Setting the scope of callbacks with Extjs

I recently ran into a problem where I was attempting to access a record in a collection being iterated over from inside a GDownloadUrl (Google Maps API) callback whose request would be executed as part of that loop. It took me a while to figure out the reason the last record was always getting passed to the callback no matter which callback was executing - GDownloadUrl is asynchronous and by the time any of the callbacks are executed the iteration was usually over!

Extjs provides an easy alternative to GDownloadUrl that allows access to the iteraiton record (or any other object) used when executing the callback - Ext.Ajax. All that needs to be done to retain the object required in the callback is to call createDelegate on the callback and then access the object in the callback using "this". An example is provided below:

for (var i = 0; i < activeLayersStore.getCount(); i++) {

    var record = activeLayersPanel.getStore().getAt(i);

    Ext.Ajax.request({
        url: url,
        timeout        : 180000,
        success: function(response, options) {
        alert("The record for this iteration is: "+ this.get('TypeName'));
        }.createDelegate(record),
        failure: function(response, options) {
            alert("Error requesting data" + response.statusText);
        }
    });

}

The documentation for Extjs Function.createDelegate can be found here: http://dev.sencha.com/deploy/dev/docs/?class=Function&member=createDelegate

Additionally,  if multiple objects are required for use in the callback, the createDelegate may be used as follows:

yourFunction.createDelegate({ o1: obj1, o2: obj2 });

or alternatively, if the signature of your handler is flexible you can pass parameters as follows:

yourFunction.createDelegate(scope, [scope2], 2); 

Image Zoom in Javascript

I recently spent some time investigating how to achieve image zoom using Javascript (as Extjs doesn't support it as of v3). There may be simpler jquery or other ways to achieve this but I wanted some simple Javascript. Suppose we have a couple of synced Extjs images and panels defined as follows:

        /**
         * The first image (BoxComponent)
         */
        var imgBox1 = new Ext.ux.Image({
            id: 'img_box1',   
            src:'http://earthobservatory.nasa.gov/Features/BlueMarble/Images/land_shallow_topo_2048.jpg'
        });

       
        /**
         * Panel for the first image
         */
        var imgPanel1 = new Ext.Panel({
            id: 'img_panel1',
            title: "Migrated",
            height: 280,
            autoScroll: true,
            items:[imgBox1],
            listeners: {
                render: function(p){
                    //sync scrolling between image panel 1 and 2
                    p.body.on('scroll', function(e){
                        var panel2 = Ext.getCmp('img_panel2').body.dom;
                        panel2.scrollLeft = e.target.scrollLeft; 
                        panel2.scrollTop = e.target.scrollTop;
                    }, p);
                  }
                }
        });
       
        /**
         * The second image (BoxComponent)
         */
        var imgBox2 = new Ext.ux.Image({
            id: 'img_box2',
            src:'http://earthobservatory.nasa.gov/Features/BlueMarble/Images/land_shallow_topo_2048.jpg'
        });
       
        /**
         * Panel for the second image
         */
        var imgPanel2 = new Ext.Panel({
            id: 'img_panel2',
            title: "Stacked",
            height: 280,
            autoScroll: true,
            items:[imgBox2],
            listeners: {
                render: function(p){
                    //sync scrolling between image panel 1 and 2
                    p.body.on('scroll', function(e){
                        var panel1 = Ext.getCmp('img_panel1').body.dom;
                        panel1.scrollLeft = e.target.scrollLeft; 
                        panel1.scrollTop = e.target.scrollTop;
                    }, p);
                  }
                }
        });

Zoom can be achieved as follows:

    /**
     * Add zoom functionality to image panels
     */
    'addZoom': function() {
   
        var zooming=function(e){
            e=window.event ||e;
            var o=this,data=e.wheelDelta || -e.detail*40,zoom,size;
                
