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Data: advanced techniques
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Listing 9.1
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SKDatabase, a new sqlite3 database class
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#import "SKDatabase.h" #import "sqlite3.h" @implementation SKDatabase - (id)initWithFile:(NSString *)dbFile { self = [super init]; NSString *paths = [[NSBundle mainBundle] resourcePath]; NSString *path = [paths stringByAppendingPathComponent:dbFile]; int result = sqlite3_open([path UTF8String], &dbh); NSAssert1(SQLITE_OK == result, NSLocalizedStringFromTable (@"Unable to open the sqlite database (%@).", @"Database", @""), [NSString stringWithUTF8String:sqlite3_errmsg(dbh)]); return self; } - (void)close { if (dbh) { sqlite3_close(dbh); } } - (sqlite3 *)dbh { return dbh; } - (sqlite3_stmt *)prepare:(NSString *)sql { const char *utfsql = [sql UTF8String]; sqlite3_stmt *statement; if (sqlite3_prepare([self dbh],utfsql,-1,&statement,NULL)==SQLITE_OK) { return statement; } else { return 0; } } Looks up - (id)lookupSingularSQL:(NSString *)sql forType: SQL results (NSString *)rettype { sqlite3_stmt *statement; Calls prepare id result; function if (statement = [self prepare:sql]) { if (sqlite3_step(statement) == SQLITE_ROW) { if ([rettype compare:@"text"] == NSOrderedSame) { result = [NSString stringWithUTF8String: (char *)sqlite3_column_text (statement,0)]; } else if ([rettype compare:@"integer"] == NSOrderedSame) { result = (id)sqlite3_column_int (statement,0); } } } sqlite3_finalize(statement); return result; }
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@end
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Using SQLite
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The header file (not shown) includes one variable declaration for the dbh (database handle) variable, the database handle. That s the one variable you want to always have available to your class, because it gives access to the database. Now you re ready to start working on the source code file. The initWithFile: method uses some of the file commands that you learned in the previous section to find the database file, which is in the main bundle (but remember, you ll want to copy this to the Documents directory if you make changes to your database). It then opens the file using sqlite3_open, the first of several sqlite3 API commands. Note that the NSString for the path has to be converted with the UTF8String method. This must be done throughout the class, because the SQLite API doesn t use the Objective-C classes you re familiar with. The next few methods are pretty simple. close signals the end of the database lifecycle, dbh is a getter for the class s one variable, and prepare turns an SQL statement into a prepared statement. The lookupSingularSQL: method is where things get interesting, because it shows off the lifecycle of a complete SQL function call B. Note that this function allows only a simple SQL call that returns one column from one row of information. That s all you need for the database view controller, but you ll doubtless need more complexity for a larger application. The function starts by turning the SQL statement into a prepared statement C. Then it steps to the first row. Depending on the type of lookup, it fetches either a string or an int. Finally, it cleans up the statement with a finalize. In a more complex class, you d doubtless want to write methods that execute SQL calls without any returns, that return multiple columns from a row, and that return multiple rows, but we ll leave that for now (because we don t need any of those features for this example) and move on to the menu class. The SQLite API has more information on these features if you need them.
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THE MENU CLASS
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The next class, SKMenu, acts as an intermediary. At the frontend, it accepts requests for information about the menu that will fill the table view. On the backend, it turns those requests into SQL queries. It s been designed in this way to create an opaque interface: you never have to know that a database is being used, just that the SKMenu class returns results for a table view. The code of SKMenu is shown in listing 9.2. It mainly illustrates how to use the SKDatabase class in listing 9.6.
Listing 9.2
SKMenu, an interface to the SKDatabase class
#import "SKMenu.h" @implementation SKMenu - (id)initWithFile:(NSString *)dbFile { self = [super init]; myDB = [[SKDatabase alloc] initWithFile:dbFile]; return self;
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