Ext.util.Observable usage

[LesJ]

I did a search of the entire Ext JS code base to see how the Ext.util.Observable class is used...

The mixin pattern is used the most frequently, but a small number of Ext JS classes extend Observable.

Is there a reason for this inconsistency? I think the mixin pattern should be preferred because it makes the prototype chain shorter.

Ext.define('Ext.state.Provider', {
    mixins: {
        observable: 'Ext.util.Observable'
    },
    ...

Ext.define('Ext.selection.Model', {
    extend: 'Ext.util.Observable',
    alternateClassName: 'Ext.AbstractSelectionModel',
    requires: ['Ext.data.StoreManager'],
    mixins: {
        bindable: 'Ext.util.Bindable'    
    },
    ....

[burnnat]

I can't speak authoritatively (of course), but if I were to offer a guess, my first impression is that it could be because Ext.util.Observable provides a constructor that is automatically inherited by subclasses, but has to be explicitly called when used as a mixin. As a result, strict subclasses have somewhat cleaner code:

Ext.define('MixinObservable', {
    mixins: {
        observable: 'Ext.util.Observable'
    },
    
    constructor: function() {
        var me = this;
        me.mixins.observable.constructor.call(me);
    }
})

// ... versus ...

Ext.define('SubclassObservable', {
    extend: 'Ext.util.Observable'
});

Whether the shorter prototype chain is worth the tradeoff with more verbose code is a separate question, I think. And this is just a guess anyway.