ZK Developer's Reference/Overture/Architecture Overview

From the Application Developer's Perspective

The ZK application runs on the server. It can access to backend resources, assemble UI with components, listen to user's activity, and then manipulate components to update UI. All of the above activities can be accomplished on the server. The synchronization of the states of the components between the browser and the server is done automatically by ZK and transparently to the application.

When running on the server, the application can access to full Java technology stacks. User activities, such as Ajax and Server Push, are abstracted to event objects. UI are composed by POJO-like components. It is the most productive approach to develop a modern Web application.

With ZK's Server+client Fusion architecture, your application will never stop running on the server. The application can enhance the interactivity by adding optional client-side functionality, such as client-side event handling, visual effect customizing or even UI composing without server-side coding. ZK enables seamless fusions ranging from pure server-centric to pure client-centric. You can have the best of two worlds: productivity and flexibility.

From the Component Developer's Perspective

Each UI object in ZK consists of a component and a widget. A component is a Java object running on the server representing a UI object which can be manipulated by a Java application. A component has all the behavior of a UI object except that it does not have a visual part. A widget is a JavaScript object running at the client. This object represents the UI object which interacts with the user. Therefore, a widget usually has a visual appearance and it handles events happening at the client.

The relationship between a component and a widget is one-to-one. However, if a component is not attached to a page, there will not be a corresponding widget at the client. On the other hand, the application is allowed to instantiate widgets at the client directly without a corresponding component.

How to synchronize the states and distribute the load is really up to the component. ZK Client Engine and Update Engine will work together to provide an elegant and robust channel to simplify the implementation.

For example, assume that we want to implement a component that allows a user to click on a DOM element to show some detailed information of a component. There are at least two approaches to implement it. Firstly, we could load detailed information to the client when the widget is instantiated, and then show the details with pure-client code. Alternatively, we may choose not to load detailed information from the beginning before sending a request back to the server for fulfilling the details when the user clicks on it.

Obviously, the first approach requires more bandwidth for the initial request but it also provides faster responses when users click on it. This is generally more transparent to the application developers, and the implementation of a component can be enhanced later as the project progresses.

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Execution Flow of Loading a Page

1. When a user types a URL or clicks a hyperlink in the browser, a request is sent to the Web server. If the request URL matches ZK's configured URL pattern, the ZK loader will be invoked to serve this request.
2. ZK loader loads a specified page and interprets it to create proper components accordingly.
3. After interpreting the whole page, ZK loader renders the result into a HTML page. The HTML page is then sent back to the browser accompanied with ZK Client Engine.
4. ZK Client engine renders the widgets into DOM elements and inserts the DOM elements into the browser's DOM tree to make them visible to users.
5. Then, ZK Client engine sits at the browser to serve the requests made by user, widgets or the application. If it goes to another page, the execution flow will start over again. If it is going to send a Ajax request back, another execution flow will start as described in the following section.

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Execution Flow of Serving an Ajax Request

1. The execution flow is started from a widget or the application. This is usually caused by the user's activity (or application's requirement) and is done by posting a client-side event (Event) to a widget (Widget.fire(Event, int)).
2. The event is then bubbled up to the widget's parent, parent's parent, and finally ZK Client engine. ZK Client engine then decides whether and when to send the event back to the server in an Ajax request.
3. If necessary, ZK Client engine will send an Ajax request to ZK Update engine on the server.
4. Upon receiving Ajax requests, ZK Update engine will invoke ComponentCtrl.service(AuRequest, boolean) for handling the request.
5. The handling is up to a component. But, it usually updates the states, if necessary, and then notifies the application by posting events (Events.postEvent(Event)).
6. If any event is posted, ZK Update engine will process them one-by-one by invoking the event listeners.
7. The event listener, provided by an application, may choose either to update the backend resources or the components or to post other events.
8. Finally, ZK Update engine collects all updates of components, including states change, attachment and detachment for optimization and then send back a collection of commands back to the client.
9. ZK Client engine evaluates each of these commands to update the widgets accordingly. Then the widgets will update the browser's DOM tree to make them available to the user.

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When to Send an Ajax Request

When ZK Client engine receives a bubbled-up client-side event (Event), it will decide whether and when to send it back to the server for further processing:

1. If there is a non-deferrable event listener registered on the server, the request will be sent immediately.
2. If there is a deferrable event listener registered on the server, the request will be queued at the client and it will be sent when another event is triggered and a non-deferrable event listener registered for it.
3. If the widget declares that the event is important (ComponentCtrl.CE_IMPORTANT), the event will be queued for later transmitting too.
4. If none of the above case or the widget has no corresponding component on the server, the event will be dropped.

A non-deferred event listener is an event listener (EventListener) that doesn't implement Deferrable. In other words, to minimize the traffic from the client, you might implement an event listener with Deferrable if applicable.

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