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Parameter arrays
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Parameter arrays are a feature with no direct parallel in Java. The use of a parameter array allows the caller of a method to provide a variable number of arguments when invoking a method; these are passed into the method in the form of an array. A method is declared to use a parameter array using the params keyword as follows:
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5. Data Types public void MyMethod(string someParam, params byte[] args) { // implementation code }
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As illustrated here, if a method takes multiple parameters, the parameter array must be the rightmost parameter declared. The params declaration must indicate the type of array that the method expects to receive; the array can be composed of simple or reference types. Within the body of the method, the params argument is processed as a normal array. The method just shown can be invoked in the following ways:
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byte[] b = {5, 2, 6, 8} ; string test = "hello" ; MyMethod(test); MyMethod(test, 3); MyMethod(test, 2,7,1); MyMethod(test, b); // // // // call call call call with no parameters with a single parameter with more than one parameter using an actual array
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Events
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Events are a formalization of the observer pattern, providing a generic mechanism through which a collection of registered listeners is notified when events occur. For example, program elements might need to know when a user clicks a button or closes a window. Events leverage delegates as the mechanism for event distribution. Objects interested in receiving event notifications register a delegate instance with the event. When triggered, the event invokes all registered delegates. We discussed delegates in the "Types" section earlier in this chapter. Like delegates, events do not provide functionality that cannot be implemented in Java using the appropriate patterns and interfaces; the observer pattern is used extensively throughout the Java class library. However, events do provide a clean syntax, freeing the programmer from implementing listener management and event distribution mechanisms.
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Declaration
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An event declaration takes the following form: [ attributes ] [ modifiers ] event type identifier [{ access-declarations }]; The event type must be an already defined and accessible delegate type. The optional accessdeclarations element provides the functionality for adding and removing event listeners. If this element is omitted, the compiler provides a default implementation suitable for most purposes. We'll discuss custom access-declarations later in this section.
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Event invocation and usage
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An event can be triggered only from within the type that declared it, irrespective of the accessibility modifiers applied to the event. An event is triggered as if invoking a delegate of the type used to declare the event. Triggering an event causes all delegate instances registered with the event to be invoked with the specified argument values. It makes no sense to trigger an event that has no registered listeners. An event will evaluate to null if it has no registered
5. Data Types
listeners. An if statement can be used to determine whether it's necessary to trigger an event. To remove all registered listeners from an event, set the value of the event to null. The following code demonstrates the use of events. The example defines a TempSource class that has a read-only property Temp for setting the current temperature. TempSource notifies all registered listeners when the temperature is set. We define a TempListener class to listen to temperature change events and output a message to the console in response.
using System; // Declare a delegate for event notifications public delegate void TemperatureEventHandler (string source, int temp); // Declare the event source object public class TempSource { private string Name; private int temp = 0; public event TemperatureEventHandler TemperatureChange; //Constructor takes a name for the Temperature Source public TempSource (string name) { Name = name; } // Declare a property to set the current temperature public int Temp { set { temp = value; // set the temperature // Raise the event if there are any listeners if (TemperatureChange != null) { TemperatureChange(Name, temp); } } } // Declare a method to remove all registered listeners public void Reset() { TemperatureChange = null; } } // Declare the event listener public class TempListener { private string Name; // Constructor that takes Listener name and an array of source public TempListener(string name, params TempSource[] sources) { Name = name; // Register with each of the temperature sources foreach (TempSource t in sources) { t.TemperatureChange += new TemperatureEventHandler(this.TempChanged); } }
5. Data Types public void TempChanged(string src, int temp) { Console.WriteLine(Name + " : Temp is " + temp + " F in the " + src); } public static void Main() { TempSource g = new TempSource("garden"); TempSource r = new TempSource("refrigerator"); new TempListener("Listener1", new TempSource[] {g, r}); new TempListener("Listener2", new TempSource[] {g, r}); g.Temp = 34; r.Temp = 16; } }
This example demonstrates multiple listeners registering with a single source as well as a listener receiving events from multiple sources. When run, the example produces the following output:
Listener1 Listener2 Listener1 Listener2 : : : : Temp Temp Temp Temp is is is is 34 34 16 16 F F F F in in in in the the the the garden. garden. refrigerator. refrigerator.
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