Dr. Dobb's Journal - March 2008 - (Page 56)

Discover a New World in Programming code, but are implemented at linktime or runtime. Wt Widgets: Interaction with Browsers Figure 2 illustrates Wt’s widget hierarchy. At the top, WWidget exposes methods for organizing the layout and look of each widget. Both the layout and visual aspects are defined in terms of CSS. A drawback is that the layout model of CSS was designed for the markup of documents, and therefore not always intuitive for designing UIs. Wt’s core widgets, which map directly onto HTML building blocks, all inherit from WWebWidget. WWebWidget implementations specify how to render widget changes in terms of the HTML Document Object Model (DOM), and expose all capabilities of these HTML elements. For example, HTML buttons are represented by class WPushButton, HTML text becomes WText, and HTML tables become class WTable. Properties of the HTML blocks (attributes and content) are turned into methods of the classes, named consistently throughout the library. Widgets whose corresponding HTML element supports interaction with the keyboard or mouse, are descendants of WInteractWidget, which exposes these events as Wt::Signal objects. Form control widgets that may receive keyboard focus and can be enabled/disabled are descendants of WFormWidget, and expose additional events. The library propagates information in two directions between the browser DOM and the server-side widget tree: When the server-side application modifies a widget attribute (color change, enable/disable, or modification of the widget tree), this change is rendered in the web browser (through AJAX, if available). In the other direction, any change or event initiated by users on the DOM is automatically reflected in the widget (such as changes to the edited text of an HTML text input field, which corresponds to a WLineEdit widget). Events are propagated using a signal/slot mechanism (which is a particular implementation of the Observer pattern). This is both for events initiated by user action, such as text changes, mouse Looking for a career with an innovative company where you can use your programming skills to make a difference in the world? Join ESRI and design and develop the next generation of our world-leading geographic information system (GIS) mapping software. We are seeking software developers with solid core programming skills and a passion for inventing new technology. We have opportunities to work on everything from database and Web development to graphics, 2D–3D rendering, and core server technology. Our developers work with the latest technologies including Java, .NET, and Flash/Flex. At ESRI you’ll have exceptional benefits, opportunities for growth, and the satisfaction of developing software that is making a difference. Join us and be a part of changing the world. Figure 1: A running Wt application. Learn more about ESRI and apply online at www.esri.com/ddj. clicks, or focus changes, as well as events initiated by application code. Next to WWebWidget, there is WCompositeWidget. While WWebWidgets expose as much as possible all capabilities available in their browser-side implementations, WCompositeWidgets implement advanced widget functionality whose implementation details are completely encapsulated. For example, WTreeNode, which defines a node in a tree hierarchy, is internally implemented using WImage and WText widgets laid out in a 2×2 WTable. This internal organization is not exposed by inheriting from WCompositeWidget, and can thus be changed at any point in the future. The look and organization of a new WCompositeWidget is specified by composition of existing widgets (which may be either WWebWidget or WCompositeWidgets), while the behavior is defined by connecting slots to various signals exposed by these widgets. New widgets are not limited to server-side event handling only, but can use the dynamic C++-to-JavaScript translation feature to bypass the server roundtrip for rendering visual updates (more on this later). For example, the WTreeNode uses this feature to implement client-side expand/collapse toggling. Deployment Architecture Communication over the HTTP protocol with a web browser is abstracted in Wt using a Connector implementation. Two Connector implementations are available in Wt 2.x. One connector uses the FastCGI protocol, which lets Wt applications run in conjunction with many web servers via FastCGI Copyright © 2007 ESRI. All rights reserved. ESRI, the ESRI globe logo, and www.esri.com, are trademarks, registered trademarks, or service marks of ESRI in the United States, the European Community, or certain other jurisdictions. Other companies and products mentioned herein may be trademarks or registered trademarks of their respective trademark owners. ESRI is an Equal Opportunity Employer. 56 Dr. Dobb’s Journal l www.ddj.com l March 2008 http://www.esri.com/ddj http://www.esri.com/ddj http://www.ddj.com

Table of Contents Feed for the Digital Edition of Dr. Dobb's Journal - March 2008

Dr. Dobb's Journal - March 2008 Contents Hmmmm Alia Vox Developer Diaries Developer’s Notebook Social Networks and Software Development Conversations Detecting Bugs in Safety-Critical Code Change Code Without Fear Continuous Integration and Performance Testing Wt: A Web Toolkit Automating Release Notifications The Agile Edge Effective Concurrency Swaine’s Flames

Dr. Dobb's Journal - March 2008

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