如何在运行时在extJS中提供口令验证

Ext.application({
    name: 'MyApp',
    launch: function () {
        Ext.create('Ext.form.Panel', {
            title: 'Login',
            width: 300,
            bodyPadding: 10,
            renderTo: Ext.getBody(),
            items: [{
                xtype: 'textfield',
                fieldLabel: 'Username',
                name: 'username',
                allowBlank: false
            }, {
                xtype: 'textfield',
                fieldLabel: 'Password',
                name: 'password',
                inputType: 'password',
                allowBlank: false
            }],
            buttons: [{
                text: 'Login',
                handler: function () {
                    var form = this.up('form').getForm();
                    if (form.isValid()) {
                        form.submit({
                            url: 'backend/login', // 后端接口地址
                            success: function (form, action) {
                                Ext.Msg.alert('Success', action.result.message);
                            },
                            failure: function (form, action) {
                                Ext.Msg.alert('Failure', action.result.message);
                            }
                        });
                    }
                }
            }]
        });
    }
});