The source code

    /**
     * @class Ext.form.FormPanel
     * @extends Ext.Panel
     * <p>Standard form container.</p>
     *
     * <p><b><u>Layout</u></b></p>
     * <p>By default, FormPanel is configured with <tt>layout:'form'</tt> to use an {@link Ext.layout.FormLayout}
     * layout manager, which styles and renders fields and labels correctly. When nesting additional Containers
     * within a FormPanel, you should ensure that any descendant Containers which host input Fields use the
     * {@link Ext.layout.FormLayout} layout manager.</p>
     *
     * <p><b><u>BasicForm</u></b></p>
     * <p>Although <b>not listed</b> as configuration options of FormPanel, the FormPanel class accepts all
     * of the config options required to configure its internal {@link Ext.form.BasicForm} for:
     * <div class="mdetail-params"><ul>
     * <li>{@link Ext.form.BasicForm#fileUpload file uploads}</li>
     * <li>functionality for {@link Ext.form.BasicForm#doAction loading, validating and submitting} the form</li>
     * </ul></div>
     *
     * <p><b>Note</b>: If subclassing FormPanel, any configuration options for the BasicForm must be applied to
     * the <tt><b>initialConfig</b></tt> property of the FormPanel. Applying {@link Ext.form.BasicForm BasicForm}
     * configuration settings to <b><tt>this</tt></b> will <b>not</b> affect the BasicForm's configuration.</p>
     *
     * <p><b><u>Form Validation</u></b></p>
     * <p>For information on form validation see the following:</p>
     * <div class="mdetail-params"><ul>
     * <li>{@link Ext.form.TextField}</li>
     * <li>{@link Ext.form.VTypes}</li>
     * <li>{@link Ext.form.BasicForm#doAction BasicForm.doAction <b>clientValidation</b> notes}</li>
     * <li><tt>{@link Ext.form.FormPanel#monitorValid monitorValid}</tt></li>
     * </ul></div>
     *
     * <p><b><u>Form Submission</u></b></p>
     * <p>By default, Ext Forms are submitted through Ajax, using {@link Ext.form.Action}. To enable normal browser
     * submission of the {@link Ext.form.BasicForm BasicForm} contained in this FormPanel, see the
     * <tt><b>{@link Ext.form.BasicForm#standardSubmit standardSubmit}</b></tt> option.</p>
     *
     * @constructor
     * @param {Object} config Configuration options
     * @xtype form
     */
    Ext.FormPanel = Ext.extend(Ext.Panel, {
        /**
         * @cfg {String} formId (optional) The id of the FORM tag (defaults to an auto-generated id).
         */
        /**
         * @cfg {Boolean} hideLabels
         * <p><tt>true</tt> to hide field labels by default (sets <tt>display:none</tt>). Defaults to
         * <tt>false</tt>.</p>
         * <p>Also see {@link Ext.Component}.<tt>{@link Ext.Component#hideLabel hideLabel}</tt>.
         */
        /**
         * @cfg {Number} labelPad
         * The default padding in pixels for field labels (defaults to <tt>5</tt>). <tt>labelPad</tt> only
         * applies if <tt>{@link #labelWidth}</tt> is also specified, otherwise it will be ignored.
         */
        /**
         * @cfg {String} labelSeparator
         * See {@link Ext.Component}.<tt>{@link Ext.Component#labelSeparator labelSeparator}</tt>
         */
        /**
         * @cfg {Number} labelWidth The width of labels in pixels. This property cascades to child containers
         * and can be overridden on any child container (e.g., a fieldset can specify a different <tt>labelWidth</tt>
         * for its fields) (defaults to <tt>100</tt>).
         */
        /**
         * @cfg {String} itemCls A css class to apply to the x-form-item of fields. This property cascades to child containers.
         */
        /**
         * @cfg {Array} buttons
         * An array of {@link Ext.Button}s or {@link Ext.Button} configs used to add buttons to the footer of this FormPanel.<br>
         * <p>Buttons in the footer of a FormPanel may be configured with the option <tt>formBind: true</tt>. This causes
         * the form's {@link #monitorValid valid state monitor task} to enable/disable those Buttons depending on
         * the form's valid/invalid state.</p>
         */

