The source code

    /**
     * @class Ext.form.CheckboxGroup
     * @extends Ext.form.Field
     * <p>A grouping container for {@link Ext.form.Checkbox} controls.</p>
     * <p>Sample usage:</p>
     * <pre><code>
    var myCheckboxGroup = new Ext.form.CheckboxGroup({
        id:'myGroup',
        xtype: 'checkboxgroup',
        fieldLabel: 'Single Column',
        itemCls: 'x-check-group-alt',
        // Put all controls in a single column with width 100%
        columns: 1,
        items: [
            {boxLabel: 'Item 1', name: 'cb-col-1'},
            {boxLabel: 'Item 2', name: 'cb-col-2', checked: true},
            {boxLabel: 'Item 3', name: 'cb-col-3'}
        ]
    });
     * </code></pre>
     * @constructor
     * Creates a new CheckboxGroup
     * @param {Object} config Configuration options
     * @xtype checkboxgroup
     */
    Ext.form.CheckboxGroup = Ext.extend(Ext.form.Field, {
        /**
         * @cfg {Array} items An Array of {@link Ext.form.Checkbox Checkbox}es or Checkbox config objects
         * to arrange in the group.
         */
        /**
         * @cfg {String/Number/Array} columns Specifies the number of columns to use when displaying grouped
         * checkbox/radio controls using automatic layout.  This config can take several types of values:
         * <ul><li><b>'auto'</b> : <p class="sub-desc">The controls will be rendered one per column on one row and the width
         * of each column will be evenly distributed based on the width of the overall field container. This is the default.</p></li>
         * <li><b>Number</b> : <p class="sub-desc">If you specific a number (e.g., 3) that number of columns will be
         * created and the contained controls will be automatically distributed based on the value of {@link #vertical}.</p></li>
         * <li><b>Array</b> : Object<p class="sub-desc">You can also specify an array of column widths, mixing integer
         * (fixed width) and float (percentage width) values as needed (e.g., [100, .25, .75]). Any integer values will
         * be rendered first, then any float values will be calculated as a percentage of the remaining space. Float
         * values do not have to add up to 1 (100%) although if you want the controls to take up the entire field
         * container you should do so.</p></li></ul>
         */
        columns : 'auto',
        /**
         * @cfg {Boolean} vertical True to distribute contained controls across columns, completely filling each column
         * top to bottom before starting on the next column.  The number of controls in each column will be automatically
         * calculated to keep columns as even as possible.  The default value is false, so that controls will be added
         * to columns one at a time, completely filling each row left to right before starting on the next row.
         */
        vertical : false,
        /**
         * @cfg {Boolean} allowBlank False to validate that at least one item in the group is checked (defaults to true).
         * If no items are selected at validation time, {@link @blankText} will be used as the error text.
         */
        allowBlank : true,
        /**
         * @cfg {String} blankText Error text to display if the {@link #allowBlank} validation fails (defaults to "You must
         * select at least one item in this group")
         */
        blankText : "You must select at least one item in this group",

        // private
        defaultType : 'checkbox',

        // private
        groupCls : 'x-form-check-group',

        // private
        initComponent: function(){
            this.addEvents(
                /**
                 * @event change
                 * Fires when the state of a child checkbox changes.
                 * @param {Ext.form.CheckboxGroup} this
                 * @param {Array} checked An array containing the checked boxes.
                 */
                'change'
            );
            this.on('change', this.validate, this);
            Ext.form.CheckboxGroup.superclass.initComponent.call(this);
        },

        // private
        onRender : function(ct, position){
            if(!this.el){
                var panelCfg = {
                    autoEl: {
                        id: this.id
                    },
                    cls: this.groupCls,
                    layout: 'column',
                    renderTo: ct,
                    bufferResize: false // Default this to false, since it doesn't really have a proper ownerCt.
                };
                var colCfg = {
                    xtype: 'container',
                    defaultType: this.defaultType,
                    layout: 'form',
                    defaults: {
                        hideLabel: true,
                        anchor: '100%'
                    }
                };

                if(this.items[0].items){

                    // The container has standard ColumnLayout configs, so pass them in directly

                    Ext.apply(panelCfg, {
                        layoutConfig: {columns: this.items.length},
                        defaults: this.defaults,
                        items: this.items
                    });
                    for(var i=0, len=this.items.length; i<len; i++){
                        Ext.applyIf(this.items[i], colCfg);
                    }

                }else{

                    // The container has field item configs, so we have to generate the column
                    // panels first then move the items into the columns as needed.

