Customizing the Event Implementation in Visual C#

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N THE PRECEDING CHAPTER ,
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Why Events
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13: Events
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in the form of a chain, sometimes known as a multicast delegate With a multicast delegate, you can call a method chain via a single method object, create variables that refer to a method s chain, and use those data types as parameters to pass methods The C# implementation of multicast delegates is a common pattern that would otherwise require significant manual code Known as the observer or publish-subscribe pattern, it represents scenarios where notifications of single events, such as a change in object state, are broadcast to multiple subscribers
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Consider a temperature control example, where a heater and a cooler are hooked up to the same thermostat In order for a unit to turn on and off appropriately, you notify the unit of changes in temperature One thermostat publishes temperature changes to multiple subscribers the heating and cooling units The next section investigates the code1
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Defining Subscriber Methods Begin by defining the Heater and Cooler objects (see Listing 131)
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Listing 131: Heater and Cooler Event Subscriber Implementations
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class Cooler { public Cooler(float temperature) { Temperature = temperature; } public float Temperature { get{return _Temperature;} set{_Temperature = value;} } private float _Temperature; public void OnTemperatureChanged(float newTemperature)
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1 In this example, I use the term thermostat because people more commonly think of it in the context of heating and cooling systems Technically, however, thermometer would be more appropriate
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{ if (newTemperature > Temperature) { SystemConsoleWriteLine("Cooler: On"); } else { SystemConsoleWriteLine("Cooler: Off"); } } } class Heater { public Heater(float temperature) { Temperature = temperature; } public float Temperature { get { return _Temperature; } set { _Temperature = value; } } private float _Temperature; public void OnTemperatureChanged(float newTemperature) { if (newTemperature < Temperature) { SystemConsoleWriteLine("Heater: On"); } else { SystemConsoleWriteLine("Heater: Off"); } } }
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The two classes are essentially identical, with the exception of the temperature comparison (In fact, you could eliminate one of the classes if you used a delegate as a method pointer for comparison within the
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13: Events OnTemperatureChanged method) Each class stores the temperature for
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when to turn on the unit In addition, both classes provide an OnTemperatureChanged() method Calling the OnTemperatureChanged() method is the means to indicate to the Heater and Cooler classes that the temperature has changed The method implementation uses newTemperature to compare against the stored trigger temperature to determine whether to turn on the device The OnTemperatureChanged() methods are the subscriber methods It is important that they have the parameters and a return type that matches the delegate from the Thermostat class, which I will discuss next
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Defining the Publisher The Thermostat class is responsible for reporting temperature changes to the heater and cooler object instances The Thermostat class code appears in Listing 132
Listing 132: Defining the Event Publisher, Thermostat
public class Thermostat { // Define the delegate data type public delegate void TemperatureChangeHandler( float newTemperature); // Define the event publisher public TemperatureChangeHandler OnTemperatureChange { get{ return _OnTemperatureChange;} set{ _OnTemperatureChange = value;} } private TemperatureChangeHandler _OnTemperatureChange; public float CurrentTemperature { get{return _CurrentTemperature;} set { if (value != CurrentTemperature) { _CurrentTemperature = value; } } } private float _CurrentTemperature; }
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The first member of the Thermostat class is the TemperatureChangeHandler delegate Although not a requirement, ThermostatTemperatureChangeHandler is a nested delegate because its definition is specific to the Thermostat class The delegate defines the signature of the subscriber methods Notice, therefore, that in both the Heater and the Cooler classes, the OnTemperatureChanged() methods match the signature of TemperatureChangeHandler In addition to defining the delegate type, Thermostat includes a property called OnTemperatureChange that is of the OnTemperatureChangeHandler delegate type OnTemperatureChange stores a list of subscribers Notice that only one delegate field is required to store all the subscribers In other words, both the Cooler and the Heater classes will receive notifications of a change in the temperature from this single publisher The last member of Thermostat is the CurrentTemperature property This sets and retrieves the value of the current temperature reported by the Thermostat class
Hooking Up the Publisher and Subscribers Finally, put all these pieces together in a Main() method Listing 133 shows a sample of what Main() could look like