133: Connecting the Publisher and Subscribers in C#

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Listing 133: Connecting the Publisher and Subscribers
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class Program { public static void Main() { Thermostat thermostat = new Thermostat(); Heater heater = new Heater(60); Cooler cooler = new Cooler(80); string temperature; // Using C# 20 or later syntax thermostatOnTemperatureChange += heaterOnTemperatureChanged; thermostatOnTemperatureChange += coolerOnTemperatureChanged; ConsoleWrite("Enter temperature: "); temperature = ConsoleReadLine(); thermostatCurrentTemperature = intParse(temperature); } }
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13: Events
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The code in this listing has registered two subscribers (heaterOnTemperatureChanged and coolerOnTemperatureChanged) to the OnTemperatureChange delegate by directly assigning them using the += operator As noted in the comment, you need to use the new operator with the TemperatureChangeHandler constructor if you are only using C# 10 By taking the temperature value the user has entered, you can set the CurrentTemperature of thermostat However, you have not yet written any code to publish the change temperature event to subscribers
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Invoking a Delegate Every time the CurrentTemperature property on the Thermostat class changes, you want to invoke the delegate to notify the subscribers (heater and cooler) of the change in temperature To do this, modify the CurrentTemperature property to save the new value and publish a notification to each subscriber The code modification appears in Listing 134
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Listing 134: Invoking a Delegate without Checking for null
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public class Thermostat { public float CurrentTemperature { get{return _CurrentTemperature;} set { if (value != CurrentTemperature) { _CurrentTemperature = value; // INCOMPLETE: Check for null needed // Call subscribers OnTemperatureChange(value); } } } private float _CurrentTemperature; }
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Now the assignment of CurrentTemperature includes some special logic to notify subscribers of changes in CurrentTemperature The call to notify all subscribers is simply the single C# statement, OnTemperatureChange(value) This single statement publishes the temperature change to the cooler and
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Coding the Observer Pattern with Multicast Delegates heater objects Here, you see in practice that the ability to notify multiple
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subscribers using a single call is why delegates are more specifically known as multicast delegates
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Check for null One important part of publishing an event code is missing from Listing 134 If no subscriber registered to receive the notification, then OnTemperatureChange would be null and executing the OnTemperatureChange(value) statement would throw a NullReferenceException To avoid this, it is necessary to check for null before firing the event Listing 135 demonstrates how to do this
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Listing 135: Invoking a Delegate
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public class Thermostat { public float CurrentTemperature { get{return _CurrentTemperature;} set { if (value != CurrentTemperature) { _CurrentTemperature = value; // If there are any subscribers // then notify them of changes in // temperature TemperatureChangeHandler localOnChange = OnTemperatureChange; if(localOnChange != null) { // Call subscribers localOnChange(value); } } } } private float _CurrentTemperature; }
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Instead of checking for null directly, first assign OnTemperatureChange to a second delegate variable, handlerCopy This simple modification ensures that if all OnTemperatureChange subscribers are removed (by a different thread) between checking for null and sending the notification, you will not fire a NullReferenceException
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13: Events
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One more time: Remember to check the value of a delegate for null before invoking it
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ADVANCED TOPIC = Operator for a Delegate Returns a New Instance Given that a delegate is a reference type, it is perhaps somewhat surprising that assigning a local variable and then using that local variable is sufficient for making the null check thread-safe Since localOnChange points at the same location that OnTemperatureChange points, one would think that any changes in OnTemperatureChange would be reflected in localOnChange as well This is not the case, because effectively, any calls to OnTemperatureChange = <listener> will not simply remove a delegate from OnTemperatureChange so that it contains one less delegate than before Rather, it will assign an entirely new multicast delegate without having any effect on the original multicast delegate to which localOnChange also points Delegate Operators To combine the two subscribers in the Thermostat example, you used the += operator This takes the first delegate and adds the second delegate to the chain so that one delegate points to the next Now, after the first delegate s method is invoked, it calls the second delegate To remove delegates from a delegate chain, use the = operator, as shown in Listing 136
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