asp.net ean 13 Deriving New Widgets in Font

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Deriving New Widgets
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The purpose of this chapter is to teach you how to create new GObject types by deriving from those that already exist. The best way to learn how to derive new objects is by example. In this section, we will be creating a new widget called MyIPAddress, which will allow the user to enter an IP address. This allows you to restrict the user from entering anything but a valid IP address.
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Note While we are deriving a new widget in this section, the method outlined here applies to all objects derived from GObject. Therefore, you are not limited to deriving new widgets but can derive a new type from any other type that is derived directly or indirectly from GObject.
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CHAPTER 11 CREATING CUSTOM WIDGETS
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A screenshot of the GtkIPAddress widget created in this section is shown in Figure 11-1. Notice that all of the one-digit and two-digit numbers are aligned to the right.
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Figure 11-1. A MyIPAddress widget
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Creating the MyIPAddress Header File
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The first step in deriving any type of GObject is to create the header file. This file allows you to set up the basic function calls required by each object. It is also a good place to plan out your widget, since the header file will contain public functions that are available to any code using your new object. In order to accommodate C++ compilers, you should bracket the content of your header file with G_BEGIN_DECLS and G_END_DECLS. These two macros add extern "C" around the content, which will force all functions to use their names for symbol names when compiled as in C. An example of the shell of the header file is shown in Listing 11-1. Listing 11-1. MyIPAddress Header File (myipaddress.h) #ifndef __MY_IP_ADDRESS_H__ #define __MY_IP_ADDRESS_H__ #include <glib.h> #include <glib-object.h> #include <gtk/gtkentry.h> G_BEGIN_DECLS ... G_END_DECLS #endif /* __MY_IP_ADDRESS_H__ */
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Note Unlike most examples in this book, this chapter s examples have been split up into small parts instead of displaying them as a whole, because they span so many lines. You can download the full files of source code from the book s web site at www.gtkbook.com.
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CHAPTER 11 CREATING CUSTOM WIDGETS
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Along with the necessary structures and functions, you need to define five macros for every new widget, as shown in Listing 11-2. All of the function and structure declarations in the rest of the header file should be placed between G_BEGIN_DECLS and G_END_DECLS. The macros in Listing 11-2 follow a standard naming scheme that every GObject uses. Standard naming makes object inheritance a lot simpler. Listing 11-2. GObject Directives (my_ip_address_get_type ()) (G_TYPE_CHECK_INSTANCE_CAST ((obj), \ MY_IP_ADDRESS_TYPE, MyIPAddress)) #define MY_IP_ADDRESS_CLASS(klass) (G_TYPE_CHECK_CLASS_CAST ((klass), \ MY_IP_ADDRESS_TYPE, MyIPAddressClass)) #define IS_MY_IP_ADDRESS(obj) (G_TYPE_CHECK_INSTANCE_TYPE ((obj), \ MY_IP_ADDRESS_TYPE)) #define IS_MY_IP_ADDRESS_CLASS(klass) (G_TYPE_CHECK_CLASS_TYPE ((klass), \ MY_IP_ADDRESS_TYPE)) All of the functions used by your new object should be prefixed with the library name such as GTK_ or GDK_, followed by the widget name. The first macro you need to define is MY_IP_ADDRESS_TYPE. This macro returns the GType structure corresponding to your object. We will define my_ip_address_get_type() later in the header file. The next macro, MY_IP_ADDRESS(), is used to cast the object as MyIPAddress. This is similar to casting an object with functions such as GTK_WIDGET(), GTK_ENTRY(), or G_OBJECT(). G_TYPE_CHECK_INSTANCE_CAST() performs two tasks. It first checks whether the object is of the correct type. If it is not, a warning will be emitted. Otherwise, the object will be cast as a MyIPAddress widget and returned. MY_IP_ADDRESS_CLASS() is used in the same way as MY_IP_ADDRESS(), except it is used to cast an object as MyIPAddressClass. You will soon learn the difference between these two types. #define MY_IP_ADDRESS_TYPE #define MY_IP_ADDRESS(obj)
Note Since your code may be compiled with a C++ compiler as well, you should always use klass
instead of class to refer to the widget class type, since the latter is a C++ keyword.
The last two macros, IS_MY_IP_ADDRESS() and IS_MY_IP_ADDRESS_CLASS(), are used to check whether the object is the correct type. Each function will return TRUE if the object is of the specified type. The next step is to define both the MyIPAddress and MyIPAddressClass structures, as in Listing 11-3. The widget content is held by _MyIPAddress. The first member in the new widget s structure should always be an instance of the type that you are deriving from. As you can see in the listing, the MyIPAddress widget is going to be derived from GtkEntry.
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