Dojo concept for javascript developer

Introduction
If you're a Java programmer coming to Dojo with little or noexperience of JavaScript, chances are you're going to struggle withsome of the concepts that enable it to work. The main concerns withDojo are that — at the time of writing — it is still
in its infancy(version 1.0 was only released in February 2008) and thedocumentation available is still somewhat limited. This articlehelps you bridge the gap from Java code to Dojo so that you can getup to speed quickly and use the toolkit when developing
yourapplications.
This article does not describe how to obtain the Dojo toolkit orthe necessary statements needed to use it, because there are manyother resources available that provide that information. Thearticle is written for Web developers who are coming to
Dojo from aservlet development background.
The JavaScripthash
One of the first challenges is to understand the syntax that isused when invoking Dojo functions, in particular the use of the"hash" or JavaScript object. A hash is expressed as a set ofattributes delimited by commas between curly braces. A simpleexample
is shown in Listing 1, declaring a hash consisting of 6attributes: a string, an integer, a boolean, an undefinedattribute, another hash, and a function.

Listing 1. Example JavaScript hash

var myHash = { str_attr : "foo", int_attr : 7, bool_attr : true, undefined_attr : null, hash_attr : {}, func_attr : function() {} };

It is important to remember that JavaScript is weakly typed, soalthough each attribute has been initialized to a value linked toits name, there is no reason why the initial

str_attr

attributecannot
be subsequently set to an integer or boolean (or any othertype for that matter). Each attribute in the hash can be accessedor set using the dot operator (as shown in Listing 2).

Listing 2. Accessing and setting hash attributes

// Accessing a hash attribute... console.log(myHash.str_attr); // Setting a hash attribute... myHash.str_attr = "bar";

The first four attributes of

myHash

shouldbe self-explanatory. The fact that a hash can have attributes thatare also hashes should be no surprise.
(This can be thought of asanalogous to Java classes referencing both primitives and objects.)It is the final attribute that is most important to understand.
Functions areobjects
Although in Java code there is a

java.reflection.Method

class,it essentially acts as wrapper to a method. In JavaScript thefunction is an object
like any other that can be set, referenced,and passed as an argument to other functions. Often it is necessaryto declare new functions in function calls in much the same way asan anonymous inner class can be declared in a Java method call.
Another important difference between Java methods and JavaScriptfunctions is that JavaScript functions can be run in differentcontexts. In Java programming the use of the keyword

this

refersto
the current instance of the class where it is used. When used ina JavaScript function,

this

refersto the context in which that function is running. A
function willrun in the closure that defines it unless otherwise specified.
In the simplest terms, a closure can be considered to be anyJavaScript code contained within curly braces (

{}

).A function declared inside a JavaScript file can use

this

toaccess
any variable declared in the main body of that file, but afunction declared inside a hash can only use

this

toreference variables declared inside that
hash, unless it isprovided with an alternative context to work in.
Because enclosed functions are often required as arguments to Dojofunctions, understanding how to set their context will save a lotof unnecessary debugging.
The main Dojo function that is used to assign contextis

dojo.hitch

.You may never use

dojo.hitch

,but
it is important to understand that it is one of thecornerstones of Dojo, and many other functions are invoking itunder the covers.
Listing 3 shows how context hitching works (its output is shown inFigure 1):

A variable is defined at the global context (

globalContextVariable

)and another variable is declared in the context of a hash(

enclosedVariable

).

The function

accessGlobalContext()

cansuccessfully access

globalContextVariable

anddisplay
its value.

But,

enclosedFunction()

canonly access its local variable

enclosedVariable

(notethat
the value of

globalContextVariable

isdisplayed as "undefined").

Using

dojo.hitch

to"hitch"

enclosedFunction()

tothe
global context allows

globalContextVariable

tobe displayed (note however that

enclosedVariable

isnow
undefined because it is not declared in the contextthat

enclosedFunction()

isrunning.

