ZK in Action [1] : MVVM - Form Binding

This is the second episode in our efforts to build a ZK application from the ground up. The previous post dealt with loading and rendering of data into a table using MVVM. In this post, we'll be introduced to ZK MVVM's form binding.

Objective

We'll build an "Add" function that would enable us to save new entries to the inventory.

A Form Appears When "Add" is Clicked

New Entry is Added When "Save" is Clicked

ZK Features in Action

• MVVM : Save, Form Binding, Conditional Binding

Add New Entries with MVVM Form Binding

we'll need to implement these parts:

• Enhance our ViewModel POJO
• Add UI markup to present a form and decorate the markup with the appropriate annotations

The ViewModel Class

public class InventoryVM {

    private List<item> items;
    private Item newItem;
 
    @NotifyChange("newItem")
    @Command
    public void createNewItem(){
        newItem = new Item("", "",0, 0,new Date());
    }
 
    @NotifyChange({"newItem","items"})
    @Command
    public void saveItem() throws Exception{
        DataService.getInstance().saveItem(newItem);
        newItem = null;
        items = getItems();
    }
  
    @NotifyChange("newItem")
    @Command
    public void cancelSave() throws Exception{
        newItem = null;
    }
 
    public List<item> getItems() throws Exception{
        items = DataService.getInstance().getAllItems();
        return items;
        }
    }

• Line 4, we declare an Item object named newItem which will reference the Item instance to be saved to the database.
• Line 6, @NotifyChange informs the binder to update the UI on the state of the associated ViewModel's property.
  In our UI markup shown below, at line 8, we have a Groupbox annotated with visible="@load(not empty vm.newItem), hence the Groupbox will become visible once createNewItem assigns an instance of Item to newItem.
  Simply put, @NotifyChange refreshes the UI with respect to the updates on ViewModel's properties.
• Line 7, we annotate the createNewItem method with @Command and in our UI markup shown below, at line 4, we have a Toolbarbutton with onClick="@commnad(createNewItem)". So when the Toolbarbutton is clicked, the createNewItem method will be invoked.
  
• Similarly, from line 12 to 18, we have a saveItem method which is called when its corresponding onClick event is triggered. Once the new Item object is saved to the database cache, we reset the newItem to null and retrieve the new list of items. The changes made to the ViewModel properties newItem (now null again) and items (now with an extra entry) are reflected to the UI using @NotifyChange as before.

The Markup

<window apply="org.zkoss.bind.BindComposer" 
 viewModel="@id('vm') @init('lab.sphota.zk.ctrl.InventoryVM')">
<toolbar>
 <toolbarbutton label="Add" onClick="@command('createNewItem')" />
</toolbar>
<groupbox form="@id('itm') @load(vm.newItem) 
        @save(vm.newItem, before='saveItem')"
 visible="@load(not empty vm.newItem)">
 <caption label="New Item"></caption>
 <grid width="50%">
  <rows>
   <row>
    <label value="Item Name" width="100px"></label>
    <textbox id="name" value="@bind(itm.name)" />
   </row>
   <row>
    <label value="Model" width="100px"></label>
    <textbox value="@bind(itm.model)" />
   </row>
   <row>
    <label value="Unit Price" width="100px"></label>
    <decimalbox value="@bind(itm.price)" format="#,###.00"
     constraint="no empty, no negative" />
   </row>
   <row>
    <label value="Quantity" width="100px"></label>
    <spinner value="@bind(itm.qty)"
     constraint="no empty,min 0 max 999: 
    Quantity Must be Greater Than Zero" />
   </row>
   <row>
    <cell colspan="2" align="center">
     <button width="80px" label="Save"
      onClick="@command('saveItem')" mold="trendy" />
     <button width="80px" label="Cancel"
      onClick="@command('cancelSave')" mold="trendy" />
    </cell>
   </row>
  </rows>
 </grid>
</groupbox>
<listbox>
...
</listbox>
</window>

• Line 1, we apply ZK's default implementation of its BindComposer. It is responsible for instantiating our ViewModel and Binder instances.
• Line 2, we supply the full class name of the ViewModel we wish to instantiate and give it an ID for future reference
• Line 4, we assign our ViewModel's "command method" createNewItem as the onClick event handler for the toolbar button.
• Line 6, the property newItem in ViewModel is made referenceable throughout the Groupbox using the ID "itm".
• Line 6,7, by using form binding, to avoid invalid or incomplete data saved to the ViewModel property, entries in the form are saved to a temporary object until the command method saveItem is called.
  
• Line 8, we show the Groupbox to enter a new Item entry only user has clicked the "Add" button; which in turn invokes createNewItem method and assigns the VM property newItem an instance of Item with default value(empty strings and 0s).
  
• Line 14, 18, 22, 27, we bind the Item properties with the input elements. @bind is effectively equivalent to @load plus @save.

