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setting up jquery for drawing with Javascript - part 1 in a series

Updated on March 29, 2011

marking where the user has clicked


For the next thing, I want to demonstrate, I want to use a mouse for my program to receive input from the user. While handling mouse events is possible in native Javascript, javascript libraries make the task a lot easier. So, I will use one. JQuery seems to have the most momemtum. So, I will use that one.

This will be the first hub in another series. In this first hub, I will just get things, setup.

Putting a dot where the user clicks should be sufficient to get the mechanics I will want for the
next hub, in place.

jQuery setup is covered in lots of other places. The only twist I am adding is to merge it in with the graphing calculator application I developed and explained in my last series. Later I will expand how the user interacts with the graph using jQuery.

style sheet stuff

First I add a style for the target of mouse events.

.mousetarget { position: absolute; top:50; left:50; z-index: 2; opacity: "0%"; cursor:pointer; }

including jquery

Next, I include the jQuery library.  The commented out inclusion is to retrieve the jQuery library from a public server, but I want to be able to work while off-line.  So, I made a local copy.  The 1.4.4 and 1.4.2 are version numbers.  The latest is 1.5.1, as of this writing, but I doubt it will matter for what I want to do.

the hand-rolled javascript

<!--<script type="text/javascript" 

<script type="text/javascript" src="./js/jquery-1.4.2.min.js"></script>

I add a container for the mouse target style.

<div id="mousetarget" class="mousetarget" ></div>

Finally, I add some script to do what I want; put a dot where the user clicks:

<script type="text/javascript" >
$( document ).ready( function () {
	alert( "hello jquery world!" );
	$( "#mousetarget" ).css( "top", unitsquare.screeny1 );
	$( "#mousetarget" ).css( "left", unitsquare.screenx0 );
	$( "#mousetarget" ).height( unitsquare.screeny0 - unitsquare.screeny1 );
	$( "#mousetarget" ).width( unitsquare.screenx1 - unitsquare.screenx0 );
	$( "#mousetarget" ).bind( "click", function (e) {
		drawrectangle( "blackrectangle", e.clientY + 2, e.clientX - 2, 4, 4 );

jquery usage explained

The alert just lets me know everything is setup ok.  No typos that keep javascript from running, etc.  $ is the cryptic version of the jQuery function.  "#mousetarget" is an id based selector, that describes what DOM element I want jQuery to opperate on.

The jQuery function includes lots of methods that do useful things.  I only used four of them: css, height, width and bind.  I'm not sure why the "top" and "left" style properties don't have their own functions, like height and width do.  bind associates a function with mouse-click events, if the mouse is over my target object when it is clicked.  drawrectangle is a function I wrote and explained in my previous series of hubs.  Anyway, it is what puts a black square where the user clicked.


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