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17: Delegates and Events |
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Example A-50. One solution to Exercise 17-1 (continued)
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Solution to Exercise 17-2. Change the program you wrote in Exercise 17-1 to ensure that the event can be published to multiple handlers safely. The event keyword guarantees that the event the CountDownClock is publishing can only be subscribed to or unsubscribed from. This change is easy, so we won t show the whole code here. The relevant portion is shown in Example A-51.
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Example A-51. The solution to Exercise 17-2
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// the delegate public delegate void TimesUpEventHandler ( object countDownClock, CountDownClockEventArgs alarmInformation ); // an instance of the delegate public event TimesUpEventHandler TimeExpired;
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Solution to Exercise 17-3. Rewrite the observer class in Exercise 17-2 to use an anonymous method. You may have noticed that the event handler method in Exercises 17-1 and 17-2 is pretty simple; all it does is output the message to the console. This is a perfect candidate for an anonymous method. To make the method anonymous, you need to delete the code that instantiates the handler, and instead move the call to Console. WriteLine to the line that registers the handler. The only changes here are to the observer class, so that s all that Example A-52 shows.
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Example A-52. The solution to Exercise 17-3
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public class CountDownTimerDisplay { public CountDownTimerDisplay(CountDownClock cdc) { // register the event handler and start the timer cdc.TimeExpired += delegate(object theClock, CountDownClockEventArgs e) { Console.WriteLine("You requested to receive this message: {0}", e.message); }; } }
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Appendix: Answers to Quizzes and Exercises
Solution to Exercise 17-4. Rewrite the observer class in Exercise 17-3 to use a lambda expression instead of an anonymous method. Again, the only change you have to make is to the event registration code in the observer class. You need to remove the keyword delegate and insert the lambda operator =>. You don t actually need the braces, since the method is only one statement, but it doesn t hurt to leave them in, either. Example A-53 shows the solution.
Example A-53. The solution to Exercise 17-4
public class CountDownTimerDisplay { public CountDownTimerDisplay(CountDownClock cdc) { // register the event handler and start the timer cdc.TimeExpired += (object theClock, CountDownClockEventArgs e) => { Console.WriteLine("You requested to receive this message: {0}", e.message); }; } }
18: Creating Windows Applications
Quiz Solutions
Solution to Question 18-1. The various widgets on a Windows form are all known as controls. Solution to Question 18-2. To add a Button control to a Windows form in Visual Studio, simply drag the Button from the Toolbox onto the form wherever you want it. Visual Studio takes care of the code to initialize the control. Solution to Question 18-3. To set the properties of a control, click on the control on the form. This causes the control s properties to appear in the Properties window. From the Properties window, you can set a number of available properties for each control. Solution to Question 18-4. Windows is an event-driven environment in that the code initializes forms that wait to respond to user or system actions events rather than running procedurally from start to finish. Solution to Question 18-5. To make a button respond to being clicked, you need to create an event handler to handle the Click event for the button.
18: Creating Windows Applications |
Solution to Question 18-6. The two ways to create an event handler in Visual Studio are as follows: Go to the Properties window, click on the lightning bolt button to open the events, and then fill in a name or double-click next to the event to let Visual Studio create the name. Double-click on the control to create the default handler with a name provided by Visual Studio. Solution to Question 18-7. When you create an event handler for a control on your form, Visual Studio automatically takes you to the code page of your form. Solution to Question 18-8. The partial keyword indicates that the code page you see in Visual Studio is not the complete class for the form. Visual Studio hides the initialization code for the controls, so you don t need to worry about it. Solution to Question 18-9. Call the Application.Exit( ) method to close the application. Solution to Question 18-10. Recursion is a method that is calling itself (such as calling MethodA( ) from within the body of MethodA( )).
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