.NET/Java PDF, Tiff, Barcode SDK Library

To control in which group of widgets your widget appears, the name of the group is returned from the group() method. The method implementation is shown in Listing 6-30. Listing 6-30. The group to join in Designer QString CircleBarPlugin::group() const { return "Book Widgets"; } To help Designer create a widget, you need to implement a factory method, which is named createWidget(QWidget*) and is shown in Listing 6-31. Listing 6-31. Creating a widget instance QWidget *CircleBarPlugin::createWidget( QWidget *parent ) { return new CircleBar( parent ); } The final step is to actually export the plugin class as a plugin by using the Q_EXPORT_ PLUGIN2 macro, as shown in Listing 6-32. This line is added to the end of the implementation file. Listing 6-32. Exporting the plugin Q_EXPORT_PLUGIN2( circleBarPlugin, CircleBarPlugin ) To build a plugin, you must create a special project file, which is shown in Listing 6-33. The important lines are highlighted in the listing. What they do is tell QMake to use a template for building a library; then the CONFIG line tells QMake that you need the designer and plugin modules. The last line configures the output of the build to end up in the right place using the DESTDIR variable. Listing 6-33. The project file for a Designer plugin TEMPLATE = lib CONFIG += designer plugin release DEPENDPATH += . TARGET = circlebarplugin HEADERS += circlebar.h circlebarplugin.h SOURCES += circlebar.cpp circlebarplugin.cpp DESTDIR = $$[QT_INSTALL_DATA]/plugins/designer

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The first time someone tries to modify a plane s SpeedInMilesPerHour this will print out a message that includes the identifier, for example:

Now that you have the controls in your Toolbox, you can drag and drop them onto your web forms. For the rest of this chapter, I ll discuss these controls and their object models, and in 7 you will start using these controls in hands-on examples.

Unfortunately, the developer who wrote this clearly wasn t the sharpest tool in the box he used the += operator to build that debug string, which will end up modifying the Identifier property. So, the plane now thinks its identifier is that whole text, including the part about the speed. And if we modified the speed again, we d see:

While it might be interesting to see the entire modification history, the fact that we ve messed up the Identifier is bad. Example 3-15 was able to do this because the SpeedInMilesPerHour property is part of the Plane class, so it can still use the private setter. We can fix this (up to a point) by making the property read-only rather than merely making the setter private, we can leave it out entirely. However, we can t just write the code in Example 3-16.

class Plane { // Wrong! public string Identifier { get; }

After you build the plugin, you can check whether Designer has found it by accessing the Help About Plugins menu item. This will bring up the dialog shown in Figure 6-9. In the figure, you can see that the plugin has been loaded and that the widget has been found.


That won t work because there s no way we could ever set Identifier not even in the constructor. Auto properties cannot be read-only, so we must write a getter with code. Example 3-17 will compile, although as we re about to see, the job s not done yet.

class Plane { public Plane(string newIdentifier) { _identifier = newIdentifier; } public string Identifier { get { return _identifier; } } private string _identifier; ...

Figure 6-9. The plugin has been loaded. Creating widget plugins for Designer is simply a matter of filling out a given interface. The job is easy, but it can be quite tedious.


The ScriptManager control is at the heart of Atlas. This control, as its name suggests, manages the deployment of the various JavaScript libraries that implement the client-side runtime functionality of Atlas.

This turns out to give us two problems. First, the original constructor from Example 3-6 would no longer compile it set Identifier, but that s now read-only. That was easy to fix, though Example 3-17 just sets the explicit backing field we ve added. More worryingly, this hasn t solved the original problem the developer who wrote the code in Example 3-15 has cleverly realized that he can fix his code by doing exactly the same thing as the constructor. As Example 3-18 shows he has just used the _identifier field directly.

public double SpeedInMilesPerHour { get { return SpeedInKilometersPerHour / kilometersPerMile; } set { _identifier += ": speed modified to " + value; Console.WriteLine(Identifier); SpeedInKilometersPerHour = value * kilometersPerMile; } }


That seemed like a long journey for no purpose. However, we can fix this problem we can modify the backing field itself to be read-only, as shown in Example 3-19.

private readonly string _identifier;

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