JGrapes

By Michael N. Lipp

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Overview

JGrapes

Web Console

JGrapes Web Console

Overview

The server side of the web console consists of several JGrapes components that drive a single page application (SPA) on the server. The design is highly modular and allows the adaption of the web console to different purposes.

Web Console Demo

A JGrapes Web Console consists —from the user’s point of view— of a fixed frame with configurable content. The frame provides some means to add content (typically by using a dropdown menu) and to configure global settings such as the locale.

The content of the frame is provided by web console display components or “conlets” for short. These components typically support a summary or preview display that can be put on an overview panel in a dashboard style and a large view that is supposed to fill the complete frame.

Tabs or a menu in a side bar can be used to switch between the overview panel(s) and the large views of the different conlets.

The architecture of the server side is explained in detail in the package description of the base component. The additional information provided here focuses on the SPA in the browser and on how to build your own console and additional conlets.

SPA Frame

The SPA frame is provided by a class derived from ConsoleWeblet. If you like (or can live with) the Freemarker template engine, you should use FreeMarkerConsoleWeblet as base class. Using the latter class, all you have to do is implement the constructor and provide the required templates.

The project currently includes three sample SPA providers:

Styling Conlets

At least for simple conlets, it should be possible to use them in differently styled consoles. This requirement implies that conlets are styled independent of a particular CSS framework.

Traditionally, CSS frameworks are “invasive” in the sense that the framework’s presentation classes (and even worse, additional divs) spread all over your HTML. Only a few “lightweight frameworks” such as Picnic base their styling on the native HTML. The problem is that even semantic HTML 5 doesn’t provide enough context to reliably style GUI widgets. If however, you add WAI-ARIA attributes to the markup (as you should anyway), it turns out that almost all styling can be based on the HTML without adding presentation classes.

Using “ARIA augmented semantic HTML” is therefore the preferred approach for authoring conlets. Combined with a web console’s CSS stylesheet that uses rules based on this kind of content this approach should lead to satisfactory results in typical cases.

Implementation Notes

Dynamic modularity for the SPA

Web applications are nowadays mostly developed as modular application. However, the modularity focuses on the code base. The various modules are then bundled by some tool and provided as monolithic resources. Optimization steps in this process may even remove JavaScript code from libraries if analysis shows that it isn’t invoked, thus making libraries only partially available.

The JGrapes web console objectives include the support for dynamic addition of conlets. Adding a conlet to a running system may require adding resources on the server side as well as in the SPA. The server side can easily be handled by a framework such as OSGi. Support in the SPA turns out to be a bit more difficult to implement.

Dynamically adding CSS

This can be implemented by adding additional link nodes to the head node in the DOM with JavaScript. The added links cause the browser to load the respective style sheets. Style sheets are applied as they become available, so the asynchronous loading may, in the worst case, result in a visible change of the pages’ appearance after its initial display.

Adding link nodes in the SPA is triggered on the server side by firing AddPageResources events.

Dynamically adding JavaScript

If everybody used ES6 modules, this wouldn’t be a problem either. An import statement in ES6 JavaScript causes the interpreter to block until the required module has been loaded. If ES6 modules aren’t used, we have to resort to adding a script node to the head node in the DOM. In this case the application has to make sure that required resources are loaded before the requiring JavaScript.

In the JGrapes web console, the necessary dependency tracking and ordered insertion of the script nodes is handled by a class that obtains the required information from ScriptResource instances as described in AddPageResources.

To be continued