        /**
         * @cfg {Number} minButtonWidth Minimum width of all buttons in pixels (defaults to <tt>75</tt>).
         */
        minButtonWidth : 75,

        /**
         * @cfg {String} labelAlign The label alignment value used for the <tt>text-align</tt> specification
         * for the <b>container</b>. Valid values are <tt>"left</tt>", <tt>"top"</tt> or <tt>"right"</tt>
         * (defaults to <tt>"left"</tt>). This property cascades to child <b>containers</b> and can be
         * overridden on any child <b>container</b> (e.g., a fieldset can specify a different <tt>labelAlign</tt>
         * for its fields).
         */
        labelAlign : 'left',

        /**
         * @cfg {Boolean} monitorValid If <tt>true</tt>, the form monitors its valid state <b>client-side</b> and
         * regularly fires the {@link #clientvalidation} event passing that state.<br>
         * <p>When monitoring valid state, the FormPanel enables/disables any of its configured
         * {@link #buttons} which have been configured with <code>formBind: true</code> depending
         * on whether the {@link Ext.form.BasicForm#isValid form is valid} or not. Defaults to <tt>false</tt></p>
         */
        monitorValid : false,

        /**
         * @cfg {Number} monitorPoll The milliseconds to poll valid state, ignored if monitorValid is not true (defaults to 200)
         */
        monitorPoll : 200,

        /**
         * @cfg {String} layout Defaults to <tt>'form'</tt>.  Normally this configuration property should not be altered.
         * For additional details see {@link Ext.layout.FormLayout} and {@link Ext.Container#layout Ext.Container.layout}.
         */
        layout : 'form',

        // private
        initComponent : function(){
            this.form = this.createForm();
            Ext.FormPanel.superclass.initComponent.call(this);

            this.bodyCfg = {
                tag: 'form',
                cls: this.baseCls + '-body',
                method : this.method || 'POST',
                id : this.formId || Ext.id()
            };
            if(this.fileUpload) {
                this.bodyCfg.enctype = 'multipart/form-data';
            }
            this.initItems();

            this.addEvents(
                /**
                 * @event clientvalidation
                 * If the monitorValid config option is true, this event fires repetitively to notify of valid state
                 * @param {Ext.form.FormPanel} this
                 * @param {Boolean} valid true if the form has passed client-side validation
                 */
                'clientvalidation'
            );

            this.relayEvents(this.form, ['beforeaction', 'actionfailed', 'actioncomplete']);
        },

        // private
        createForm : function(){
            var config = Ext.applyIf({listeners: {}}, this.initialConfig);
            return new Ext.form.BasicForm(null, config);
        },

        // private
        initFields : function(){
            var f = this.form;
            var formPanel = this;
            var fn = function(c){
                if(formPanel.isField(c)){
                    f.add(c);
                }else if(c.findBy && c != formPanel){
                    formPanel.applySettings(c);
                    //each check required for check/radio groups.
                    if(c.items && c.items.each){
                        c.items.each(fn, this);
                    }
                }
            };
            this.items.each(fn, this);
        },

        // private
        applySettings: function(c){
            var ct = c.ownerCt;
            Ext.applyIf(c, {
                labelAlign: ct.labelAlign,
                labelWidth: ct.labelWidth,
                itemCls: ct.itemCls
            });
        },

        // private
        getLayoutTarget : function(){
            return this.form.el;
        },

        /**
         * Provides access to the {@link Ext.form.BasicForm Form} which this Panel contains.
         * @return {Ext.form.BasicForm} The {@link Ext.form.BasicForm Form} which this Panel contains.
         */
        getForm : function(){
            return this.form;
        },

        // private
        onRender : function(ct, position){
            this.initFields();
            Ext.FormPanel.superclass.onRender.call(this, ct, position);
            this.form.initEl(this.body);
        },

