Web Programming Step by Step

Lecture 16
Events

Reading: 9.1 - 9.2

Except where otherwise noted, the contents of this presentation are Copyright 2010 Marty Stepp and Jessica Miller.

JavaScript events

• the click event (onclick) is just one of many events that can be handled
• problem: events are tricky and have incompatibilities across browsers
  • reasons: fuzzy W3C event specs; IE disobeying web standards; etc.
• solution: Prototype includes many event-related features and fixes

Attaching event handlers the Prototype way

element.onevent = function;
element.observe("event", "function");

// call the playNewGame function when the Play button is clicked
$("play").observe("click", playNewGame);

• to use Prototype's event features, you must attach the handler using the DOM element object's observe method (added by Prototype)
• pass the event name as a string, and the function name to call
• handlers must be attached this way for Prototype's event features to work

• observe substitutes for addEventListener (not supported by IE)

Attaching multiple event handlers with $$

// listen to clicks on all buttons with class "control" that
// are directly inside the section with ID "game"
window.onload = function() {
	var gameButtons = $$("#game > button.control");
	for (var i = 0; i < gameButtons.length; i++) {
		gameButtons[i].observe("click", gameButtonClick);
	}
};

function gameButtonClick() { ... }

• you can use $$ and other DOM walking methods to unobtrusively attach event handlers to a group of related elements in your window.onload code

The event object

function name(event) {
	// an event handler function ...
}

• Event handlers can accept an optional parameter to represent the event that is occurring. Event objects have the following properties / methods:

• *   replaces non-standard srcElement and which properties
• ** replaces non-standard return false;, stopPropagation, etc.

Mouse events (9.2.2)

Mouse event objects

The event passed to a mouse handler has these properties:

• *   replaces non-standard properties pageX and pageY
• ** replaces non-standard properties button and which

Mouse event example

<pre id="target">Move the mouse over me!</pre>

window.onload = function() {
	$("target").observe("mousemove", showCoords);
};

function showCoords(event) {
	$("target").innerHTML = 
		  "pointer: (" + event.pointerX() + ", " + event.pointerY() + ")\n"
		+ "screen : (" + event.screenX + ", " + event.screenY + ")\n"
		+ "client : (" + event.clientX + ", " + event.clientY + ")";
}

Move the mouse over me!

The keyword this (8.1.3)

this.fieldName                  // access field
this.fieldName = value;          // modify field

this.methodName(parameters);    // call method

• all JavaScript code actually runs inside of an object
• by default, code runs in the global window object (so this === window)
  • all global variables and functions you declare become part of window
• the this keyword refers to the current object

Event handler binding

window.onload = function() {
	$("textbox").observe("mouseout", booyah);   // bound to text box here
	$("submit").observe("click", booyah);       // bound to submit button here
};

function booyah() {           // booyah knows what object it was called on
	this.value = "booyah";
}

• event handlers attached unobtrusively are bound to the element
• inside the handler, that element becomes this (rather than the window)

Fixing redundant code with this

<fieldset>
	<label><input type="radio" name="ducks" value="Huey"  /> Huey</label>
	<label><input type="radio" name="ducks" value="Dewey" /> Dewey</label>
	<label><input type="radio" name="ducks" value="Louie" /> Louie</label>
</fieldset>

function processDucks() {
	if ($("huey").checked) {
		alert("Huey is checked!");
	} else if ($("dewey").checked) {
		alert("Dewey is checked!");
	} else {
		alert("Louie is checked!");
	}
	alert(this.value + " is checked!");
}

• if the same function is assigned to multiple elements, each gets its own bound copy

Page/window events (9.2.5)

• The above can be handled on the window object. An alternative to window.onload:

window.onload = function() { ... };
document.observe("dom:loaded", function() {
	// attach event handlers, etc.
});

Form events (9.1.6, 9.2.4)

• Prototype adds the following methods to form controls' DOM objects:

Prototype form shortcuts (9.1.6)

$("formID")["name"]

• gets parameter with given name from form with given id

$F("controlID")

• $F function returns the value of a form control with the given id

  if ($F("username").length < 4) {
  	$("username").clear();
  	$("login").disable();
  }
  

Stopping an event

<form id="exampleform" action="http://foo.com/foo.php">...</form>

window.onload = function() {
	$("exampleform").observe("submit", checkData);
};

function checkData(event) {
	if ($F("city") == "" || $F("state").length != 2) {
		alert("Error, invalid city/state.");  // show error message 
		event.stop();
		return false;
	}
}

• to abort a form submit or other event, call Prototype's stop method on the event

Keyboard/text events (9.2.3)

• focus: the attention of the user's keyboard (given to one element at a time)

Key event objects

• issue: if the event you attach your listener to doesn't have the focus, you won't hear the event
  • possible solution: attach key listener to entire page body, outer element, etc.