The source code

    /**
     * @class Ext.form.Checkbox
     * @extends Ext.form.Field
     * Single checkbox field.  Can be used as a direct replacement for traditional checkbox fields.
     * @constructor
     * Creates a new Checkbox
     * @param {Object} config Configuration options
     * @xtype checkbox
     */
    Ext.form.Checkbox = Ext.extend(Ext.form.Field,  {
        /**
         * @cfg {String} focusClass The CSS class to use when the checkbox receives focus (defaults to undefined)
         */
        focusClass : undefined,
        /**
         * @cfg {String} fieldClass The default CSS class for the checkbox (defaults to 'x-form-field')
         */
        fieldClass : 'x-form-field',
        /**
         * @cfg {Boolean} checked <tt>true</tt> if the checkbox should render initially checked (defaults to <tt>false</tt>)
         */
        checked : false,
        /**
         * @cfg {String} boxLabel The text that appears beside the checkbox
         */
        boxLabel: '&#160;',
        /**
         * @cfg {String/Object} autoCreate A DomHelper element spec, or true for a default element spec (defaults to
         * {tag: 'input', type: 'checkbox', autocomplete: 'off'})
         */
        defaultAutoCreate : { tag: 'input', type: 'checkbox', autocomplete: 'off'},
        /**
         * @cfg {String} inputValue The value that should go into the generated input element's value attribute
         */
        /**
         * @cfg {Function} handler A function called when the {@link #checked} value changes (can be used instead of
         * handling the check event). The handler is passed the following parameters:
         * <div class="mdetail-params"><ul>
         * <li><b>checkbox</b> : Ext.form.Checkbox<div class="sub-desc">The Checkbox being toggled.</div></li>
         * <li><b>checked</b> : Boolean<div class="sub-desc">The new checked state of the checkbox.</div></li>
         * </ul></div>
         */
        /**
         * @cfg {Object} scope An object to use as the scope ('this' reference) of the {@link #handler} function
         * (defaults to this Checkbox).
         */

        // private
        actionMode : 'wrap',

    	// private
        initComponent : function(){
            Ext.form.Checkbox.superclass.initComponent.call(this);
            this.addEvents(
                /**
                 * @event check
                 * Fires when the checkbox is checked or unchecked.
                 * @param {Ext.form.Checkbox} this This checkbox
                 * @param {Boolean} checked The new checked value
                 */
                'check'
            );
        },

        // private
        onResize : function(){
            Ext.form.Checkbox.superclass.onResize.apply(this, arguments);
            if(!this.boxLabel && !this.fieldLabel){
                this.el.alignTo(this.wrap, 'c-c');
            }
        },

        // private
        initEvents : function(){
            Ext.form.Checkbox.superclass.initEvents.call(this);
            this.mon(this.el, {
                scope: this,
                click: this.onClick,
                change: this.onClick
            });
        },

        /**
         * @hide
         * Overridden and disabled. The editor element does not support standard valid/invalid marking.
         * @method
         */
        markInvalid : Ext.emptyFn,
        /**
         * @hide
         * Overridden and disabled. The editor element does not support standard valid/invalid marking.
         * @method
         */
        clearInvalid : Ext.emptyFn,

        // private
        onRender : function(ct, position){
            Ext.form.Checkbox.superclass.onRender.call(this, ct, position);
            if(this.inputValue !== undefined){
                this.el.dom.value = this.inputValue;
            }
            this.wrap = this.el.wrap({cls: 'x-form-check-wrap'});
            if(this.boxLabel){
                this.wrap.createChild({tag: 'label', htmlFor: this.el.id, cls: 'x-form-cb-label', html: this.boxLabel});
            }
            if(this.checked){
                this.setValue(true);
            }else{
                this.checked = this.el.dom.checked;
            }
            // Need to repaint for IE, otherwise positioning is broken
            if (Ext.isIEQuirks) {
                this.wrap.repaint();
            }
            this.resizeEl = this.positionEl = this.wrap;
        },

        // private
        onDestroy : function(){
            Ext.destroy(this.wrap);
            Ext.form.Checkbox.superclass.onDestroy.call(this);
        },

        // private
        initValue : function() {
            this.originalValue = this.getValue();
        },

        /**
         * Returns the checked state of the checkbox.
         * @return {Boolean} True if checked, else false
         */
        getValue : function(){
            if(this.rendered){
                return this.el.dom.checked;
            }
            return this.checked;
        },

    	// private
        onClick : function(){
            if(this.el.dom.checked != this.checked){
                this.setValue(this.el.dom.checked);
            }
        },

        /**
         * Sets the checked state of the checkbox, fires the 'check' event, and calls a
         * <code>{@link #handler}</code> (if configured).
         * @param {Boolean/String} checked The following values will check the checkbox:
         * <code>true, 'true', '1', or 'on'</code>. Any other value will uncheck the checkbox.
         * @return {Ext.form.Field} this
         */
        setValue : function(v){
            var checked = this.checked,
                inputVal = this.inputValue;

            if (v === false) {
                this.checked = false;
            } else {
                this.checked = (v === true || v === 'true' || v == '1' || (inputVal ? v == inputVal : String(v).toLowerCase() == 'on'));
            }

            if(this.rendered){
                this.el.dom.checked = this.checked;
                this.el.dom.defaultChecked = this.checked;
            }
            if(checked != this.checked){
                this.fireEvent('check', this, this.checked);
                if(this.handler){
                    this.handler.call(this.scope || this, this, this.checked);
                }
            }
            return this;
        }
    });
    Ext.reg('checkbox', Ext.form.Checkbox);