Question

I need to be able to instantiate an object of a class in Dojo at runtime and mix it into another object (kind of like specifying an extends or an implements in Java, but at runtime). and I came up with the following solution:

var declaredClassBackup = this.declaredClass;  // backup the "declaredClass" 

var mixinObject = null;
try {
    dojo.require(kwArgs.mixinClassName);

    /*
     * Eval the mixinClassName variable to get the Function reference, 
     * then call it as a constructor with our mixinSettings
     */
    mixinObject = new (eval(kwArgs.mixinClassName))(kwArgs.mixinSettings);
} catch (e){
    if(console){
        console.error("%s could not be loaded as a mixin.", 
                kwArgs.mixinClassName);
    }
    mixinObject = new package.path.DefaultMixin(kwArgs.mixinSettings);
}
dojo.mixin(this, mixinObject);

/*
 * Re-set the declaredClass name back to that of this class.
 */
this.declaredClass = declaredClassBackup;

What could go wrong with this type of code, if anything? (How would you make it more robust?) Also, is there something I could ve just missed in dojo that would ve done this for me more gracefully?

Answer 1

At least two things can go wrong:

Your code assumes that a dynamically loaded module is loaded synchronously with dojo.require(). It is true only for the default loader. The XD loader will load things asynchronously breaking your logic.
An object is instantiated and its properties copied using dojo.mixin(), which by necessity will flatten it. It means:
- It may override some internals (you preserve declaredClass, but there may be others).
- OOP helpers (like this.inherited()) will be broken for copied methods.

But if these restrictions fit your use case, you should be fine.

It is hard to suggest improvements because it is not clear what you are trying to achieve. If you want to add flat mixins to an object, the only thing you need to make sure that the objects are truly flat.

Minor improvements to your code:

declaredClass is defined on object s prototype, not on object itself ⇒ you don t need to preserve it. Just delete it from the object itself:

//var declaredClassBackup = this.declaredClass;  // backup the "declaredClass"
// no need
// the rest of your code
...
/*
 * Re-set the declaredClass name back to that of this class.
 */
//this.declaredClass = declaredClassBackup;
// no need
delete this.declaredClass;

Instead of dojo.mixin() you can use dojo.safeMixin(), which skips constructor and decorate methods. This method is available since Dojo 1.4 (including the current trunk).

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