Listing 3. Closures and context

var globalContextVariable = "foo"; function accessGlobalContext() { // This will successfully output "foo"... console.log(this.globalContextVariable); }; var myHash = { enclosedVariable : "bar", enclosedFunction : function() { // Display global context variable... console.log(this.globalContextVariable); // Display enclosed context variable... console.log(this.enclosedVariable); } }; console.log("Calling accessGlobalContext()..."); accessGlobalContext(); console.log("Calling myHash.enclosedFunction()..."); myHash.enclosedFunction(); console.log("Switch the context using dojo.hitch..."); var switchContext = dojo.hitch(this, myHash.enclosedFunction); switchContext();

Figure 1. How contect hitching works



Declaringclasses

Class declaration tips

While

myClass

isa perfectly valid name, it is a better practice to declare namesusing a fully qualified class name style, for example,

com.ibm.dojo.myClass

.This
does not mean that the class should be deployed to the filesystem under the relative path of "./com/ibm/dojo/"; it simplyreduces the chances of there being naming collisions with any otherclasses.

There must never be a

,

(comma)after the last attribute because some browsers will ignore it(FireFox), but others will blow-up (Internet Explorer). This
rulealso applies to declarations of hash objects anywhere.

The reason why this hitching is so important will become apparentas soon as you start to declare Dojo classes or to create your ownwidgets. One of the greatest powers of Dojo is the ability to"wire" objects together through the use of the

dojo.connect

functionand
the built-in pub/sub model.
Declaring a class requires three objects:

A unique name for the class

A parent class to extend function from (plus any "mix-in" classesto simulate multiple inheritance)

A hash defining all the attributes and functions.

The simplest possible class that can be declared in shown inListing 4, and its instantiation in Listing 5.

Listing 4. Basic class declaration

dojo.declare( "myClass", null, {} );

Listing 5. Basic class instantiation

var myClassInstance = new myClass();

If you want to declare a "real" (that is, useful) Dojo class, it'simportant to understand constructors. In Java code you can declaremultiple overloaded constructors to enable the instantiation of theclass by a variety of different signatures. In
a Dojo class you candeclare a

preamble

,a

constructor

,and a

postscript

,but
in the majority of cases you will only need to declare aconstructor.

Unless you're mixing-in other classes to simulate multipleinheritances, you are unlikely to require

preamble

,as it allows you to manipulate your

constructor

argumentsbefore
they are actually passed to the extended and mixed-inclasses.

postscript

drivesthe Dojo widget life cycle methods, but provides no benefit to astandard Dojo class.

It is not essential to declare any of them, but to pass any valuesinto an instance of the class the

constructor

functionmust be declared as
a minimum. If the

constructor

argumentsare to be accessed by any other method of the class, they must beassigned to declared attributes. Listing 6 shows
a class thatassigns only one of its

constructor

argumentsto a class attribute and then attempts to reference both in anothermethod.

Listing 6. Assigning constructor arguments

dojo.declare( "myClass", null, { arg1 : "", constructor : function(arg1, arg2) { this.arg1 = arg1; }, myMethod : function() { console.log(this.arg1 + "," + this.arg2); } } ); var myClassInstance = new myClass("foo", "bar"); myClassInstance.myMethod();

Figure 2. Output from assigning constructorarguments



Complex attributerules
Class attributes can be initialized when declared, but if theattribute is initialized with a complex object type (such as a hashor an array) that attribute becomes analogous to a public staticvariable in a Java class. This means that whenever any
instanceupdates it, the change will be reflected in all other instances. Toavoid this problem, complex attributes should be initialized in theconstructor; however, this is not necessary for simple attributessuch as strings, booleans, and so on.

Listing 7. Global class attributes

dojo.declare( "myClass", null, { globalComplexArg : { val : "foo" }, localComplexArg : null, constructor : function() { this.localComplexArg = { val:"bar" }; } } ); // Create instances of myClass A and B... var A = new myClass(); var B = new myClass(); // Output A's attributes... console.log("A's global val: " + A.globalComplexArg.val); console.log("A's local val: " + A.localComplexArg.val); // Update both of A's attributes... A.globalComplexArg.val = "updatedFoo"; A.localComplexArg.val = "updatedBar"; // Update B's attributes... console.log("A's global val: " + B.globalComplexArg.val); console.log("A's local val: " + B.localComplexArg.val);

