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Tuesday, November 18, 2008

Ext JS - How to hack the JsonReader

I have a new job ( hooooray?! ) and I suggested Ext JS framework as web UI to focus more about Ajax, XML + XSLT data interactions rather than problems with CSS, events delegations, etc ... and I guess I am doing well, so well, that here I am with a simple tiny trick to hack an instance, specially the root and the totalProperty params:

// directly from Ext JS 2.2 API site

totalProperty: "results", // The property which contains the total dataset size (optional)
root: "rows", // The property which contains an Array of row objects
id: "id" // The property within each row object that provides an ID for the record (optional)

Especially for the paginator toolbar, the JsonReader is a must to surf a big amount of data without stressing too much both server and client sides.

One nice feature, or one clever way to make the root node customizable, is the usage of evaluated code via a new Function call.

If the root property ontains a dot, that property is retrieved via nested objec properties.


Thanks to this feature, it is possible to pre parse and pre generate the list that will be assigned as root Array, the one used inside the Grid, DataView, or whatever.Component is managing your interactions.

The trick to pass the returned object to an arbitrary function is this:


The callback suppose to be a valid function with a global scope that will return a filtered list of objects compatible with the column model or the data manager we chose.

The trick is based on their regexp that checks simply a dot or a square bracket "[" in the passed string.

That's it, let me discover better tricky stuff in the source and I'll post them :D


Kirill said...

Thank You for pointing to this

Balázs Endrész said...

getJsonAccessor: function(){
var re = /[\[\.]/;
return function(expr) {
try {
return(re.test(expr)) ?
new Function("obj", "return obj." + expr) :
return obj[expr];
} catch(e){}
return Ext.emptyFn;

So this is supposed to be a commercial product and this very basic feature is still not documented... and on my first day working with Ext I had to spend my time reading the source to find this. Also, the "clever" way they wrote it makes me wonder too:

* an unnecessary closure for referencing a regex used only once
* a nice little try block to prevent the user finding bugs
* and I can't even pass a simple function!

Andrea Giammarchi said...

Balázs Endrész, welcome into The Daily Ext JS WTF! :D

Balázs Endrész said...

To be fair, having spent a few days with at least a dozen of components, I haven't had any other issues like this (yet).

Also, I've found a bit nicer solution for pre parsing json data by overriding the readRecords method:

jsonReader.readRecords = function(obj){
return, obj);

Andrea Giammarchi said...

well, let's talk in few months ;)
V3 anyway solved lot of of problems I had before, but not everything