Mixins

Just as Dojo adds class-based inheritance to JavaScript, so it adds support for multiple inheritance. We do this through Dojo mixins. The methods and properties of a mixed-in class are simply added to each instance.

In pure object-oriented languages like Java, you must use typecasts to make an object "act like" its mixed-in class (in Java, this is through interfaces). Not in Dojo. You can use the mixed-in properties directly.

Suppose, for example, you have a class called VanillaSoftServe, and classes MandMs and CookieDough. Here's how to make a Blizzard:

/* GeSHi (C) 2004 - 2007 Nigel McNie (http://qbnz.com/highlighter) */ .geshifilter {font-family: monospace;} .geshifilter .imp {font-weight: bold; color: red;} .geshifilter .kw1 {color: #000066; font-weight: bold;} .geshifilter .kw2 {color: #003366; font-weight: bold;} .geshifilter .kw3 {color: #000066;} .geshifilter .co1 {color: #009900; font-style: italic;} .geshifilter .coMULTI {color: #009900; font-style: italic;} .geshifilter .es0 {color: #000099; font-weight: bold;} .geshifilter .br0 {color: #66cc66;} .geshifilter .st0 {color: #3366CC;} .geshifilter .nu0 {color: #CC0000;} .geshifilter .me1 {color: #006600;} .geshifilter .re0 {color: #0066FF;}

dojo.declare("VanillaSoftServe",null, {

    constructor: function() { console.debug ("mixing in Vanilla"); }

});

dojo.declare("MandMs",null, {

    constructor: function() { console.debug("mixing in MandM's"); },

    kind: "plain"

});

dojo.declare("CookieDough",null, {

    chunkSize: "medium"

});

dojo.declare("Blizzard", [VanillaSoftServe, MandMs, CookieDough], {

        constructor: function() {

             console.debug("A blizzard with "+

                 this.kind+" M and Ms and "+

                 this.chunkSize+" chunks of cookie dough."

             );

        }

});

Then the following:

new Blizzard();

Will first print "mixing in Vanilla" on the debug console because VanillaSoftServe is the superclass of Blizzard. In fact, VanillaSoftServe is the only superclass of Blizzard - the first mixin is always the superclass. Next the constructors of the mixins are called, so "mixing in MandMs" will appear. Then "A blizzard with plain M and Ms and medium chunks of cookie dough." will appear.

Mixins are used a lot in defining Dijit classes, with most classes extending Dijit._Widget and mixing in Dijit._Templated.