.NET/ASP.NET/C#/VB.NET PDF Document SDK

Let s start by having a look at the filter plugin shown in Figure 11-5. The class is called Flip (its declaration is shown in Listing 11-17). The header file includes the filter interface class declaration so the plugin knows how to define the class according to the interface s specification. As shown in the listing, Flip inherits QObject and FilterInterface. It is important that QObject is inherited first; otherwise the meta-object compiler will fail. The class declaration then starts with the Q_OBJECT macro followed by a Q_INTERFACES macro, indicating that the class implements the FilterInterface interface. Following the macro declarations you ll find the required methods. Since the base class contains only pure virtual methods, all methods must be implemented here. If not, the plugin class can t be instantiated.

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array s contents enclosed in braces, which is called an initializer list. But this requires you to know exactly what you want in the array when you write the code. You will often work with information that your program discovers at runtime, perhaps from a database or a web service. So C# offers an alternative mechanism that lets you choose the array s size at runtime. For example, suppose I decide I d like to display the events in my calendar as a numbered list. I already have an array of event names, but I d like to build a new string array that adds a number to the event text. Example 7-3 shows how to do this.

static string[] AddNumbers(string[] names) { string[] numberedNames = new string[names.Length]; for (int i = 0; i < names.Length ; ++i) { numberedNames[i] = string.Format("{0}: {1}", i, names[i]); } return numberedNames; }

This AddNumbers method doesn t know up front what will be in the array it creates it s building a modified copy of an existing array. So instead of creating a fixed list of items, it uses this syntax: new ElementType[arrayLength]. This specifies the two things that are fixed when you create a new array: the type and number of elements. When you create an array with this minimal syntax, the elements all start out with their default values. With the string type used here, the default is null; an array of numbers created this way would contain all zeros. So Example 7-3 immediately goes on to populate the newly created array with some useful values. In fact, that s also what happened in Example 7-1 when you provide an array with a list of initial contents, the C# compiler turns it into the sort of code shown in Example 7-4.

Listing 11-17. The class declaration of the filter Flip #include "filterinterface.h" class Flip : public QObject, FilterInterface { Q_OBJECT Q_INTERFACES(FilterInterface) public: QString name() const; QImage filter( const QImage &image ) const; }; The implementation of the name method is pretty straightforward. Because the name is used in the user interface, it is passed in a more human-readable form than just Flip. The source code can be seen in Listing 11-18. Listing 11-18. The full name of Flip is "Flip Horizontally" QString Flip::name() const { return "Flip Horizontally"; } The filter method is slightly more complex (see the implementation source code in Listing 11-19). The resulting image is created from the dimensions and format of the given input image. Then the flip is made before the resulting image is returned. Listing 11-19. The filter method flips the given image and returns the result. QImage Flip::filter( const QImage &image ) const { QImage result( image.width(), image.height(), image.format() ); for( int y=0; y<image.height(); ++y ) for( int x=0; x<image.width(); ++x ) result.setPixel( x, image.height()-1-y, image.pixel( x, y ) ); return result; } Before you finish the implementation of the Flip filter, you must tell Qt that the class implements the interface of the plugin. This is done by using the Q_EXPORT_PLUGIN2, just as with the image file format plugin (see Listing 11-20). Listing 11-20. It is important to tell Qt that Flip is the plugin interface. Q_EXPORT_PLUGIN2( flip, Flip )

string[] eventNames = new string[5]; eventNames[0] = "Swing Dancing at the South Bank"; eventNames[1] = "Saturday Night Swing"; eventNames[2] = "Formula 1 German Grand Prix"; eventNames[3] = "Swing Dance Picnic"; eventNames[4] = "Stompin' at the 100 Club";

The array initialization syntax in Example 7-1 is really just convenient shorthand the .NET Framework itself always expects to be told the number and type of elements, so it s just a matter of whether the C# compiler works out the element count for you. The Example 7-1 shorthand works only at the point at which you declare a variable. If your program decides to put a new array into an existing variable, you ll find that the syntax no longer works (see Example 7-5).

// Won't compile! eventNames = { "Dean Collins Shim Sham Lesson", "Intermediate Lindy Hop Lesson", "Wild Times - Social Dancing at Wild Court" };