.net qr code The Ramifications of Using an Observer Pattern for Ticket Sales in VB.NET

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The Ramifications of Using an Observer Pattern for Ticket Sales
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The Observer pattern as it has been implemented is great for informing the client of the newest ticket sales. However, this results in the ticket sales data being maintained in two separate locations. Contrast this to the approach where the total ticket sales are calculated on the fly and the only state is the collection of tickets. Using the Observer pattern, there are two states, running total and tickets in the collection. The proxy needs to keep the two states in sync, which generally isn t a problem since the proxy manages both states. A data corruption is only possible if the proxy doesn t do its job properly.
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Let s look a bit closer at the method TicketBuilder.CreateCollection. In the last iteration, a delegate was required to implement the Observer pattern. The method TicketBuilder. CreateCollection requires a delegate because the constructor of the class StatisticsCounter requires it. Let s say that you don t care about the statistics. In this case, it s tedious to define a delegate for something you don t need or want to provide. The simplest solution is to modify the TicketBuilder.CreateCollection to include a method that doesn t require a delegate. This would then mean the StatisticsCounter class would have to provide a constructor without parameters. Implementing both changes isn t complicated, and the structure without implementations of the classes would appear as follows: public class TicketsBuilder { private class StatisticsCounter { private double _runningTotal; private RunningTotalBroadcast _delegateRunningTotal; public } public } public } public } StatisticsCounter() { StatisticsCounter(RunningTotalBroadcast delegateRunningTotal) { void ClosureAddMethod(Ticket ticket) { void ClosureRemoveMethod( Ticket ticket) {
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} public static IList<Ticket> CreateCollection() { } public static IList<Ticket> CreateCollection(RunningTotalBroadcast runningTotal) { } } Looking at the implementation of StatisticsCounter and TicketsBuilder, it s obvious that implementing all methods would require a copy-and-paste operation from the other method. Such is the case for the CreateCollection methods, for which the code would have to instantiate an IList<> implementation. Ideally, it would be better if one CreateCollection method called the other CreateCollection method, as in this example: public static IList<Ticket> CreateCollection() { CreateCollection( null); } public static IList<Ticket> CreateCollection(RunningTotalBroadcast runningTotal) { } The proposed solution will work, and it means that the StatisticsCounter class would not require any changes. However, it does require that the StatisticsCounter constructor may be called with a null value. And that brings up the recommendation originally presented in 2: null values should be used as little as possible since they don t indicate a consistent state.
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Using the Null Object Pattern
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The Null Object pattern5 addresses the issue of not wanting to use null values. As outlined in 3, null values cause developers to have to check for valid conditions, empty conditions, and null conditions. For a large percentage of applications, empty conditions and null conditions are the same thing. Yet when writing code, both conditions need to be accounted for, causing more complex logic. A Null Object pattern is a specialization of the Proxy pattern in which the surrogate has no implemented functionality. You can employ the Null Object pattern if there is a base type, and that base type is defined using either an interface or base class. It s possible to use the Null Object pattern when the client does want to receive statistics information. For the movie ticket application, you need to implement the delegate RunningTotalBroadcast as follows: public class NullRunningTotalBroadcast { static void NothingRunningTotalBroadcast( double runningTotal) { } public static RunningTotalBroadcast GetInstance() { return new RunningTotalBroadcast( NothingRunningTotalBroadcast); } } NullRunningTotalBroadcast has declared two static methods. The method NothingRunningTotalBroadcast is the delegate for RunningTotalBroadcast. The method GetInstance retrieves a delegate instance that can be used by TicketBuilder. Following is the rewritten client code that uses the Null Object implementation: IList<Cinema.Ticket> list = Cinema.Implementations.TicketsBuilder.CreateCollection( Cinema.NullRunningTotalBroadcast.GetInstance()); list.Add(new Cinema.Ticket(10.0, 12)); list.Add(new Cinema.Ticket(10.0, 12)); list.RemoveAt( 1); The CreateCollection method receives as a parameter the result from NullRunning TotalBroadcast.GetInstance(), which is a delegate that does nothing. The client code illustrates how it s possible to use the existing types and methods, and yet implement new logic.
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