The source code

    /**
     * @class Ext.form.Field
     * @extends Ext.BoxComponent
     * Base class for form fields that provides default event handling, sizing, value handling and other functionality.
     * @constructor
     * Creates a new Field
     * @param {Object} config Configuration options
     * @xtype field
     */
    Ext.form.Field = Ext.extend(Ext.BoxComponent,  {
        /**
         * <p>The label Element associated with this Field. <b>Only available after this Field has been rendered by a
         * {@link form Ext.layout.FormLayout} layout manager.</b></p>
         * @type Ext.Element
         * @property label
         */
        /**
         * @cfg {String} inputType The type attribute for input fields -- e.g. radio, text, password, file (defaults
         * to 'text'). The types 'file' and 'password' must be used to render those field types currently -- there are
         * no separate Ext components for those. Note that if you use <tt>inputType:'file'</tt>, {@link #emptyText}
         * is not supported and should be avoided.
         */
        /**
         * @cfg {Number} tabIndex The tabIndex for this field. Note this only applies to fields that are rendered,
         * not those which are built via applyTo (defaults to undefined).
         */
        /**
         * @cfg {Mixed} value A value to initialize this field with (defaults to undefined).
         */
        /**
         * @cfg {String} name The field's HTML name attribute (defaults to '').
         * <b>Note</b>: this property must be set if this field is to be automatically included with
         * {@link Ext.form.BasicForm#submit form submit()}.
         */
        /**
         * @cfg {String} cls A custom CSS class to apply to the field's underlying element (defaults to '').
         */

        /**
         * @cfg {String} invalidClass The CSS class to use when marking a field invalid (defaults to 'x-form-invalid')
         */
        invalidClass : 'x-form-invalid',
        /**
         * @cfg {String} invalidText The error text to use when marking a field invalid and no message is provided
         * (defaults to 'The value in this field is invalid')
         */
        invalidText : 'The value in this field is invalid',
        /**
         * @cfg {String} focusClass The CSS class to use when the field receives focus (defaults to 'x-form-focus')
         */
        focusClass : 'x-form-focus',
        /**
         * @cfg {Boolean} preventMark
         * <tt>true</tt> to disable {@link #markInvalid marking the field invalid}.
         * Defaults to <tt>false</tt>.
         */
        /**
         * @cfg {String/Boolean} validationEvent The event that should initiate field validation. Set to false to disable
          automatic validation (defaults to 'keyup').
         */
        validationEvent : 'keyup',
        /**
         * @cfg {Boolean} validateOnBlur Whether the field should validate when it loses focus (defaults to true).
         */
        validateOnBlur : true,
        /**
         * @cfg {Number} validationDelay The length of time in milliseconds after user input begins until validation
         * is initiated (defaults to 250)
         */
        validationDelay : 250,
        /**
         * @cfg {String/Object} autoCreate <p>A {@link Ext.DomHelper DomHelper} element spec, or true for a default
         * element spec. Used to create the {@link Ext.Component#getEl Element} which will encapsulate this Component.
         * See <tt>{@link Ext.Component#autoEl autoEl}</tt> for details.  Defaults to:</p>
         * <pre><code>{tag: 'input', type: 'text', size: '20', autocomplete: 'off'}</code></pre>
         */
        defaultAutoCreate : {tag: 'input', type: 'text', size: '20', autocomplete: 'off'},
        /**
         * @cfg {String} fieldClass The default CSS class for the field (defaults to 'x-form-field')
         */
        fieldClass : 'x-form-field',
        /**
         * @cfg {String} msgTarget <p>The location where the message text set through {@link #markInvalid} should display.
         * Must be one of the following values:</p>
         * <div class="mdetail-params"><ul>
         * <li><code>qtip</code> Display a quick tip containing the message when the user hovers over the field. This is the default.
         * <div class="subdesc"><b>{@link Ext.QuickTips#init Ext.QuickTips.init} must have been called for this setting to work.</b></div</li>
         * <li><code>title</code> Display the message in a default browser title attribute popup.</li>
         * <li><code>under</code> Add a block div beneath the field containing the error message.</li>
         * <li><code>side</code> Add an error icon to the right of the field, displaying the message in a popup on hover.</li>
         * <li><code>[element id]</code> Add the error message directly to the innerHTML of the specified element.</li>
         * </ul></div>
         */
        msgTarget : 'qtip',
        /**
         * @cfg {String} msgFx <b>Experimental</b> The effect used when displaying a validation message under the field
         * (defaults to 'normal').
         */
        msgFx : 'normal',
        /**
         * @cfg {Boolean} readOnly <tt>true</tt> to mark the field as readOnly in HTML
         * (defaults to <tt>false</tt>).
         * <br><p><b>Note</b>: this only sets the element's readOnly DOM attribute.
         * Setting <code>readOnly=true</code>, for example, will not disable triggering a
         * ComboBox or DateField; it gives you the option of forcing the user to choose
         * via the trigger without typing in the text box. To hide the trigger use
         * <code>{@link Ext.form.TriggerField#hideTrigger hideTrigger}</code>.</p>
         */
        readOnly : false,
        /**
         * @cfg {Boolean} disabled True to disable the field (defaults to false).
         * <p>Be aware that conformant with the <a href="http://www.w3.org/TR/html401/interact/forms.html#h-17.12.1">HTML specification</a>,
         * disabled Fields will not be {@link Ext.form.BasicForm#submit submitted}.</p>
         */
        disabled : false,
        /**
         * @cfg {Boolean} submitValue False to clear the name attribute on the field so that it is not submitted during a form post.
         * Defaults to <tt>true</tt>.
         */
        submitValue: true,