                    var numCols, cols = [];

                    if(typeof this.columns == 'string'){ // 'auto' so create a col per item
                        this.columns = this.items.length;
                    }
                    if(!Ext.isArray(this.columns)){
                        var cs = [];
                        for(var i=0; i<this.columns; i++){
                            cs.push((100/this.columns)*.01); // distribute by even %
                        }
                        this.columns = cs;
                    }

                    numCols = this.columns.length;

                    // Generate the column configs with the correct width setting
                    for(var i=0; i<numCols; i++){
                        var cc = Ext.apply({items:[]}, colCfg);
                        cc[this.columns[i] <= 1 ? 'columnWidth' : 'width'] = this.columns[i];
                        if(this.defaults){
                            cc.defaults = Ext.apply(cc.defaults || {}, this.defaults);
                        }
                        cols.push(cc);
                    };

                    // Distribute the original items into the columns
                    if(this.vertical){
                        var rows = Math.ceil(this.items.length / numCols), ri = 0;
                        for(var i=0, len=this.items.length; i<len; i++){
                            if(i>0 && i%rows==0){
                                ri++;
                            }
                            if(this.items[i].fieldLabel){
                                this.items[i].hideLabel = false;
                            }
                            cols[ri].items.push(this.items[i]);
                        };
                    }else{
                        for(var i=0, len=this.items.length; i<len; i++){
                            var ci = i % numCols;
                            if(this.items[i].fieldLabel){
                                this.items[i].hideLabel = false;
                            }
                            cols[ci].items.push(this.items[i]);
                        };
                    }

                    Ext.apply(panelCfg, {
                        layoutConfig: {columns: numCols},
                        items: cols
                    });
                }

                this.panel = new Ext.Container(panelCfg);
                this.panel.ownerCt = this;
                this.el = this.panel.getEl();

                if(this.forId && this.itemCls){
                    var l = this.el.up(this.itemCls).child('label', true);
                    if(l){
                        l.setAttribute('htmlFor', this.forId);
                    }
                }

                var fields = this.panel.findBy(function(c){
                    return c.isFormField;
                }, this);

                this.items = new Ext.util.MixedCollection();
                this.items.addAll(fields);
            }
            Ext.form.CheckboxGroup.superclass.onRender.call(this, ct, position);
        },

        initValue : function(){
            if(this.value){
                this.setValue.apply(this, this.buffered ? this.value : [this.value]);
                delete this.buffered;
                delete this.value;
            }
        },

        afterRender : function(){
            Ext.form.CheckboxGroup.superclass.afterRender.call(this);
            this.eachItem(function(item){
                item.on('check', this.fireChecked, this);
                item.inGroup = true;
            });
        },

        // private
        doLayout: function(){
            //ugly method required to layout hidden items
            if(this.rendered){
                this.panel.forceLayout = this.ownerCt.forceLayout;
                this.panel.doLayout();
            }
        },

        // private
        fireChecked: function(){
            var arr = [];
            this.eachItem(function(item){
                if(item.checked){
                    arr.push(item);
                }
            });
            this.fireEvent('change', this, arr);
        },

        /**
         * Runs CheckboxGroup's validations and returns an array of any errors. The only error by default
         * is if allowBlank is set to true and no items are checked.
         * @return {Array} Array of all validation errors
         */
        getErrors: function() {
            var errors = Ext.form.CheckboxGroup.superclass.getErrors.apply(this, arguments);

            if (!this.allowBlank) {
                var blank = true;

                this.eachItem(function(f){
                    if (f.checked) {
                        return (blank = false);
                    }
                });

                if (blank) errors.push(this.blankText);
            }

            return errors;
        },

        // private
        isDirty: function(){
            //override the behaviour to check sub items.
            if (this.disabled || !this.rendered) {
                return false;
            }

            var dirty = false;

            this.eachItem(function(item){
                if(item.isDirty()){
                    dirty = true;
                    return false;
                }
            });

            return dirty;
        },

        // private
        setReadOnly : function(readOnly){
            if(this.rendered){
                this.eachItem(function(item){
                    item.setReadOnly(readOnly);
                });
            }
            this.readOnly = readOnly;
        },

        // private
        onDisable : function(){
            this.eachItem(function(item){
                item.disable();
            });
        },

        // private
        onEnable : function(){
            this.eachItem(function(item){
                item.enable();
            });
        },

        // private
        onResize : function(w, h){
            this.panel.setSize(w, h);
            this.panel.doLayout();
        },

