.net qr code reader Reading Ticket Sales Data in Visual Basic .NET

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Reading Ticket Sales Data
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The code for calculating the grand total for the ticket sales works. But there is a very big problem in that no client can ever retrieve the value of the ticket sales. TicketBuilder creates an instance of StatisticsCounter, but the instance of StatisticsCounter isn t saved for reference. In other words, statistics are being generated, but no client has access to those values. The simplest solution for the ticket sales total problem is to create a property called RunningTotal. But where should this property be created Associating RunningTotal with TicketBuilder is a shortcut as TicketBuilder instantiates object hierarchies only. Associating RunningTotal with the IList<> instance would work, but is kludgy, as the consumer of IList<> has to know that the RunningTotal property exists. This would bind the consumer of IList<> to the closure functors, which is also not part of the overall design.
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Recall from the oven monitoring example back in 5 how it was possible to get the temperature of the oven using a callback. In that example, it seemed simpler to create a property that referenced the temperature. This time though, a property is neither desirable nor possible, since it would create impractical interface bindings. And remember, the idea of a proxy is to make the client think it s using the actual implementation. Adding a property isn t possible. If in 5 the argument against properties seemed weak, now you should be convinced otherwise.
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Implementing the Observer Pattern
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The solution to the ticket sales problem is to use the Observer pattern.4 Implementing the Observer pattern is easy because it s a delegate in .NET. The oven temperature example in 5 used the Observer pattern. It s possible to use .NET events, but in this example, they complicate the solution. The theory of the Observer pattern is to allow a client to be informed of data changes via an indirect callback mechanism identical to a publish and subscribe process. The event that will be generated is defined by the following delegate: public delegate void RunningTotalBroadcast( double runningTotal); RunningTotalBroadcast has a single parameter, which represents the running total of all ticket sales. Every client that is interested in ticket sales defines a delegate and informs StatisticsCounter. It s important to realize that the delegate sends the running total, and not the value of an individual ticket sale. Broadcasting the running total means that the publisher keeps the current state, and not the subscriber of the data, which is often, but not always, the case. The following source code represents the modified TicketBuilder that uses delegates to broadcast the running total: public class TicketsBuilder { private class StatisticsCounter { private double _runningTotal; private RunningTotalBroadcast _delegateRunningTotal; public StatisticsCounter(RunningTotalBroadcast delegateRunningTotal) { _runningTotal = 0.0; _delegateRunningTotal = delegateRunningTotal; } public void ClosureAddMethod(Ticket ticket) { _runningTotal += ticket.Price; _delegateRunningTotal(_runningTotal); } public void ClosureRemoveMethod( Ticket ticket) { _runningTotal -= ticket.Price; _delegateRunningTotal( _runningTotal); } } public static IList<Ticket>
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4. Design Patterns: Elements of Reusable Object-Oriented Software, p. 293.
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CreateCollection(RunningTotalBroadcast runningTotal) { StatisticsCounter cls = new StatisticsCounter( runningTotal); IList<Ticket> parent = new ClosureAddProxy< Ticket>( new List< Ticket>(), new DelegateClosure< Ticket>( cls.ClosureAddMethod)); return new ClosureRemoveProxy<Ticket>( parent, new DelegateClosure< Ticket>( cls.ClosureRemoveMethod)); } } The method TicketBuilder.CreateCollection has been modified to require a parameter. The parameter runningTotal is a delegate that is passed to StatisticsCounter and is called whenever the methods ClosureAddMethod and ClosureRemoveMethod are called. Next, let s look at a test method that illustrates how to add tickets: [TestFixture] public class TestMovie { private void RunningTotalMethod(double runningTotal) { Console.WriteLine("Running Total " + runningTotal); } [Test] public void TestCallback() { IList<Cinema.Ticket> list = Cinema.Implementations.TicketsBuilder.CreateCollection( new Cinema.RunningTotalBroadcast( this.RunningTotalMethod)); list.Add(new Cinema.Ticket(10.0, 12)); list.Add(new Cinema.Ticket(10.0, 12)); list.RemoveAt( 1); } } The method TestCallback instantiates the IList<> variable list using the method TicketsBuilder.CreateCollection. The method RunningTotalMethod is a callback that receives the updates whenever a ticket is added to the collection. When an update is received, RunningTotalMethod displays the ticket sales running total. What is important about the TestCallback method is that the client only interacts with an IList<> type. Therefore, whenever the methods Add or RemoveAt are called, the closure functors react and process the data. The client only needs to understand the IList<> interface and the events that are propagated.
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