        // private
        isFormField : true,

        // private
        msgDisplay: '',

        // private
        hasFocus : false,

        // private
        initComponent : function(){
            Ext.form.Field.superclass.initComponent.call(this);
            this.addEvents(
                /**
                 * @event focus
                 * Fires when this field receives input focus.
                 * @param {Ext.form.Field} this
                 */
                'focus',
                /**
                 * @event blur
                 * Fires when this field loses input focus.
                 * @param {Ext.form.Field} this
                 */
                'blur',
                /**
                 * @event specialkey
                 * Fires when any key related to navigation (arrows, tab, enter, esc, etc.) is pressed.
                 * To handle other keys see {@link Ext.Panel#keys} or {@link Ext.KeyMap}.
                 * You can check {@link Ext.EventObject#getKey} to determine which key was pressed.
                 * For example: <pre><code>
    var form = new Ext.form.FormPanel({
        ...
        items: [{
                fieldLabel: 'Field 1',
                name: 'field1',
                allowBlank: false
            },{
                fieldLabel: 'Field 2',
                name: 'field2',
                listeners: {
                    specialkey: function(field, e){
                        // e.HOME, e.END, e.PAGE_UP, e.PAGE_DOWN,
                        // e.TAB, e.ESC, arrow keys: e.LEFT, e.RIGHT, e.UP, e.DOWN
                        if (e.{@link Ext.EventObject#getKey getKey()} == e.ENTER) {
                            var form = field.ownerCt.getForm();
                            form.submit();
                        }
                    }
                }
            }
        ],
        ...
    });
                 * </code></pre>
                 * @param {Ext.form.Field} this
                 * @param {Ext.EventObject} e The event object
                 */
                'specialkey',
                /**
                 * @event change
                 * Fires just before the field blurs if the field value has changed.
                 * @param {Ext.form.Field} this
                 * @param {Mixed} newValue The new value
                 * @param {Mixed} oldValue The original value
                 */
                'change',
                /**
                 * @event invalid
                 * Fires after the field has been marked as invalid.
                 * @param {Ext.form.Field} this
                 * @param {String} msg The validation message
                 */
                'invalid',
                /**
                 * @event valid
                 * Fires after the field has been validated with no errors.
                 * @param {Ext.form.Field} this
                 */
                'valid'
            );
        },

