.NET/Java PDF, Tiff, Barcode SDK Library

The last part to add is the text below the shapes This is done by setting up a large font and then using it in a call to addText The addText works like drawText but allows the text to start only from a given point (that is, no texts contained in rectangles) This forms a whole bunch of closed regions that form the text: QFont font = QApplication::font(); fontsetPixelSize( 40 ); pathaddText( 20, 180, font, "Path" ); When the painter path is complete, all that s left to do is stroke it with a painter In the following code, you configure a pen and a brush for a painter Then the drawPath method is used to draw the actual painter path Figure 7-14 shows that when the regions overlap, the brush is not applied This makes it possible to create hollow paths by putting other paths inside them.

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With this code in place, you would see the message printed out by that constructor just once at the beginning of the program static constructors run exactly once.

In case you re wondering, yes, static fields can be marked as readonly. And just as a normal readonly field can only be modified in a constructor, a static readonly field can only be modified in a static constructor.

But when exactly do static constructors run We know when regular members get initialized and when normal constructors run that happens when we new up the object. Everything gets initialized to zero, and then our constructor(s) are called to do any other initialization that we need doing. But what about static initialization The static constructor will run no later than the first time either of the following happens: you create an instance of the class; you use any static member of the class. There are no guarantees about the exact moment the code will run it s possible you ll see them running earlier than you would have expected for optimization reasons. Field initializers for static fields add some slight complication. (Remember, a field initializer is an expression that provides a default value for a field, and which appears in the field declaration itself, rather than the constructor. Example 3-44 shows some examples.) .NET initializes the statics in the order in which they are declared. So, if you

The designer then generates the <ErrorTemplate> code for you. Here is the code associated with the custom error template shown in Figure 6-12:

reference one static field from the initializer for another static field in the same class, you need to be careful, or you can get errors at runtime. Example 3-44 illustrates how this can go wrong. (Also, the .NET Framework is somewhat noncommittal about exactly when field initializers will run in theory it has more freedom than with a static constructor, and could run them either later or earlier than you might expect, although in practice, it s not something you d normally need to worry about unless you re writing multithreaded code that depends on the order in which static initialization occurs.)

paintersetPen( Qt::black ); paintersetBrush( Qt::gray ); painterdrawPath( path ); Paths can consist of more shapes than the ones used in the preceding source code The following list mentions some of the methods that you can use to add shapes to your path: addEllipse: Adds an ellipse or circle addRect: Adds a rectangle addText: Adds text addPolygon: Adds a polygon When building a region from lines, arcs, and other components, the following methods can be useful: moveTo: Moves the current position lineTo: Draws a line to the next position arcTo: Draws an arc to the next position cubicTo: Draws a cubic Bezier curve (a smooth line) to the next point closeSubpath: Closes the current region by drawing a straight line from the current position to the starting point.

class Bar { public bool myField; } // Bad - null reference exception on construction class Foo { public static bool field2 = field1.myField; public static Bar field1 = new Bar(); } // OK - initialized in the right order class Foo { public static Bar field1 = new Bar(); public static bool field2 = field1.myField; }

Summary

We saw how to define classes from which we can create instances called objects, and that this can be useful when attempting to model real-world entities. We can also define value types, using the struct keyword, and the main difference is that when we assign variables or pass arguments, value types always copy the whole value, whereas ordinary classes (which are reference types) only copy a reference to the underlying object. We also saw a simpler kind of type: enum. This lets us define named sets of constant values, and is useful when we need a value representing a choice from a fixed set of options. So, now we know how to abstract basic ideas of information storage (through fields and simple properties) and manipulation (through functions and calculated properties), using classes and objects. In the next chapter, we re going to look at how we can extend these ideas further using a concept called polymorphism to model a hierarchy of related classes that can extend or refine some basic contract.

Paths can be very useful for representing shapes that you need to draw over and over again, but their true potential is shown when they are combined with brushes (discussed next)..