Echo Web Framework

A mixin is a collection of functionality, defined as methods and variables in an object, which may be included in multiple classes such that each class gains that functionality. A mixin itself is not capable of being instantiated.

A mixin is defined as an object. For example, the following mixin is defined to include a method toDebugString() which will create a string based on the values of all variables in an instance of a class:

var DebugString = {
    toDebugString: function() {
        var out = "";
        for (var i in this) {
            if (!(this[i] instanceof Function)) {
                out += i + " = " + this[i].toString() + "\n";
            }
        }
    }
};

The mixin can then be included in classes defined using the $include property in Core.extend(). The value of the $include property is a Array of mixins:

MixinExample = Core.extend({

    $include: [DebugString],

    instanceVariable1: "foo",

    instanceVariable2: "oof",

    instanceMethod1: function() {
        return this.instanceVariable1 + this.instanceVariable2;
    }
});

By "mixing in" or mixin object, we are now able to invoke toDebugString() on instances of MixinExample.

Override Issues

It is important to note that a mixin will never override an existing property of a class. If for example, we had already defined a toDebugString() method in MixinExample, the version from the DebugString mixin would not be added.

In the case where multiple mixins are specified in the $include array, there is the possibility that multiple mixins will define the same properties to be mixed-in to the class. In such a case, a mixin specified later in the array will override that defined earlier in the array.