In my last two postings on this subject, I outlined a Logger class used by my Parser class for logging errors. In this article, I show a more sophisticated approach for the application to register and unregister as an interested Observer with Logger, so it is notified when an error message is posted to Logger.
First, the Observer class must derive from the
ILoggerObserver interface:
///
/// Interface to be implemented by objects who wish to
/// observe changes on the Logger object's data.
///
public interface ILoggerObserver
{
void didAddLogMessage(string message);
}
In my original posting, I showed the following code snippet showing the Observer register and unregister sequence:
public class MyParser : ILoggerObserver
{
...
public void MyParseLogic()
{
// Register to log any parsing errors
Logger log = Logger.Instance;
log.Attach(this);
// Parse my data file
Parser p = new Parser(my_file_path_to_be_parsed);
p.Parse(); // will call the same Logger instance
log.Detach(this); // resign as a Logger observer
...
}
/// Implement the ILoggerObserver interface
public void didAddLogMessage(string message)
{
// Output the message to a TextBlock control or
// pop up a Message Box.
...
}
} // end class MyParser
Note that before parsing is done, the
MyParser class calls Logger's Attach method to register as an observer, and after finishing with the parsing, calls Detach to unregister. But I later came up with a better design so Detach can be done automatically. I accomplish this by taking advantage of the .NET IDisposable interface.I created a new container class named
LoggerObserverDisposer, derived from IDisposable. Its only job is to call Logger's Detach for the Observer when it's being disposed by .NET. The target Observer object is provided to its constructor and remembered for when the Detach is done.Here is that new Disposer class:
/// Container class for an observer, which terminates its observing
/// when out of scope.
public class LoggerObserverDisposer : IDisposable
{
protected ILoggerObserver _observer = null;
public LoggerObserverDisposer(ILoggerObserver observer)
{
// Remember the observer object to be disposed
_observer = observer;
}
public void Dispose()
{
Dispose(true); // call our Dispose method below
// prevent redundant Dispose during garbage collection
GC.SuppressFinalize(this);
}
protected virtual void Dispose(bool disposing)
{
if (disposing)
{
// Get the Logger object
Logger logger = Logger.Instance;
// Make operation atomic
Monitor.Enter(logger);
if (_observer != null)
{
// Remove the observer from Logger
logger.Detach(_observer);
_observer = null;
}
Monitor.Exit(logger);
}
}
} // end class LoggerObserverDisposer
Previously, here is what Logger's
Attach method looked like:
/// Add an Observer object to the list of those
/// to be notified of changes.
public void Attach(ILoggerObserver observer)
{
if (null != observer)
{
// Add this observer to the list, but just once
Detach(observer);
_observers.Add(observer);
}
}
Now, it is changed to make use of our new
LoggerObserverDisposer class. Attach returns a new LoggerObserverDisposer instance for the Observer, but only if requested by the caller.
/// Add an Observer object to the list of those to be notified
/// of changes, and return a Disposer object for the observer,
/// if requested.
///
/// Params:
/// "observer" - The Observer object to add to the list.
/// "doMakeDisposer" - true to return a Disposer object.
/// Returns: A Disposer object, if requested, or null
public LoggerObserverDisposer Attach(ILoggerObserver observer,
bool doMakeDisposer = false)
{
if (null != observer)
{
// Add this observer to the list
Detach(observer);
_observers.Add(observer);
}
// Return an object the caller can include in a
// 'using' statement so the observer is automatically
// removed when the Disposer goes out of scope
return (doMakeDisposer) ?
new LoggerObserverDisposer(observer) : null;
}
Finally, the
MyParser class changes to use this new version of Logger's Attach, but never needs to explicitly call Detach:
public void MyParseLogic()
{
// Register to log any parsing errors
Logger log = Logger.Instance;
using (logger.Attach(this, true))
{
// Parse my data file
Parser p = new Parser(my_file_path_to_be_parsed);
p.Parse(); // will call the same Logger instance
}
...
}
Dispose method is called. In our case, the new Attach method returns a LoggerObserverDisposer object (rather than void as before), which remains in scope throughout the "using", even though I never assign it to a variable. Thus, LoggerObserverDisposer::Dispose is called when the "using" block loses scope, which in turn calls Logger's Detach for the Observer MyParser class.This logic saves the programmer from having to remember to manually
Detach, and possibly leave a dangling Observer that has since been destroyed.