            //TODO: Zooming in IE doesn't zoom to the correct point?
            if(!+'\v1'){//IE
                var oldWidth=o.offsetWidth;
                var oldHeight=o.offsetHeight;       
   
                zoom = parseInt(o.style.zoom) || 100;
                zoom += data / 12;
                if(zoom > zooming.min)
                    o.style.zoom = zoom + '%';
                e.returnValue=false;
   
                var newWidth=o.offsetWidth*zoom/100;
                var newHeight=o.offsetHeight*zoom/100;
                var scrollLeft = (o.parentNode.scrollLeft/oldWidth)*newWidth;
                var scrollTop = (o.parentNode.scrollTop/oldHeight)*newHeight;
               
                o.parentNode.scrollLeft = scrollLeft;
                o.parentNode.scrollTop = scrollTop;
            }else {
                size=o.getAttribute("_zoomsize").split(",");
                zoom=parseInt(o.getAttribute("_zoom")) ||100;
                zoom+=data/12;
               
                var oldWidth=o.offsetWidth;
                var oldHeight=o.offsetHeight;
                var newWidth=size[0]*zoom/100;
                var newHeight=size[1]*zoom/100;
                var scrollLeft = (o.parentNode.scrollLeft/oldWidth)*newWidth;
                var scrollTop = (o.parentNode.scrollTop/oldHeight)*newHeight;  
   
                if(zoom>zooming.min){
                    o.setAttribute("_zoom",zoom);
                    o.style.width=newWidth+"px";
                    o.style.height=newHeight+"px";
                    //TODO: Zoom is very jerky when setting scrollbars this way, when
                    // either scrollbar is not at position 0. Need to fix it.
                    o.parentNode.scrollLeft = scrollLeft;
                    o.parentNode.scrollTop = scrollTop;
                }
                e.preventDefault();
                e.stopPropagation();//for firefox3.6
            }
        };
   
        zooming.add=function(obj,min){// obj = image box, min defines the minimum image zoom size ,defaults to 50
            zooming.min=min || 50;
            obj.onmousewheel=zooming;
            if(/Firefox/.test(navigator.userAgent))//if Firefox
                obj.addEventListener("DOMMouseScroll",zooming,false);
            if(-[1,]){//if not IE
                    obj.setAttribute("_zoomsize",obj.naturalWidth+","+obj.naturalHeight);
            }
        };
       
        zooming.add(document.getElementById("img_box1"));  
        zooming.add(document.getElementById("img_box2"));      
    }

The above zoom function is as far as I got with it before having to put it aside. As you may notice from the comments, it doesn't work that nicely in IE and setting of the scrollbars is quite jerky when setting both scrollLeft and scrollTop to somthing other than position 0. This is something I will need to investigate further and update here in future.

Learning Ext JS

Ext JS is a javascript library for building web applications, originally an extension of YUI.

The API documentation can be found at http://dev.sencha.com/deploy/dev/docs/

An excellent source for learning Ext JS are the YouTube tutorials by Jay Garcia from TDG-innovations (http://tdg-i.com/ has the screencasts with a better quality than those on YouTube). Some topics they have covered include:

• Ext.extend - subclassing with Ext JS
• Ext.apply - a utility that allows one to easily copy properties over from one object to anothe
• Ext.each - an alternative to a for loop, used to iteratie over an array (this one gets a little hairy in the screencast, I'm not sure how useful it really is)
• Containers (Ext.Panel, Ext.Window - add, remove, doLayout, Ext.Element, Ext.Fx - slideOut, fadeOut)
•  
  

Advantages and Disadvantages of Javascript

Created by Netscape in 1996, JavaScript has little to do with Java (confusing as the naming is) except for the fact that some of the syntax looks similar. Javascript is a browser-based programming language that runs client-side.

Advantages:

• Enables data validation on the client side resulting in a smoother user experience and less load on the server
• It is supported by most browsers due to its popularity. No extra downloads are required to view JavaScript
• It does not require any special compilers or editors
• JavaScript excels in the creation of dynamic effects (eg: rollover images)
• JavaScript load time is faster than some other front-end technologies (eg: Flash, Java Applets)

Disadvantages:

• Not all browsers support JavaScript consistently and to the same version.
• If JavaScript is not stored separately it makes it difficult for search engine crawlers to find keywords and content in web pages.

Retarded IE javascript error messages

Error: Expected ';'
Code: 0

Could well mean that you have capitalised the first letter of a variable declaration (Var instead of var) in error, as javascript is case-sensitive, and IE is retarded.