Webdynpro provides three types of Generic Value Help Services which help us in creating value help for constants eg for Countries, for airlines, for Zip codes etc.1. Simple Value Selector (SVS): This Value help is used as a dropdown list box to display a set of constants when you bind a DropDownByKey UI element to a value attribute of the type Simple Type. This is generally used when the dropdown list is not too big and is generally not more than 30 entries. 
2. Extended Value Selector: If the dropdown list box is too long due to the large number of constant values in a simple data type, you should use the extended value selector (EVS). This value selector can display a large set of constants within a specific dialog box below a regular input field. 
3. Object Value Selected (OVS): A generic OVS user interface provides all functionality for entering and executing a search query and copying values of a selected object back to one or many input fields.
OVS Implementation can be done in three steps:Step1: Adding an OVS Extension to the Context The Webdynpro programming model API provides the WDValueServices class for adding OVS Extension to context attribute. This context attribute is bound to the input field for which we want to have OVS. Step2:Implementing the OVS Custom Controller:In the OVS Custom Controller a node of cardinality 1..1 is declared which is used to store the input data for query. Another node of Cardinality 0..n is then declared where the result of the query is stored. If the suitable RFC function is used, we have to declare the model binding in that case. Step3: Implementing the interface IWDOVSContextNotificationListener: This interface implements three hook methods as follows- a) onQuery method: This method is called after we give the input parameters for the search. In case we are using a RFC, a call to this RFC is made in this method. b) applyResult method: The query search returns a list of output based upon the input given. The value selected by the user from this list is then applied to the corresponding context attribute bound to this input method. c) applyInputValues method: Here we set the initial default value for the search if required. Value Help in Web Dynpro Application Tutorial and Advanced Value Help - OVS Tutorial give better insight into the above mentioned generic value help services along with relevant example and code. Those interested can quickly go through the tutorial for better understanding of the above mentioned brief concepts.
2. Extended Value Selector: If the dropdown list box is too long due to the large number of constant values in a simple data type, you should use the extended value selector (EVS). This value selector can display a large set of constants within a specific dialog box below a regular input field. 3. Object Value Selected (OVS): A generic OVS user interface provides all functionality for entering and executing a search query and copying values of a selected object back to one or many input fields. OVS Implementation can be done in three steps:Step1: Adding an OVS Extension to the Context The Webdynpro programming model API provides the WDValueServices class for adding OVS Extension to context attribute. This context attribute is bound to the input field for which we want to have OVS. Step2:Implementing the OVS Custom Controller:In the OVS Custom Controller a node of cardinality 1..1 is declared which is used to store the input data for query. Another node of Cardinality 0..n is then declared where the result of the query is stored. If the suitable RFC function is used, we have to declare the model binding in that case. Step3: Implementing the interface IWDOVSContextNotificationListener: This interface implements three hook methods as follows- a) onQuery method: This method is called after we give the input parameters for the search. In case we are using a RFC, a call to this RFC is made in this method. b) applyResult method: The query search returns a list of output based upon the input given. The value selected by the user from this list is then applied to the corresponding context attribute bound to this input method. c) applyInputValues method: Here we set the initial default value for the search if required. Value Help in Web Dynpro Application Tutorial and Advanced Value Help - OVS Tutorial give better insight into the above mentioned generic value help services along with relevant example and code. Those interested can quickly go through the tutorial for better understanding of the above mentioned brief concepts.
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