Communication between ViewModel and Composer

To get an in-depth understanding of the following paragraphs, it is recommended that you have read and understood the concept of global command and global command binding prior reading this article.

The global command binding usage section demonstrates how you can communicate between multiple ViewModels. We can also use the same mechanism to perform communication between a composer and a ViewModel, but of course, usage is a little different.

Posting a Command from a Composer to a ViewModel

We know that global command is triggered by binder sending events into event queue, hence, ViewModels attached with binders are able to communicate with each other by global command. However, a composer doesn't have a binder to send a global command, therefore, we provide a utility class, BindUtils, that can do this job. You can use it in a composer.

For example, after adding a product, you want to tell ShoppingCartViewModel to refresh shopping cart's items. Assuming that we do not change the default setting such that ShoppingCartViewModel's binder (receiver) subscribes to the default event queue.

Send a global command in a composer (Sender)

public class MyComposer extends SelectorComposer {

    @Listen("onAddProductOrder=#PrdoDiv #prodGrid row productOrder")
    public void addProduct(Event fe) {

        //business logic of adding a product

        BindUtils.postGlobalCommand(null, null, "updateShoppingCart", null);
  • We use BindUtils.postGlobalCommand(String queueName, String queueScope, String cmdName, Map<java.lang.String,java.lang.Object> args) to post a command. We leave first two arguments as "null" to use default queue name and default scope (desktop). The third arguments is global command's name. You can send extra parameters with a Map by the fourth argument.
  • The specified global command is then executed by all ViewModel's binders that have subscribed to the event queue in the same (desktop) scope.

In the ShoppingCartViewModel, we should declare a global command method named updateShoppingCart to receive this command request and refresh cart items. The code snippet below shows this.

Global command in a ViewModel (Receiver)

public class ShoppingCartViewModel {
    public void updateShoppingCart() {
        //update shopping cart

    // rest of the class body

Posting a Command from a ViewModel to a Composer

As a ViewModel has a binder attached to it, triggering a global command doesn't need BindUtils, we can simply use global command binding.

Assuming that we want to inform a composer to update shopping cart's items.

Bind global command in a ZUL (Sender)

<window apply="org.zkoss.bind.BindComposer" binder="@init(queueName='myqueue')"
    viewModel="@id('vm') @init('example.MyViewModel')">
    <button id="addProduct" label="Add" onClick="@global-command('updateShoppingCart')"/>
  • We set the queue name as 'myqueue' in which the binder publishes to.

As mentioned earlier, global command is sent by event queue, the composer (receiver) should subscribe to the same scope event queue to receive this global command.

To subscribe global command (Receiver) to the event queue

public class MyComposesr extends SelectorComposer<Component> {

    public void updateShoppingCart(Event evt) {
        if (evt instanceof GlobalCommandEvent) {
            if ("updateShoppingCart".equals(((GlobalCommandEvent)evt).getCommand())) {
                //update shopping cart's items