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CHAPTER 6 EMBEDDED MYSQL
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void DBEngine::StartQuery(char *QueryStatement) { if (!IteratorStarted) { results=ExecQuery(QueryStatement); if (results) { record=mysql_fetch_row(results); } } IteratorStarted=true; } void DBEngine::RunQuery(char *QueryStatement) { results=ExecQuery(QueryStatement); if (results) { record=mysql_fetch_row(results); if(!record) { mysqlError = true; } } } int DBEngine::GetNext() { //if EOF then no more records IteratorStarted=false; record=mysql_fetch_row(results); if (record) { return (1); } else { return (0); } } char *DBEngine::GetField(int fldNum) { if (record) { return (record[fldNum]); }
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else { return (""); } } MYSQL_RES *ExecQuery(char *Query) { mysql_dbug_print("ExecQuery."); mysql_free_result(results); mysql_query(mysql, Query); return (mysql_store_result(mysql)); } #pragma managed One thing you should notice about this code is all of the error handling that I ve added to make the code more robust, or hardened. While I do not have all of the possible error handlers implemented, the most important ones are. The get methods are all implemented using the same process. I first generate the appropriate query (and thereby hide the SQL statement from the client), execute the query, retrieve the result set and then the record from the query, and return the value. One method that is of interest is VendBook(). Take a moment and look through that one again. You ll see that I ve followed a similar method of generating the query, but this time I don t get the results because there aren t any. Actually, there is a result it is the number of records affected. This could be handy if you wanted to do some additional process or rule checking in your application. The rest of the methods should look familiar to you as they are all copies of the original example I showed you except this time they have error handling included. Now, let s take a look at how the user interface code calls the database class. Customer Interface (Main Form) The source code for the customer interface is very large. This is due to the auto-generated code that Microsoft places in the form.h file. I m including only those portions that I wrote. I ve included this section to show you how you can write your own .NET (or other) user interfaces. Aside from the code in the button events, I am using only four additional methods that I need to complete the user interface. The first method, DisplayError(), is defined as void DisplayError() I use this function as a means to detect errors in the database class and to present the error message to the user. The implementation of the method is a typical call to the MessageBox::Show() function. The second method is a helper method that completes the detail view of the book selected. The function is named LoadDetails(). I abstracted this method because I realized I would be repeating the code for all ten buttons.3 Abstracting in this manner minimizes the code and permits easier debugging. This method is defined as
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3. There was only one feature of Visual Basic I found really cool: control arrays. Alas, they are a thing of the past.
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CHAPTER 6 EMBEDDED MYSQL
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void LoadDetails(int Slot) The method takes as a parameter the slot number (which corresponds to the button number). It queries the database using the database class methods and populates the detail interface elements. This is where most of the heavy lifting of communicating with the database engine class occurs.
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Note You may be wondering what all that gnarly code is surrounding the character strings. It turns out the
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.NET string class is not compatible with the C-style character strings. The extra code I included is designed to marshal the strings between these formats.
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The third method is a helper method named Delay() and is defined as void Delay(int secs) The function causes a delay in processing for the number of seconds passed as a parameter. While not something you would want to include in your own application, I added it to simulate the vending process. This is an excellent example of how you can use stubbed functionality to demonstrate an application. This can be especially helpful in prototyping a new interface. The fourth method, CheckAvailability(), is used to turn the buttons on the interface on or off depending on whether there is sufficient quantity of the product available. This method is defined as void CheckAvailability() The function makes a series of calls to the database engine to check the quantity for each slot. If the slot is empty (quantity == 0), then the button is disabled. Listing 6-14 shows an excerpt of the source code for the customer interface. I ve omitted a great deal of the auto-generated code (represented as ...). Notice at the top of the file I reference the database engine header using the #include "DBEngine.h" directive. Also notice that I ve defined a variable of type DBEngine. I use this object throughout the code. Since it is local to the form, I can use it in any event or method. I use the ... to indicate portions of the auto-generated code and comments omitted from the listing. Listing 6-14. Main Form Source Code (MainForm.h) #pragma once #include "DBEngine.h" #include <stdio.h> #include <stdlib.h> #include <string> #include "vcclr.h" #include <time.h>
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