how to print barcode in c# net What About Error Handling in Font

Print QR-Code in Font What About Error Handling

Some of you may be wondering about error handling. Specifically, how can you detect problems with the embedded server and handle them gracefully A number of the embedded library calls have error codes that you can interrogate and act on. The previous sections described the return values for the functions I ll be using. Although I didn t include much error handling in the first embedded MySQL examples, I will in the next example. Take note of how I capture the errors and handle sending the errors to the client.

The previous examples showed you how to create a basic embedded MySQL application. While the examples showed how to connect and read data from a dedicated MySQL installation, they aren t good models for building your own embedded application because they lack enough coverage for all but the most trivial requirements. Oh, and they don t have any error handling! The example in this chapter, while fictional, is all about providing you with the tools you need to build a real embedded application. This application, called the Book Vending Machine (BVM), is an embedded system designed to run on a dedicated Microsoft Windows based PC with a touch screen. The system and its other input devices are housed in a specialized mechanical vending machine designed to dispense books. The idea behind the BVM is to allow publishers to offer their most popular titles in a semi-mobile package that the vendor can configure and replenish as needed. The BVM would allow publishers to install their vending machine in areas where space is at a premium. Examples include trade shows, airports, and shopping malls. These areas usually have high traffic consisting of customers interested in purchasing printed books. The BVM saves publishers money by reducing the need for a storefront and personnel to staff it.

Note I ve often found myself wondering if this idea has ever been given consideration. I ve read several

articles predicting the continued rise of print-on-demand, but seldom have I seen anything written about how a book vending machine would work. I understand there are a few prototype installations by some publishers, but these trials have not generated much enthusiasm. I chose to use this example as a means to add some realism. I too read technical books and often find myself bored with unrealistic or trivial examples. Here is an example that I hope you agree is at least plausible.

This application has a need for a dual interface; one for the normal vending machine activity and one to allow vendors to restock the vending machine adjusting the information as needed. The vending machine interface is designed to provide the customer with an array of buttons providing a thumbnail of books for specific slots in the vending machine. Since most modern vending machines use product buttons that are illuminated when the product is available and dimmed or turned off when the product is depleted, the BVM interface enables the button when the product in that slot is available and disables it when the product is depleted. When the customer clicks a product button, the screen changes to a short, detailed display that describes the book and its price. If the customer wants to purchase the book, she can click Purchase and is prompted for payment. This application is written to simulate those activities. A real implementation would call the appropriate hardware control library to receive payment, validate the payment, and engage the mechanical part of the vending machine to disperse the product from the indicated slot. Figure 6-3 shows the main interface for the book vending machine. Figure 6-4 shows the effect of low quantity for some of the books.