeKarelForWalk();
}

With this code, the makeKarelHappy function will be called every 1000 milliseconds. That function then calls each of Karel's other functions. Notice that in the Random Circles program, the timer calls the draw function, which then calls drawCircle.