.NET/Java PDF, Tiff, Barcode SDK Library

If you look at the code behind the designer for the UpdatePanel control from the previous example, you ll see the following: <atlas:UpdatePanel ID="UpdatePanel1" runat="server"> <ContentTemplate> <asp:Label ID="lblResult" runat="server" Text="Label"></asp:Label> </ContentTemplate> </atlas:UpdatePanel> The <atlas:UpdatePanel> tag supports two child tags: the <ContentTemplate> tag and the <Triggers> tag.

// A reference to Bill, the trainee Firefighter bill = new TraineeFirefighter { Name = "Bill" }; bill.ExtinguishFire();

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Signals and slots are very loose types of connection, so the only thing that matters is that the arguments types match between the signal and the slot. The called class does not need to know anything about the calling class, and vice versa. That means that the simple example class can be put to the test letting it interact with a set of Qt s classes. The plan is to put MyClass between a widget that lets the user enter text, QLineEdit, and a widget that shows text, QLabel. A widget is a visual component such as a button, a slider, a menu item, or anything else that is a part of a graphical user interface. (Widgets are described in some detail in 3.) You can make the MyClass object work as a bridge carrying text from the user editable field to the label by connecting the textChanged signal from the QLineEdit object to the setText slot of the MyClass object and then connecting the textChanged signal from the MyClass object to the QLabel object s setText slot. The entire setup is shown in Figure 1-7.

If we compile and run, we see the following output:

Joe is putting out the fire! Training the hose on the fire. The fire is going out. Bill is putting out the fire! Training the hose on the fire. The fire is going out.

Well done, Bill; all that training came in handy, exactly as we d expect. Although it works, you ll notice that we ve duplicated some code from our base class into our derived class the bit that actually does the work in each of those methods. It would be better if we could just call on our base class implementation to do the job for us. As you d expect, C# has this in hand, with the base keyword.

If we ever want to call on the implementation of a member in our base class (bypassing any of our own overrides), we can do so through the special base name:

Figure 1-7. MyClass acting as a bridge between QLineEdit and QLabel The main function of this example can be split into three parts: creating the involved object instances, making the connections, and then running the application. Listing 1-10 shows how the involved components are created. First, there is a QApplication object. For all graphical Qt applications, there must be one (and only one) application instance available. The application object contains what is called the event loop. In this loop, the application waits for something to happen for an event to occur. For example, the user presses a key or moves the mouse, or a certain period of time has passed. As soon as an event has occurred, it is transformed into a call to an appropriate QObject. For instance, the key press event would go to the widget having keyboard focus. The event is processed by the receiving object, and sometimes a signal is emitted. In the key press scenario, a textChanged signal is emitted; in the case of a button and the key being entered or space, a pressed signal is emitted. The signals are then connected to slots performing the actual tasks of the application. Take a moment to review Listing 1-10. The QApplication object is created, along with three widgets: a plain QWidget, a QLineEdit, and a QLabel. The QWidget acts as a container for the other two. That is why you create a QVBoxLayout it is a vertical box layout that stacks its widgets on top of each other. Then you put the line edit and label in the box layout before assigning the layout to the widget. The resulting widget is shown in Figure 1-8.

base.CallOnTheBase();

Using that, we can reimplement our TraineeFirefighter and remove that duplicate code, as shown in Example 4-12.

The <ContentTemplate> tag simply defines the HTML or ASP .NET that will get updated by the UpdatePanel control. You can use the designer to generate this HTML. If, for example, you drag and drop a Calendar control onto the UpdatePanel control s content template area (see Figure 6-18), it will be defined within the <ContentTemplate> tag area.

class TraineeFirefighter : Firefighter { private bool hoseTrainedOnFire; protected override void TurnOnHose() { if (hoseTrainedOnFire) { // Call on the base implementation base.TurnOnHose(); } else { Console.WriteLine("There's water going everywhere!"); } } protected override void TrainHoseOnFire() { hoseTrainedOnFire = true; base.TrainHoseOnFire(); }

All Qt widgets are paint devices, so you can create a QPainter object and use it to draw to a widget. However, this can be done only from the paintEvent(QPaintEvent*) method.

}