The source code

    /**
     * @class Ext.util.Observable
     */
    Ext.apply(Ext.util.Observable.prototype, function(){
        // this is considered experimental (along with beforeMethod, afterMethod, removeMethodListener?)
        // allows for easier interceptor and sequences, including cancelling and overwriting the return value of the call
        // private
        function getMethodEvent(method){
            var e = (this.methodEvents = this.methodEvents ||
            {})[method], returnValue, v, cancel, obj = this;

            if (!e) {
                this.methodEvents[method] = e = {};
                e.originalFn = this[method];
                e.methodName = method;
                e.before = [];
                e.after = [];

                var makeCall = function(fn, scope, args){
                    if((v = fn.apply(scope || obj, args)) !== undefined){
                        if (typeof v == 'object') {
                            if(v.returnValue !== undefined){
                                returnValue = v.returnValue;
                            }else{
                                returnValue = v;
                            }
                            cancel = !!v.cancel;
                        }
                        else
                            if (v === false) {
                                cancel = true;
                            }
                            else {
                                returnValue = v;
                            }
                    }
                };

                this[method] = function(){
                    var args = Array.prototype.slice.call(arguments, 0),
                        b;
                    returnValue = v = undefined;
                    cancel = false;

                    for(var i = 0, len = e.before.length; i < len; i++){
                        b = e.before[i];
                        makeCall(b.fn, b.scope, args);
                        if (cancel) {
                            return returnValue;
                        }
                    }

                    if((v = e.originalFn.apply(obj, args)) !== undefined){
                        returnValue = v;
                    }

                    for(var i = 0, len = e.after.length; i < len; i++){
                        b = e.after[i];
                        makeCall(b.fn, b.scope, args);
                        if (cancel) {
                            return returnValue;
                        }
                    }
                    return returnValue;
                };
            }
            return e;
        }

        return {
            // these are considered experimental
            // allows for easier interceptor and sequences, including cancelling and overwriting the return value of the call
            // adds an 'interceptor' called before the original method
            beforeMethod : function(method, fn, scope){
                getMethodEvent.call(this, method).before.push({
                    fn: fn,
                    scope: scope
                });
            },

            // adds a 'sequence' called after the original method
            afterMethod : function(method, fn, scope){
                getMethodEvent.call(this, method).after.push({
                    fn: fn,
                    scope: scope
                });
            },

            removeMethodListener: function(method, fn, scope){
                var e = this.getMethodEvent(method);
                for(var i = 0, len = e.before.length; i < len; i++){
                    if(e.before[i].fn == fn && e.before[i].scope == scope){
                        e.before.splice(i, 1);
                        return;
                    }
                }
                for(var i = 0, len = e.after.length; i < len; i++){
                    if(e.after[i].fn == fn && e.after[i].scope == scope){
                        e.after.splice(i, 1);
                        return;
                    }
                }
            },

            /**
             * Relays selected events from the specified Observable as if the events were fired by <tt><b>this</b></tt>.
             * @param {Object} o The Observable whose events this object is to relay.
             * @param {Array} events Array of event names to relay.
             */
            relayEvents : function(o, events){
                var me = this;
                function createHandler(ename){
                    return function(){
                        return me.fireEvent.apply(me, [ename].concat(Array.prototype.slice.call(arguments, 0)));
                    };
                }
                for(var i = 0, len = events.length; i < len; i++){
                    var ename = events[i];
                    me.events[ename] = me.events[ename] || true;
                    o.on(ename, createHandler(ename), me);
                }
            },

            /**
             * <p>Enables events fired by this Observable to bubble up an owner hierarchy by calling
             * <code>this.getBubbleTarget()</code> if present. There is no implementation in the Observable base class.</p>
             * <p>This is commonly used by Ext.Components to bubble events to owner Containers. See {@link Ext.Component.getBubbleTarget}. The default
             * implementation in Ext.Component returns the Component's immediate owner. But if a known target is required, this can be overridden to
             * access the required target more quickly.</p>
             * <p>Example:</p><pre><code>
    Ext.override(Ext.form.Field, {
        //  Add functionality to Field&#39;s initComponent to enable the change event to bubble
        initComponent : Ext.form.Field.prototype.initComponent.createSequence(function() {
            this.enableBubble('change');
        }),

        //  We know that we want Field&#39;s events to bubble directly to the FormPanel.
        getBubbleTarget : function() {
            if (!this.formPanel) {
                this.formPanel = this.findParentByType('form');
            }
            return this.formPanel;
        }
    });

    var myForm = new Ext.formPanel({
        title: 'User Details',
        items: [{
            ...
        }],
        listeners: {
            change: function() {
                // Title goes red if form has been modified.
                myForm.header.setStyle('color', 'red');
            }
        }
    });
    </code></pre>
             * @param {String/Array} events The event name to bubble, or an Array of event names.
             */
            enableBubble : function(events){
                var me = this;
                if(!Ext.isEmpty(events)){
                    events = Ext.isArray(events) ? events : Array.prototype.slice.call(arguments, 0);
                    for(var i = 0, len = events.length; i < len; i++){
                        var ename = events[i];
                        ename = ename.toLowerCase();
                        var ce = me.events[ename] || true;
                        if (typeof ce == 'boolean') {
                            ce = new Ext.util.Event(me, ename);
                            me.events[ename] = ce;
                        }
                        ce.bubble = true;
                    }
                }
            }
        };
    }());

    /**
     * Starts capture on the specified Observable. All events will be passed
     * to the supplied function with the event name + standard signature of the event
     * <b>before</b> the event is fired. If the supplied function returns false,
     * the event will not fire.
     * @param {Observable} o The Observable to capture events from.
     * @param {Function} fn The function to call when an event is fired.
     * @param {Object} scope (optional) The scope (<code>this</code> reference) in which the function is executed. Defaults to the Observable firing the event.
     * @static
     */
    Ext.util.Observable.capture = function(o, fn, scope){
        o.fireEvent = o.fireEvent.createInterceptor(fn, scope);
    };

    /**
     * Sets observability on the passed class constructor.<p>
     * <p>This makes any event fired on any instance of the passed class also fire a single event through
     * the <i>class</i> allowing for central handling of events on many instances at once.</p>
     * <p>Usage:</p><pre><code>
    Ext.util.Observable.observeClass(Ext.data.Connection);
    Ext.data.Connection.on('beforerequest', function(con, options) {
        console.log('Ajax request made to ' + options.url);
    });</code></pre>
     * @param {Function} c The class constructor to make observable.
     * @param {Object} listeners An object containing a series of listeners to add. See {@link #addListener}.
     * @static
     */
    Ext.util.Observable.observeClass = function(c, listeners){
        if(c){
          if(!c.fireEvent){
              Ext.apply(c, new Ext.util.Observable());
              Ext.util.Observable.capture(c.prototype, c.fireEvent, c);
          }
          if(typeof listeners == 'object'){
              c.on(listeners);
          }
          return c;
       }
    };