zxing barcode generator example c# Figure 2-8. Viewing an item in Objective-C

Maker qr barcode in Objective-C Figure 2-8. Viewing an item

// Initialized to 0 - value type // Initialized to null - reference type // Initialized to 25 // Initialized to "abcd"

In fact, much of this book is filled with examples of things you can do in method bodies, such as creating user interfaces, opening network connections, querying data, and so on. In the following sections, I ll cover the things you can do that relate to methods and objects in a general sense so that when we come to the more specific topics (databases, networks, interfaces, and so on), the examples will make more sense.

protected override bool IsObjectValid( SingletonData obj) { FileInfo file = new FileInfo( _path); if( file.Exists) { return false; } else { return true; } } protected override SingletonData InstantiateNewObject( object descriptor) { FileStream filestream = new FileStream( _path, FileMode.Open, FileAccess.Read, FileShare.None); StreamReader stream = new StreamReader( filestream); SingletonData retval = new SingletonData( stream.ReadLine()); stream.Close(); FileInfo info = new FileInfo( _path); info.Delete(); return retval; } } SingletonBuilder is responsible for validating and instantiating SingletonData. The data for SingletonData is based on a file that is defined by a configuration file entry. The details of the configuration file entry have been removed for simplicity. However, illustrated is the need for a configuration file or database, or registry entry. The implementation of IsObjectValid doesn t include the parameter obj. The validation is based on the existence of a file. If the file exists, then a false is returned, indicating that the singleton data should be updated. In the implementation of the method InstantiateNewObject, again the parameter descriptor is ignored. Then a number of file operations are performed to read the contents of the file and create a new instance of SingletonData. The new instance is returned to the caller and will serve as the new singleton data. In the example implementation, the parameters to the methods were ignored. This isn t always the case, for it depends on the scenario. For example, the input parameters could themselves contain references for validation or instantiation. In the case of InstantiateNewObject, the parameter descriptor could be an object that uses the Extension pattern for constructor information. Putting It All Together and Thinking About What All This Means It s necessary to step back and consider what the versatile Singleton pattern implementation means. The implementation of the singleton includes three main pieces: singleton container, singleton data, and singleton data builder. Separating all three makes it possible to have the singleton builder located on another machine, and yet have the singleton data in the local AppDomain. As an example, consider the following singleton data builder, which is designed to be called using .NET remoting:

error code, accessible from the getErrorCode() method. These methods are still there and can be used; but in addition to these methods, there are now subclasses specific to common SQL states. There are also two new categories for SQL exceptions: transient and nontransient. These are represented by the new SQLTransientException and SQLNonTransientException classes. Transient exceptions are those that when retried could succeed without changing anything. These exceptions include the following subclasses: SQLTimeoutException: Expired statement timeout SQLTransactionRollbackException: Database rolled back statement automatically, possibly due to deadlock (SQLState 40) SQLTransientConnectionException: Communication layer problem (SQLState 08) Nontransient exceptions are those that will fail again on retry until the underlying cause of the problem is corrected. There are six subclasses of SQLNonTransientException: SQLDataException: Data error, such as an invalid argument (SQLState 22) SQLFeatureNotSupportedException: JDBC driver doesn t support feature (SQLState 0A) SQLIntegrityConstraintViolationException: Constraint on a key was violated (SQLState 23) SQLInvalidAuthorizationSpecException: Invalid authorization credentials presented during connection (SQLState 28) SQLNonTransientConnectionException: Communication layer problem that cannot be corrected (SQLState 08) SQLSyntaxErrorException: Query violated SQL syntax (SQLState 42) In some cases, such as SQLFeatureNotSupportedException, the fix for the problem is to get a new database driver, not to necessarily change anything in code. The connection exception can be transient or nontransient, depending upon what the problem is hence the shared SQLState value of 08.