Introduction to HTML5 Canvas - HTML5 Canvas - page 17

HTML5 Canvas “Hello World!”
As we just mentioned, one of the first things we need to do when putting Canvas on an
HTML5 page is test to see whether the entire page has loaded and all HTML elements are
present before we start performing any operations. This will become essential when we start
working with images and sounds in Canvas.
Todothis,youneedtoworkwith events inJavaScript.Eventsaredispatchedbyobjectswhen
a defined event occurs. Other objects listen for events so that they can do something based on
the event. Some common events that an object in JavaScript might listen for are keystrokes,
mouse movements, and when something has finished loading.

The first event we need to listen for is a window object's load event, which occurs when the
HTML page has finished loading.

To add a listener for an event, use the addEventListener() method that belongs to objects
thatarepartoftheDOM.Because window representstheHTMLpage,itisthetoplevelofthe
DOM.

The addEventListener() function accepts three arguments:

Event: load load

Thisisthenamedeventforwhichweareaddingalistener.Eventsforexistingobjectslike
window are already defined.

Event handler function: eventWindowLoaded() eventWindowLoaded()

Call this function when the event occurs. In our code, we will then call the canvasApp()
function, which will start our main application execution.

useCapture useCapture : true true or false false

This sets the function to capture this type of event before it propagates lower in the DOM
tree of objects. We will always set this to false .

The final code we will use to test to see whether the window has loaded is as follows:

window . addEventListener ( "load" , eventWindowLoaded , false false );

function function eventWindowLoaded () {

canvasApp ();

}

Alternatively, you can set up an event listener for the load event in a number of other ways: