Understanding WPF User Interface Events

argument is always the sender (the source) of the event, and the second argument is always an EventArgs argument (or a class derived from EventArgs).

With the sender argument, you can reuse a single method for multiple events. The del-

egated method can examine the sender argument and respond accordingly. For example, you can use the same method to subscribe to the Click event for two buttons (you add the

same method to two different events). When the event is raised, the code in the method can examine the sender argument to ascertain which button was clicked.

You learn more about how to handle events for WPF controls in Chapter 22, “Introducing Windows Presentation Foundation.”

Using Events

In the following exercise, you will use events to simplify the program you completed in the first exercise. You will add an event field to the Ticker class and delete its Add and Remove methods. You will then modify the Clock.Start and Clock.Stop methods to subscribe to the event. You will also examine the Timer object, used by the Ticker class to obtain a pulse once

each second.

Rework the digital clock application

1.Return to the Visual Studio 2008 window displaying the Delegates project.

2.Display the Ticker.cs file in the Code and Text Editor window.

This file contains the declaration of the Tick delegate type in the Ticker class:

public delegate void Tick(int hh, int mm, int ss);

3.Add a public event called tick of type Tick to the Ticker class, as shown in bold type in the following code:

class Ticker

{

public delegate void Tick(int hh, int mm, int ss); public event Tick tick;

...

}

4.Comment out the following delegate variable tickers near the bottom of the Ticker class definition because it is now obsolete:

// private Tick tickers;

5.Comment out the Add and Remove methods from the Ticker class.

The add and remove functionality is automatically provided by the += and –= operators of the event object.

328Part III Creating Components

6.Locate the Ticker.Notify method. This method previously invoked an instance of the Tick delegate called tickers. Modify it so that it calls the tick event instead. Don’t forget to check whether tick is null before calling the event.

The Notify method should look like this:

private void Notify(int hours, int minutes, int seconds)

{

if (this.tick != null) this.tick(hours, minutes, seconds);

}

Notice that the Tick delegate specifies parameters, so the statement that raises the tick event must specify arguments for each of these parameters.

7. Examine the definition of the ticking variable at the end of the class:

private DispatcherTimer ticking = new DispatcherTimer();

The DispatcherTimer class can be programmed to raise an event repeatedly at a specified interval.

8. Examine the constructor for the Ticker class:

public Ticker()

{

this.ticking.Tick += new EventHandler(this.OnTimedEvent); this.ticking.Interval = new TimeSpan(0, 0, 1); // 1 second this.ticking.Start();

}

The DispatcherTimer class exposes the Tick event, which can be raised at regular intervals according to the value of the Interval property. Setting Interval to the

TimeSpan shown causes the Tick event to be raised once a second. The timer starts when you invoke the Start method. Methods that subscribe to the Tick event must match the signature of the EventHandler delegate. The EventHandler delegate has the same signature as the RoutedEventHandler delegate described earlier. The Ticker constructor creates an instance of this delegate referring to the OnTimedEvent method and

subscribes to the Tick event.

The OnTimedEvent method in the Ticker class obtains the current time by examining the static DateTime.Now property. The DateTime structure is part of the .NET Framework class library. The Now property returns a DateTime structure. This structure has several fields, including those used by the OnTimedEvent method shown in the following code and called Hour, Minute, and Second. The OnTimedEvent method uses this information in turn to raise the tick event through the Notify method:

private void OnTimedEvent(object source, EventArgs args)

{

DateTime now = DateTime.Now; int hh = now.Hour;

int mm = now.Minutes;