        /**
         * Returns the {@link Ext.form.Field#name name} or {@link Ext.form.ComboBox#hiddenName hiddenName}
         * attribute of the field if available.
         * @return {String} name The field {@link Ext.form.Field#name name} or {@link Ext.form.ComboBox#hiddenName hiddenName}
         */
        getName : function(){
            return this.rendered && this.el.dom.name ? this.el.dom.name : this.name || this.id || '';
        },

        // private
        onRender : function(ct, position){
            if(!this.el){
                var cfg = this.getAutoCreate();

                if(!cfg.name){
                    cfg.name = this.name || this.id;
                }
                if(this.inputType){
                    cfg.type = this.inputType;
                }
                this.autoEl = cfg;
            }
            Ext.form.Field.superclass.onRender.call(this, ct, position);
            if(this.submitValue === false){
                this.el.dom.removeAttribute('name');
            }
            var type = this.el.dom.type;
            if(type){
                if(type == 'password'){
                    type = 'text';
                }
                this.el.addClass('x-form-'+type);
            }
            if(this.readOnly){
                this.setReadOnly(true);
            }
            if(this.tabIndex !== undefined){
                this.el.dom.setAttribute('tabIndex', this.tabIndex);
            }

            this.el.addClass([this.fieldClass, this.cls]);
        },

        // private
        getItemCt : function(){
            return this.itemCt;
        },

        // private
        initValue : function(){
            if(this.value !== undefined){
                this.setValue(this.value);
            }else if(!Ext.isEmpty(this.el.dom.value) && this.el.dom.value != this.emptyText){
                this.setValue(this.el.dom.value);
            }
            /**
             * The original value of the field as configured in the {@link #value} configuration, or
             * as loaded by the last form load operation if the form's {@link Ext.form.BasicForm#trackResetOnLoad trackResetOnLoad}
             * setting is <code>true</code>.
             * @type mixed
             * @property originalValue
             */
            this.originalValue = this.getValue();
        },

        /**
         * <p>Returns true if the value of this Field has been changed from its original value.
         * Will return false if the field is disabled or has not been rendered yet.</p>
         * <p>Note that if the owning {@link Ext.form.BasicForm form} was configured with
         * {@link Ext.form.BasicForm}.{@link Ext.form.BasicForm#trackResetOnLoad trackResetOnLoad}
         * then the <i>original value</i> is updated when the values are loaded by
         * {@link Ext.form.BasicForm}.{@link Ext.form.BasicForm#setValues setValues}.</p>
         * @return {Boolean} True if this field has been changed from its original value (and
         * is not disabled), false otherwise.
         */
        isDirty : function() {
            if(this.disabled || !this.rendered) {
                return false;
            }
            return String(this.getValue()) !== String(this.originalValue);
        },

        /**
         * Sets the read only state of this field.
         * @param {Boolean} readOnly Whether the field should be read only.
         */
        setReadOnly : function(readOnly){
            if(this.rendered){
                this.el.dom.readOnly = readOnly;
            }
            this.readOnly = readOnly;
        },

        // private
        afterRender : function(){
            Ext.form.Field.superclass.afterRender.call(this);
            this.initEvents();
            this.initValue();
        },

        // private
        fireKey : function(e){
            if(e.isSpecialKey()){
                this.fireEvent('specialkey', this, e);
            }
        },

        /**
         * Resets the current field value to the originally loaded value and clears any validation messages.
         * See {@link Ext.form.BasicForm}.{@link Ext.form.BasicForm#trackResetOnLoad trackResetOnLoad}
         */
        reset : function(){
            this.setValue(this.originalValue);
            this.clearInvalid();
        },

        // private
        initEvents : function(){
            this.mon(this.el, Ext.EventManager.getKeyEvent(), this.fireKey,  this);
            this.mon(this.el, 'focus', this.onFocus, this);

            // standardise buffer across all browsers + OS-es for consistent event order.
            // (the 10ms buffer for Editors fixes a weird FF/Win editor issue when changing OS window focus)
            this.mon(this.el, 'blur', this.onBlur, this, this.inEditor ? {buffer:10} : null);
        },

