Mixins are used in ExtJS4 to implement multiple inheritance. The specific example code is as follows:

(function(){
Ext.onReady(function(){
Ext.define('say',{
canSay:function(){
alert("hello");
}
});
Ext.define('eat',{
caneat:function(){
alert("eating");
}
});
Ext.define("user",{
mixins:{
csay:'say',
ceat:'eat'
}
});
var ss = Ext.create("user",{});
ss.caneat();
ss.canSay();
});
})();

Note the difference between mixins and extend, which can only implement single inheritance, because extend can only be followed by a String of type arguments, not separated from a file by commas, etc.

In mixins, multiple classes can be loaded to achieve the effect of multiple inheritance.