        // private
        beforeDestroy : function(){
            this.stopMonitoring();
            this.form.destroy(true);
            Ext.FormPanel.superclass.beforeDestroy.call(this);
        },

        // Determine if a Component is usable as a form Field.
        isField : function(c) {
            return !!c.setValue && !!c.getValue && !!c.markInvalid && !!c.clearInvalid;
        },

        // private
        initEvents : function(){
            Ext.FormPanel.superclass.initEvents.call(this);
            // Listeners are required here to catch bubbling events from children.
            this.on({
                scope: this,
                add: this.onAddEvent,
                remove: this.onRemoveEvent
            });
            if(this.monitorValid){ // initialize after render
                this.startMonitoring();
            }
        },

        // private
        onAdd: function(c){
            Ext.FormPanel.superclass.onAdd.call(this, c);
            this.processAdd(c);
        },

        // private
        onAddEvent: function(ct, c){
            if(ct !== this){
                this.processAdd(c);
            }
        },

        // private
        processAdd : function(c){
            // If a single form Field, add it
            if(this.isField(c)){
                this.form.add(c);
            // If a Container, add any Fields it might contain
            }else if(c.findBy){
                this.applySettings(c);
                this.form.add.apply(this.form, c.findBy(this.isField));
            }
        },

        // private
        onRemove: function(c){
            Ext.FormPanel.superclass.onRemove.call(this, c);
            this.processRemove(c);
        },

        onRemoveEvent: function(ct, c){
            if(ct !== this){
                this.processRemove(c);
            }
        },

        // private
        processRemove: function(c){
            if(!this.destroying){
                // If a single form Field, remove it
                if(this.isField(c)){
                    this.form.remove(c);
                // If a Container, its already destroyed by the time it gets here.  Remove any references to destroyed fields.
                }else if (c.findBy){
                    Ext.each(c.findBy(this.isField), this.form.remove, this.form);
                    /*
                     * This isn't the most efficient way of getting rid of the items, however it's the most
                     * correct, which in this case is most important.
                     */
                    this.form.cleanDestroyed();
                }
            }
        },

        /**
         * Starts monitoring of the valid state of this form. Usually this is done by passing the config
         * option "monitorValid"
         */
        startMonitoring : function(){
            if(!this.validTask){
                this.validTask = new Ext.util.TaskRunner();
                this.validTask.start({
                    run : this.bindHandler,
                    interval : this.monitorPoll || 200,
                    scope: this
                });
            }
        },

        /**
         * Stops monitoring of the valid state of this form
         */
        stopMonitoring : function(){
            if(this.validTask){
                this.validTask.stopAll();
                this.validTask = null;
            }
        },

        /**
         * This is a proxy for the underlying BasicForm's {@link Ext.form.BasicForm#load} call.
         * @param {Object} options The options to pass to the action (see {@link Ext.form.BasicForm#doAction} for details)
         */
        load : function(){
            this.form.load.apply(this.form, arguments);
        },

        // private
        onDisable : function(){
            Ext.FormPanel.superclass.onDisable.call(this);
            if(this.form){
                this.form.items.each(function(){
                     this.disable();
                });
            }
        },

        // private
        onEnable : function(){
            Ext.FormPanel.superclass.onEnable.call(this);
            if(this.form){
                this.form.items.each(function(){
                     this.enable();
                });
            }
        },

        // private
        bindHandler : function(){
            var valid = true;
            this.form.items.each(function(f){
                if(!f.isValid(true)){
                    valid = false;
                    return false;
                }
            });
            if(this.fbar){
                var fitems = this.fbar.items.items;
                for(var i = 0, len = fitems.length; i < len; i++){
                    var btn = fitems[i];
                    if(btn.formBind === true && btn.disabled === valid){
                        btn.setDisabled(!valid);
                    }
                }
            }
            this.fireEvent('clientvalidation', this, valid);
        }
    });
    Ext.reg('form', Ext.FormPanel);

    Ext.form.FormPanel = Ext.FormPanel;