Learning Processing: A Beginner's Guide to Programming Images ...

342 Learning Processing
without pausing your Processing sketch, allowing the sketch to multitask and continue animating while
the data retrieval is in process.
Th e library functions in a similar manner to the Processing video library, which we became familiar with in
Chapter 16. To retrieve a web page, you must create an instance of an HTMLRequest object, passing in a
reference to the the sketch itself ( this ) and the URL you want to request.
HTMLRequest req = new HTMLRequest(this, " http://www.yahoo.com " );
Th e request will not begin, however, until you call makeRequest( ).
req.makeRequest();
Finally, to receive the data, you must write a function called netEvent( ) . Th is function will be invoked the
instant the request is fi nished and data is available.
Th is function is another example of a callback function, the same as captureEvent( ) (see Chapter 16) or
mousePressed( ) . In this case, instead of an event being triggered when the user clicks the mouse or an
image from the camera is available, the event is triggered when the HTML request fi nishes (or XML as
we will see in a moment).
Th e HTML source of the web page is returned as a String via the function readRawSource( ) .
void netEvent(HTMLRequest ml) {
String html = ml.readRawSource();
println(html);
}
Example 18-7 retrieves Yahoo’s homepage every ten seconds using the simpleML library. Th e
visualization here is arbitrary (lines are colored according to the characters in Yahoo’s HTML source);
however, what is important is to notice how the animation never pauses while waiting for the data to
arrive .
Example 18-7: Loading a URL with simpleML
import simpleML.*;
// A Request object, from the library
HTMLRequest htmlRequest;
Timer timer = new Timer(5000);
String html = " " ; // String to hold data from request
int counter = 0; // Counter to animate rectangle across window
int back = 255; // Background brightness
void setup() {
size(200,200);