        // private
        preFocus: Ext.emptyFn,

        // private
        onFocus : function(){
            this.preFocus();
            if(this.focusClass){
                this.el.addClass(this.focusClass);
            }
            if(!this.hasFocus){
                this.hasFocus = true;
                /**
                 * <p>The value that the Field had at the time it was last focused. This is the value that is passed
                 * to the {@link #change} event which is fired if the value has been changed when the Field is blurred.</p>
                 * <p><b>This will be undefined until the Field has been visited.</b> Compare {@link #originalValue}.</p>
                 * @type mixed
                 * @property startValue
                 */
                this.startValue = this.getValue();
                this.fireEvent('focus', this);
            }
        },

        // private
        beforeBlur : Ext.emptyFn,

        // private
        onBlur : function(){
            this.beforeBlur();
            if(this.focusClass){
                this.el.removeClass(this.focusClass);
            }
            this.hasFocus = false;
            if(this.validationEvent !== false && (this.validateOnBlur || this.validationEvent == 'blur')){
                this.validate();
            }
            var v = this.getValue();
            if(String(v) !== String(this.startValue)){
                this.fireEvent('change', this, v, this.startValue);
            }
            this.fireEvent('blur', this);
            this.postBlur();
        },

        // private
        postBlur : Ext.emptyFn,

        /**
         * Returns whether or not the field value is currently valid by
         * {@link #validateValue validating} the {@link #processValue processed value}
         * of the field. <b>Note</b>: {@link #disabled} fields are ignored.
         * @param {Boolean} preventMark True to disable marking the field invalid
         * @return {Boolean} True if the value is valid, else false
         */
        isValid : function(preventMark){
            if(this.disabled){
                return true;
            }
            var restore = this.preventMark;
            this.preventMark = preventMark === true;
            var v = this.validateValue(this.processValue(this.getRawValue()), preventMark);
            this.preventMark = restore;
            return v;
        },

        /**
         * Validates the field value
         * @return {Boolean} True if the value is valid, else false
         */
        validate : function(){
            if(this.disabled || this.validateValue(this.processValue(this.getRawValue()))){
                this.clearInvalid();
                return true;
            }
            return false;
        },

        /**
         * This method should only be overridden if necessary to prepare raw values
         * for validation (see {@link #validate} and {@link #isValid}).  This method
         * is expected to return the processed value for the field which will
         * be used for validation (see validateValue method).
         * @param {Mixed} value
         */
        processValue : function(value){
            return value;
        },

        /**
         * Uses getErrors to build an array of validation errors. If any errors are found, markInvalid is called
         * with the first and false is returned, otherwise true is returned. Previously, subclasses were invited
         * to provide an implementation of this to process validations - from 3.2 onwards getErrors should be
         * overridden instead.
         * @param {Mixed} The current value of the field
         * @return {Boolean} True if all validations passed, false if one or more failed
         */
         validateValue : function(value) {
             //currently, we only show 1 error at a time for a field, so just use the first one
             var error = this.getErrors(value)[0];

             if (error == undefined) {
                 return true;
             } else {
                 this.markInvalid(error);
                 return false;
             }
         },

        /**
         * Runs this field's validators and returns an array of error messages for any validation failures.
         * This is called internally during validation and would not usually need to be used manually.
         * Each subclass should override or augment the return value to provide their own errors
         * @return {Array} All error messages for this field
         */
        getErrors: function() {
            return [];
        },

        /**
         * Gets the active error message for this field.
         * @return {String} Returns the active error message on the field, if there is no error, an empty string is returned.
         */
        getActiveError : function(){
            return this.activeError || '';
        },

        /**
         * <p>Display an error message associated with this field, using {@link #msgTarget} to determine how to
         * display the message and applying {@link #invalidClass} to the field's UI element.</p>
         * <p><b>Note</b>: this method does not cause the Field's {@link #validate} method to return <code>false</code>
         * if the value does <i>pass</i> validation. So simply marking a Field as invalid will not prevent
         * submission of forms submitted with the {@link Ext.form.Action.Submit#clientValidation} option set.</p>
         * {@link #isValid invalid}.
         * @param {String} msg (optional) The validation message (defaults to {@link #invalidText})
         */
        markInvalid : function(msg){
            //don't set the error icon if we're not rendered or marking is prevented
            if (this.rendered && !this.preventMark) {
                msg = msg || this.invalidText;

