Setting up the playing field

First we setup the HTML for the canvas (playing field) and place some players on it.

<div id="dd-demo-canvas1"> <div id="dd-demo-canvas2"> <div id="dd-demo-canvas3"> <div id="dd-demo-1" class="dd-demo"><div>1</div></div> <div id="dd-demo-2" class="dd-demo"><div>2</div></div> <div id="dd-demo-3" class="dd-demo"><div>3</div></div> </div> </div> </div>

Size of the playing field

Next we have to get the size of the playing field. To do this, we will use YAHOO.util.Dom.getRegion('dd-demo-canvas'). getRegion will return something like this:

{ 0: 255, 1: 248, bottom: 432, left: 255, right: 1068, top: 248 }

Now we have the top, bottom, left and right pixel locations of the playing field, we can do some math and set some constraints.

The Math

The math for this is pretty easy, as shown in the code below:

It should be noted that the box model comes into play here. Placing padding, margins or borders on the playing field or the draggable elements will cause them to not stay fully in the playing field. They will overlap the region by the width of the margin/padding/border.

//Get the top, right, bottom and left positions var region = Dom.getRegion(this.cont); //Get the element we are working on var el = this.getEl(); //Get the xy position of it var xy = Dom.getXY(el); //Get the width and height var width = parseInt(Dom.getStyle(el, 'width'), 10); var height = parseInt(Dom.getStyle(el, 'height'), 10); //Set left to x minus left var left = xy[0] - region.left; //Set right to right minus x minus width var right = region.right - xy[0] - width; //Set top to y minus top var top = xy[1] - region.top; //Set bottom to bottom minus y minus height var bottom = region.bottom - xy[1] - height;

Now that we have the vars for top, bottom, left and right, we can set contraints on a Drag Drop instance with setXConstraint and setYConstraint

Full Javascript Source

(function() { var Dom = YAHOO.util.Dom, Event = YAHOO.util.Event, dd1, dd2, dd3; YAHOO.example.DDRegion = function(id, sGroup, config) { this.cont = config.cont; YAHOO.example.DDRegion.superclass.constructor.apply(this, arguments); }; YAHOO.extend(YAHOO.example.DDRegion, YAHOO.util.DD, { cont: null, init: function() { //Call the parent's init method YAHOO.example.DDRegion.superclass.init.apply(this, arguments); this.initConstraints(); Event.on(window, 'resize', function() { this.initConstraints(); }, this, true); }, initConstraints: function() { //Get the top, right, bottom and left positions var region = Dom.getRegion(this.cont); //Get the element we are working on var el = this.getEl(); //Get the xy position of it var xy = Dom.getXY(el); //Get the width and height var width = parseInt(Dom.getStyle(el, 'width'), 10); var height = parseInt(Dom.getStyle(el, 'height'), 10); //Set left to x minus left var left = xy[0] - region.left; //Set right to right minus x minus width var right = region.right - xy[0] - width; //Set top to y minus top var top = xy[1] - region.top; //Set bottom to bottom minus y minus height var bottom = region.bottom - xy[1] - height; //Set the constraints based on the above calculations this.setXConstraint(left, right); this.setYConstraint(top, bottom); } }); Event.onDOMReady(function() { dd1 = new YAHOO.example.DDRegion('dd-demo-1', '', { cont: 'dd-demo-canvas3' }); dd2 = new YAHOO.example.DDRegion('dd-demo-2', '', { cont: 'dd-demo-canvas2' }); dd3 = new YAHOO.example.DDRegion('dd-demo-3', '', { cont: 'dd-demo-canvas1' }); }); })();

Configuration for This Example

You can load the necessary JavaScript and CSS for this example from Yahoo's servers. Click here to load the YUI Dependency Configurator with all of this example's dependencies preconfigured.