        // inherit docs from Field
        reset : function(){
            if (this.originalValue) {
                // Clear all items
                this.eachItem(function(c){
                    if(c.setValue){
                        c.setValue(false);
                        c.originalValue = c.getValue();
                    }
                });
                // Set items stored in originalValue, ugly - set a flag to reset the originalValue
                // during the horrible onSetValue.  This will allow trackResetOnLoad to function.
                this.resetOriginal = true;
                this.setValue(this.originalValue);
                delete this.resetOriginal;
            } else {
                this.eachItem(function(c){
                    if(c.reset){
                        c.reset();
                    }
                });
            }
            // Defer the clearInvalid so if BaseForm's collection is being iterated it will be called AFTER it is complete.
            // Important because reset is being called on both the group and the individual items.
            (function() {
                this.clearInvalid();
            }).defer(50, this);
        },

        /**
         * {@link Ext.form.Checkbox#setValue Set the value(s)} of an item or items
         * in the group. Examples illustrating how this method may be called:
         * <pre><code>
    // call with name and value
    myCheckboxGroup.setValue('cb-col-1', true);
    // call with an array of boolean values
    myCheckboxGroup.setValue([true, false, false]);
    // call with an object literal specifying item:value pairs
    myCheckboxGroup.setValue({
        'cb-col-2': false,
        'cb-col-3': true
    });
    // use comma separated string to set items with name to true (checked)
    myCheckboxGroup.setValue('cb-col-1,cb-col-3');
         * </code></pre>
         * See {@link Ext.form.Checkbox#setValue} for additional information.
         * @param {Mixed} id The checkbox to check, or as described by example shown.
         * @param {Boolean} value (optional) The value to set the item.
         * @return {Ext.form.CheckboxGroup} this
         */
        setValue: function(){
            if(this.rendered){
                this.onSetValue.apply(this, arguments);
            }else{
                this.buffered = true;
                this.value = arguments;
            }
            return this;
        },

        /**
         * @private
         * Sets the values of one or more of the items within the CheckboxGroup
         * @param {String|Array|Object} id Can take multiple forms. Can be optionally:
         * <ul>
         *   <li>An ID string to be used with a second argument</li>
         *   <li>An array of the form ['some', 'list', 'of', 'ids', 'to', 'mark', 'checked']</li>
         *   <li>An array in the form [true, true, false, true, false] etc, where each item relates to the check status of
         *       the checkbox at the same index</li>
         *   <li>An object containing ids of the checkboxes as keys and check values as properties</li>
         * </ul>
         * @param {String} value The value to set the field to if the first argument was a string
         */
        onSetValue: function(id, value){
            if(arguments.length == 1){
                if(Ext.isArray(id)){
                    Ext.each(id, function(val, idx){
                        if (Ext.isObject(val) && val.setValue){ // array of checkbox components to be checked
                            val.setValue(true);
                            if (this.resetOriginal === true) {
                                val.originalValue = val.getValue();
                            }
                        } else { // an array of boolean values
                            var item = this.items.itemAt(idx);
                            if(item){
                                item.setValue(val);
                            }
                        }
                    }, this);
                }else if(Ext.isObject(id)){
                    // set of name/value pairs
                    for(var i in id){
                        var f = this.getBox(i);
                        if(f){
                            f.setValue(id[i]);
                        }
                    }
                }else{
                    this.setValueForItem(id);
                }
            }else{
                var f = this.getBox(id);
                if(f){
                    f.setValue(value);
                }
            }
        },

        // private
        beforeDestroy: function(){
            Ext.destroy(this.panel);
            if (!this.rendered) {
                Ext.destroy(this.items);
            }
            Ext.form.CheckboxGroup.superclass.beforeDestroy.call(this);

        },

        setValueForItem : function(val){
            val = String(val).split(',');
            this.eachItem(function(item){
                if(val.indexOf(item.inputValue)> -1){
                    item.setValue(true);
                }
            });
        },

        // private
        getBox : function(id){
            var box = null;
            this.eachItem(function(f){
                if(id == f || f.dataIndex == id || f.id == id || f.getName() == id){
                    box = f;
                    return false;
                }
            });
            return box;
        },

        /**
         * Gets an array of the selected {@link Ext.form.Checkbox} in the group.
         * @return {Array} An array of the selected checkboxes.
         */
        getValue : function(){
            var out = [];
            this.eachItem(function(item){
                if(item.checked){
                    out.push(item);
                }
            });
            return out;
        },

        /**
         * @private
         * Convenience function which passes the given function to every item in the composite
         * @param {Function} fn The function to call
         * @param {Object} scope Optional scope object
         */
        eachItem: function(fn, scope) {
            if(this.items && this.items.each){
                this.items.each(fn, scope || this);
            }
        },

        /**
         * @cfg {String} name
         * @hide
         */

        /**
         * @method getRawValue
         * @hide
         */
        getRawValue : Ext.emptyFn,

        /**
         * @method setRawValue
         * @hide
         */
        setRawValue : Ext.emptyFn

    });

    Ext.reg('checkboxgroup', Ext.form.CheckboxGroup);