                var mt = this.getMessageHandler();
                if(mt){
                    mt.mark(this, msg);
                }else if(this.msgTarget){
                    this.el.addClass(this.invalidClass);
                    var t = Ext.getDom(this.msgTarget);
                    if(t){
                        t.innerHTML = msg;
                        t.style.display = this.msgDisplay;
                    }
                }
            }

            this.setActiveError(msg);
        },

        /**
         * Clear any invalid styles/messages for this field
         */
        clearInvalid : function(){
            //don't remove the error icon if we're not rendered or marking is prevented
            if (this.rendered && !this.preventMark) {
                this.el.removeClass(this.invalidClass);
                var mt = this.getMessageHandler();
                if(mt){
                    mt.clear(this);
                }else if(this.msgTarget){
                    this.el.removeClass(this.invalidClass);
                    var t = Ext.getDom(this.msgTarget);
                    if(t){
                        t.innerHTML = '';
                        t.style.display = 'none';
                    }
                }
            }

            this.unsetActiveError();
        },

        /**
         * Sets the current activeError to the given string. Fires the 'invalid' event.
         * This does not set up the error icon, only sets the message and fires the event. To show the error icon,
         * use markInvalid instead, which calls this method internally
         * @param {String} msg The error message
         * @param {Boolean} suppressEvent True to suppress the 'invalid' event from being fired
         */
        setActiveError: function(msg, suppressEvent) {
            this.activeError = msg;
            if (suppressEvent !== true) this.fireEvent('invalid', this, msg);
        },

        /**
         * Clears the activeError and fires the 'valid' event. This is called internally by clearInvalid and would not
         * usually need to be called manually
         * @param {Boolean} suppressEvent True to suppress the 'invalid' event from being fired
         */
        unsetActiveError: function(suppressEvent) {
            delete this.activeError;
            if (suppressEvent !== true) this.fireEvent('valid', this);
        },

        // private
        getMessageHandler : function(){
            return Ext.form.MessageTargets[this.msgTarget];
        },

        // private
        getErrorCt : function(){
            return this.el.findParent('.x-form-element', 5, true) || // use form element wrap if available
                this.el.findParent('.x-form-field-wrap', 5, true);   // else direct field wrap
        },

        // Alignment for 'under' target
        alignErrorEl : function(){
            this.errorEl.setWidth(this.getErrorCt().getWidth(true) - 20);
        },

        // Alignment for 'side' target
        alignErrorIcon : function(){
            this.errorIcon.alignTo(this.el, 'tl-tr', [2, 0]);
        },

        /**
         * Returns the raw data value which may or may not be a valid, defined value.  To return a normalized value see {@link #getValue}.
         * @return {Mixed} value The field value
         */
        getRawValue : function(){
            var v = this.rendered ? this.el.getValue() : Ext.value(this.value, '');
            if(v === this.emptyText){
                v = '';
            }
            return v;
        },

        /**
         * Returns the normalized data value (undefined or emptyText will be returned as '').  To return the raw value see {@link #getRawValue}.
         * @return {Mixed} value The field value
         */
        getValue : function(){
            if(!this.rendered) {
                return this.value;
            }
            var v = this.el.getValue();
            if(v === this.emptyText || v === undefined){
                v = '';
            }
            return v;
        },

        /**
         * Sets the underlying DOM field's value directly, bypassing validation.  To set the value with validation see {@link #setValue}.
         * @param {Mixed} value The value to set
         * @return {Mixed} value The field value that is set
         */
        setRawValue : function(v){
            return this.rendered ? (this.el.dom.value = (Ext.isEmpty(v) ? '' : v)) : '';
        },

        /**
         * Sets a data value into the field and validates it.  To set the value directly without validation see {@link #setRawValue}.
         * @param {Mixed} value The value to set
         * @return {Ext.form.Field} this
         */
        setValue : function(v){
            this.value = v;
            if(this.rendered){
                this.el.dom.value = (Ext.isEmpty(v) ? '' : v);
                this.validate();
            }
            return this;
        },