Figure 3. Class attributes



Overridingmethods
A method of a superclass can be extended by declaring an attributewith the same name. There is no concept of overloading, asJavaScript ignores any unexpected arguments and substitutes nullfor any that are missing. In Java code, to invoke the overriddenmethod
you call the method on super (that is,

super().methodName(arg1,arg1);

), but in Dojo you use the

inherited

method(

this.inherited(arguments);

).Listing
8 shows two classes declared, where

child

extends

parent

,overriding
its

helloWorld

method,but calls

inherited

toaccess
the function of

parent

.

Listing 8. Invoking superclass method in Dojo

dojo.declare( "parent", null, { helloWorld : function() { console.log("parent says 'hello world'"); } } ); dojo.declare( "child", parent, { helloWorld : function() { this.inherited(arguments); // Call superclass method... console.log("child says 'hello world'"); } } ); var child = new child(); child.helloWorld();

Figure 4. Output from invoking superclass method inDojo



Setting methodcontext
Listing 9 shows a Java class that, upon instantiation, copies theelements from the supplied string array to an ArrayList of strings.It would not be unreasonable to assume that the same functionalitycan be provided in Dojo with the code in Listing
10. (Note theinstantiation of

targetArray

inthe constructor function to prevent it being global.)Unfortunately, it will result in the error message shown
in Figure5, because the function declared in the

dojo.forEach

methodcall creates a closure that defines

this

asreferring
to its own body.

Listing 9. Accessing class scoped variable in Javacode

import java.util.ArrayList; public class MyClass { // Declare an ArrayList of Strings... private ArrayList<String> targetArray = new ArrayList<String>(); public MyClass(String[] sourceArray) { // Copy each element of a String[] into the ArrayList... for (String val: sourceArray) { this.targetArray.add(val); } } }

Listing 10. Missing context in Dojo

dojo.declare( "myClass", null, { targetArray: null, constructor: function(source) { // Initialise in constructor to avoid making global this.targetArray = []; // Copy each element from source into target... dojo.forEach(source, function(item) { this.targetArray[this.targetArray.length] = item; }); }, } ); // This will cause an error! var myClass = new myClass(["item1","item2"]);

Figure 5. Output from missing context in Dojo



Although

targetArray

isnot defined at the context enclosed by the function, it is possibleto pass the context where it is defined as an argument
to the Dojofunction. This means the

this

keywordcan then access any objects (including functions) that have beendeclared at that context. The correct
implementation is shown inListing 11 (note the additional code in bold).

Listing 11. Setting correct context in Dojo

dojo.declare( "myClass", null, { targetArray: null, constructor: function(source) { // Initialise in constructor to avoid making global this.targetArray = []; // Copy each element from source into target... dojo.forEach(source, function(item) { this.targetArray[this.targetArray.length] = item; }, this); }, } );

Context is not always passed as the same argument in a Dojofunctions signature:

In

dojo.subscribe

thecontext is passed before the functiondeclaration
(Listing 12).

In

dojo.connect

boththe context where the trigger method is defined and the contextwhere the target method is defined should be supplied. Listing 13shows
an example where

obj1

isthe context where

methodA

isdefined
and

obj2

isthe context where

methodB

isdefined.
Calling

methodA

on

obj1

willresult in

methodB

beinginvoked
on

obj2

.

Listing 12. Setting context in dojo.subscribe

dojo.declare( "myClass", null, { subscribe : function() { dojo.subscribe("publication", this, function(pub) { this.handlePublication(pub); }); }, handlePublication : function(pub) { console.log("Received: " + pub); } } );

Listing 13. Setting context in dojo.connect

dojo.connect(obj1, "methodA", obj2, "methodB");

Conclusion
JavaScript will never come naturally to developers who have becomeused to the more structured environment of Java code. Yet theimplementation of Dojo, with its class declaration capabilities,does make the leap to client-side development considerably
easier.A good understanding of context, and when and how to set it, willsave a lot of pain for the Java developer and help them confidentlyadd JavaScript to their toolbox.