那么如何增加和减少maxLength
是的，您可以使用textfield.inputEl在focusenter和foucuseleave事件上更改。

textfield.inputEl.dom.maxLength=15//Your dynamic value here

在这个Fiddle中，我使用相同的textfield.inputEl创建了一个演示。

代码片段

Ext.application({
    name: 'Fiddle',

    launch: function () {

        function stringInsert(str, n, joinwith) {
            var ret = [],
                i = 0,
                len = str.length;;

            for (; i < len; i += n) {
                ret.push(str.substr(i, n))
            }

            return ret.join(joinwith)
        };

        Ext.create({
            xtype: 'form',
            title: 'Demo Form',
            bodyPadding: 15,
            renderTo: Ext.getBody(),
            items: [{
                xtype: 'textfield',
                name: 'email',
                fieldLabel: 'Email Address',
                vtype: 'email', // requires value to be a valid email address format,
                listeners: {
                    focusenter: function (field) {
                        var value = field.getValue().replace(/\./g, '');
                        field.setValue(value);
                        field.inputEl.dom.maxLength=15;
                    },
                    focusleave: function (field, event, eOpts) {
                        var value = this.getValue(),
                            newValue = stringInsert(value, 3, '.');

                        field.inputEl.dom.maxLength=newValue.length;
                        field.setValue(newValue);
                    }
                }
            }]
        })
    }
});