        // private, does not work for all fields
        append : function(v){
             this.setValue([this.getValue(), v].join(''));
        }

        /**
         * @cfg {Boolean} autoWidth @hide
         */
        /**
         * @cfg {Boolean} autoHeight @hide
         */

        /**
         * @cfg {String} autoEl @hide
         */
    });

    Ext.form.MessageTargets = {
        'qtip' : {
            mark: function(field, msg){
                field.el.addClass(field.invalidClass);
                field.el.dom.qtip = msg;
                field.el.dom.qclass = 'x-form-invalid-tip';
                if(Ext.QuickTips){ // fix for floating editors interacting with DND
                    Ext.QuickTips.enable();
                }
            },
            clear: function(field){
                field.el.removeClass(field.invalidClass);
                field.el.dom.qtip = '';
            }
        },
        'title' : {
            mark: function(field, msg){
                field.el.addClass(field.invalidClass);
                field.el.dom.title = msg;
            },
            clear: function(field){
                field.el.dom.title = '';
            }
        },
        'under' : {
            mark: function(field, msg){
                field.el.addClass(field.invalidClass);
                if(!field.errorEl){
                    var elp = field.getErrorCt();
                    if(!elp){ // field has no container el
                        field.el.dom.title = msg;
                        return;
                    }
                    field.errorEl = elp.createChild({cls:'x-form-invalid-msg'});
                    field.on('resize', field.alignErrorEl, field);
                    field.on('destroy', function(){
                        Ext.destroy(this.errorEl);
                    }, field);
                }
                field.alignErrorEl();
                field.errorEl.update(msg);
                Ext.form.Field.msgFx[field.msgFx].show(field.errorEl, field);
            },
            clear: function(field){
                field.el.removeClass(field.invalidClass);
                if(field.errorEl){
                    Ext.form.Field.msgFx[field.msgFx].hide(field.errorEl, field);
                }else{
                    field.el.dom.title = '';
                }
            }
        },
        'side' : {
            mark: function(field, msg){
                field.el.addClass(field.invalidClass);
                if(!field.errorIcon){
                    var elp = field.getErrorCt();
                    // field has no container el
                    if(!elp){
                        field.el.dom.title = msg;
                        return;
                    }
                    field.errorIcon = elp.createChild({cls:'x-form-invalid-icon'});
                    if (field.ownerCt) {
                        field.ownerCt.on('afterlayout', field.alignErrorIcon, field);
                        field.ownerCt.on('expand', field.alignErrorIcon, field);
                    }
                    field.on('resize', field.alignErrorIcon, field);
                    field.on('destroy', function(){
                        Ext.destroy(this.errorIcon);
                    }, field);
                }
                field.alignErrorIcon();
                field.errorIcon.dom.qtip = msg;
                field.errorIcon.dom.qclass = 'x-form-invalid-tip';
                field.errorIcon.show();
            },
            clear: function(field){
                field.el.removeClass(field.invalidClass);
                if(field.errorIcon){
                    field.errorIcon.dom.qtip = '';
                    field.errorIcon.hide();
                }else{
                    field.el.dom.title = '';
                }
            }
        }
    };

    // anything other than normal should be considered experimental
    Ext.form.Field.msgFx = {
        normal : {
            show: function(msgEl, f){
                msgEl.setDisplayed('block');
            },

            hide : function(msgEl, f){
                msgEl.setDisplayed(false).update('');
            }
        },

        slide : {
            show: function(msgEl, f){
                msgEl.slideIn('t', {stopFx:true});
            },

            hide : function(msgEl, f){
                msgEl.slideOut('t', {stopFx:true,useDisplay:true});
            }
        },

        slideRight : {
            show: function(msgEl, f){
                msgEl.fixDisplay();
                msgEl.alignTo(f.el, 'tl-tr');
                msgEl.slideIn('l', {stopFx:true});
            },

            hide : function(msgEl, f){
                msgEl.slideOut('l', {stopFx:true,useDisplay:true});
            }
        }
    };
    Ext.reg('field', Ext.form.Field);