In a Nuteshell

To sum up in point form:

• Using form binding avoids directly modifying data in ViewModel properties by saving form entries to a temporary object. Data is written to the ViewModel properties only if the condition specified is satisfied; in our example, only if the saveItem method is invoked. 
•  @Command annotation allows the binder to map UI event handlers to ViewModel command methods.
• @NotifyChange informs the binder which ViewModel properties had been modified after the command method is executed so changes in data can then be reflected on the UI.
• We can assign values to any of UI components' attributes at run-time via MVVM binding to manipulate parameters such as visibility, style, disable/enable, etc.

In this post, we've not seen how data entries are validated. Before that, we'll implement the delete and edit functionalities in the next post.

Reference

ZK Developer Reference

ZK in Action [0] : MVVM - Load and Render Data

A previous post had briefly introduced the RIA framework ZK and how its CSS Selector inspired controller mechanism alleviates some of the burdens that comes with UI changes by making the task of referencing UI components in the controller class a relatively flexible affair.

We then explored how the MVVM patterns in ZK allows a single ViewModel to serve different views in the last post.

This post marks the beginning of a series of posts that will go through steps in building a simple application from the ground up using ZK.

Objective

For now, we'll build a simple inventory management feature which is limited only to the loading and rendering of a data collection from a database into a table.

ZK Features in Action

• MVVM : Load
• Template Tag

Load and Render Data into a Table with MVVM

Assume there's a collection of objects named "Item" and there's a DataService class which takes care of caching and communication with the database (MongoDB and Morphia).

@Entity("items")
public class Item {
 @Id
 private ObjectId id;
 
 private String name;
 private String model;
 private int qty;
 private float price;
 private Date datemod;
 
        // getters & setters

To render data into a table as shown below in ZK, we'll need to implement these parts:

• A POJO that will serve as our ViewModel
• A ZK markup file as our presentation

The ViewModel Class

public class InventoryVM {

    private List<item> items;
 
    public List<item> getItems() throws Exception{
        items = DataService.getInstance().getAllItems();
        return items;
        }
    }

• Line 3,  the list of items needs to be declared as a property of the VM class
• Line 5, we need to provide a getter method so the Binder can retrieve the list of items. To recap, the Binder holds reference to the UI components and the ViewModel so it can keep data on both sides in sync as well as call command methods in ViewModel as events are triggered in View.

The Markup

<window apply="org.zkoss.bind.BindComposer" 
 viewModel="@id('vm') @init('lab.sphota.zk.ctrl.InventoryVM')">
 <listbox model="@load(vm.items) ">
  <listhead>
   <listheader label="Name" />
   <listheader label="Model" />
   <listheader label="Quantity" />
   <listheader label="Unit Price"/>
   <listheader label="Last Modified" />
  </listhead>
  <template name="model" var="item" >
   <listitem>
    <listcell>
     <textbox value="@load(item.name)" inplace="true" />
    </listcell>
    <listcell>
     <textbox value="@load(item.model)" inplace="true" />
    </listcell>
    <listcell>
     <spinner value="@load(item.qty)"  inplace="true" />
    </listcell>
    <listcell>
     <decimalbox value="@load(item.price)" inplace="true" 
     format="#,###.00"/>
    </listcell>
    <listcell label="@load(item.datemod)" />
   </listitem>
  </template>
 </listbox>
</window>

• Line 1, we apply ZK's default implementation of its BindComposer. It is responsible for instantiating our VM instance as well as the Binder instance.
• Line 2, we supply the full class name of the ViewModel we wish to instantiate and give it an ID (in this case, 'vm') for future reference
• Line 3, we assign a data model, which we made as a property of our ViewModel instance, to the Listbox.
• Line 11, we instruct the Template component to iterate through the given collection. We also declare a variable called "item" which will iteratively take on each Item object inside our collection. Alternatively, we can omit the variable declaration and use the keyword "each" to reference the data object (Item).
• Line 14, 17, 20, 23, 26, we retrieve the Item properties which we'd like to be displayed in the Listbox.
• Here we use input elements (Textbox, Spinner, Decimalbox) inside the Listcells in anticipation of our future implementation of an editable table. The attribute "inplace=true" will render these input elements as regular labels while they're not selected.

Wrap Up

ZK Binder is central to the workings of ZK MVVM. It holds references to both the UI components and the ViewModel. The ViewModel class is just a POJO where we declare and assign our data models. It exposes getter methods so Binder can retrieve and bind data to their respective annotated UI components. The template tag then allows us to iteratively render UI components with respect to the data model. In our case, a row of 5 Listcells with each cell holding a bean property is rendered iteratively through the bean collection using the template tag.

In the next post, we'll implement an "Add" feature so we can save new entries to our existing inventory using MVVM's form binding.

Reference

ZK Developer Reference