在Extjs中以动态生成的形式进行验证

I'm doing eforms framework design, In this there will be multiple products available and each product will have different kind of forms. On drag and drop it'll appear in the form panel, these form panel data's are retreived from Json file. We have json file for each forms. If i want to add validations to those fields in forms means how can I accomplish this, bcoz form fields are available in json which will be dynamically generate on drag and drop. I'm doing eforms framework design, In this ther