ASP.NET Web PDF Document Viewer/Editor Control Library

When we handled our return values, we had to propagate them up the call stack by hand, adding appropriate return values to each and every method, checking the result, and either passing it up or transforming and passing it as we go. Exceptions, on the other hand, propagate up the stack automatically. If we don t want to add a handler, we don t have to, and the next call site up gets its chance instead, until eventually we pop out at the top of Main and either break into the debugger or Windows Error Handling steps in. This means we can take a more structured approach to error handling identifying points in our application control flow where we want to handle particular types of exceptions, and gathering our error-handling code into easily identified blocks. The try, catch, and finally keywords help us to define those blocks (along with the ubiquitous braces). In our example, we have no need to handle the potential errors from each and every call to RunFor separately. Instead, we can wrap the whole set into a single set of try, catch, and finally blocks, as shown in Example 6-14.

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static void Main(string[] args) { Turtle arthurTheTurtle = new Turtle { PlatformWidth = 0.0, PlatformHeight = 10.0, MotorSpeed = 5.0 }; ShowPosition(arthurTheTurtle); try {

if( !device->isOpen() ) return 0; QImageIOPlugin::Capabilities result; if( device->isReadable() && TextImageHandler::canRead( device ) ) result |= QImageIOPlugin::CanRead; if( device->isWritable() ) result |= QImageIOPlugin::CanWrite; return result; } So how does Qt know which formats to ask for All image plugins report which formats they can handle with the keys method. The formats (or format, in this case) are put in a QStringList that is returned. The implementation is shown in Listing 11-3. Listing 11-3. Putting the image file formats in a QStringList QStringList TextImagePlugin::keys() const { return QStringList() << "ti"; } When the format is correct and can be handled, the last method comes into action. The create method shown in Listing 11-4 creates an instance of the custom TextImageIOHandler, configures it with a format and a device, and returns the result. A format is set for the handler so it can be made to handle several formats. There are many formats that are almost identical, so it can be useful to reduce the size of the source code. Listing 11-4. Creating and configuring an image IO handler QImageIOHandler *TextImagePlugin::create( QIODevice *device, const QByteArray &format ) const { QImageIOHandler *result = new TextImageHandler(); result->setDevice( device ); result->setFormat( format ); return result; } Before you can move on to the handler class, you must tell Qt that this class is a part of the plugin interface. You can do this by using the Q_EXPORT_PLUGIN2 macro, as shown in Listing 11-5. The macro is placed somewhere in the implementation file (not the header). The first argu-

// We want to proceed forwards arthurTheTurtle.LeftMotorState = MotorState.Running; arthurTheTurtle.RightMotorState = MotorState.Running; // For two seconds arthurTheTurtle.RunFor(2.0); ShowPosition(arthurTheTurtle); // Now, let's rotate clockwise for a bit arthurTheTurtle.RightMotorState = MotorState.Reversed; // PI / 2 seconds should do the trick arthurTheTurtle.RunFor(Math.PI / 2.0); ShowPosition(arthurTheTurtle);

// And let's go into reverse arthurTheTurtle.RightMotorState = MotorState.Reversed; arthurTheTurtle.LeftMotorState = MotorState.Reversed; // And run for 5 seconds arthurTheTurtle.RunFor(5); ShowPosition(arthurTheTurtle); // Then rotate back the other way arthurTheTurtle.RightMotorState = MotorState.Running; // And run for PI/4 seconds to give us 45 degrees arthurTheTurtle.RunFor(Math.PI / 4.0); ShowPosition(arthurTheTurtle); // And finally drive backwards for a bit arthurTheTurtle.RightMotorState = MotorState.Reversed; arthurTheTurtle.LeftMotorState = MotorState.Reversed; arthurTheTurtle.RunFor(Math.Cos(Math.PI / 4.0)); ShowPosition(arthurTheTurtle); } catch (InvalidOperationException e) { Console.WriteLine("Error running turtle:"); Console.WriteLine(e.Message); } finally { Console.ReadKey(); }

From here, you can select Manage Users to put your new user(s) into their specific roles. This will take you to the user administration settings (see Figure 7-18). You can see a list of users on this screen. In Figure 7-18, you can see the user Arnie that was created earlier. You can also see that Arnie isn t in any roles yet, so although he can log in and browse the wiki, he cannot be an administrator or contribute content. If you click Edit User on this screen, you ll be taken to the screen shown in Figure 7-19, which allows you to set his role. In this case, set the user to be an administrator. You can then sign in with Arnie and take a look at some more features of the wiki and in particular how you can make them Ajaxoriented using Atlas.

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ment is the class name with all characters in lowercase, whereas the second argument is the actual class name. The macro tells Qt that this class is the interface to the plugin. Each plugin can have only one interface, so this macro must be used exactly one time per plugin. Listing 11-5. Exporting the class as a plugin Q_EXPORT_PLUGIN2( textimageplugin, TextImagePlugin )