Dr. Dobb's Journal - November 2007 - (Page 44)

d11vaugh_p5db 9/17/07 8:35 AM Page 44 State of the Art TRANSPARENCY ON DEMAND Object Client MethodName Reference Fuction Body resources need be retrieved only once, which means that repeated calls to DisplayMenu will cause redundant calls to the loading functions—those calls serve their purpose just once before becoming deadweight that impinges on performance. All of these points run contrary to the very aim of loading on-demand, which is to pay only for what is used. Function Interception Figure 1: Normal object-method binding. .NET, ActiveX, and C Libraries Available The stress-free choice for document imaging is Pegasus Imaging. Pegasus Imaging brings over 15 years of imaging experience and support to your software when you accelerate development using our .NET and ActiveX toolkits. Don’t take a chance on the unknown. Choose the leader in image compression, recognition, and editing technologies. Choose Pegasus Imaging. To learn more, visit our website today. Forms Processing Zonal & Full-Page OCR ICR OMR Barcode Annotation Image Viewing Scanning File Conversion Image Compression Multi-page Document Support Image Cleanup Happily, these problems can be resolved decisively. All objects in JavaScript are implemented as associative arrays that hold name-value pairs, meaning that the interpreter accesses a given object-member by using the member’s name as a key with which to look up the associated value. Where the member-name corresponds to a method, the associated value is a reference to the body of that method, and it follows that calling the method causes the interpreter to retrieve the relevant reference, whence it executes the function body. Figure 1 illustrates this. The value that is associated with a given member name can be changed using surprisingly simple syntax, and this means that the reference to a given object-method can be replaced with a reference to a function that acts as a proxy. This will cause calls to the original method to be “intercepted” by the proxy, which can do whatever it likes thereafter; see Listing Three. Cohesion Restored This in hand, the cohesion problems of loading on demand evaporate. If the proxy calls a library-loader before invoking the original method-body, then calls to the intercepted method will trigger a download automatically and transparently, followed by execution of the method itself. Figure 2 depicts the essential concept. Critically, this technique permits complete separation of the code that creates an interception from the “interceptee,” thus removing adventitious syntax from the points at which library loading is employed, and allowing all interception-setting code to be located in one place. This disentangles continued on page 48 WWW.PEGASUSIMAGING.COM (800) 875-7009 Pegasus is a registered trademark of Pegasus Imaging Corporation in the United States. Microsoft is a registered trademark of Microsoft Corporation in the United States and/or other countries. 44 Dr. Dobb’s Journal l www.ddj.com l November 2007 http://www.pegasusimaging.com http://www.pegasusimaging.com http://www.pegasusimaging.com http://www.ddj.com

Table of Contents Feed for the Digital Edition of Dr. Dobb's Journal - November 2007

Contents Hmmmm Alia Vox Developer Diaries Developer’s Notebook Smart Compilers - But Smart Enough? Conversations Grid-Enabling Resource-Intensive Applications Distributed Computing: Windows and Linux Adobe AIR: Desktop/Web Convergence Transparency on Demand Reusable Associations Effective Concurrency The Agile Edge Swaine’s Flames

Dr. Dobb's Journal - November 2007

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