ASP.NET PDF Viewer using C#, VB/NET

The changes to the code are minimal. First, the definition of the class is altered slightly, as shown in Listing 1-2. The parent argument is also added to the constructor as a convenience because QObject has a function, setParent, which can be used to assign an object instance to a parent after creation. However, it is common and recommended to pass the parent as an argument to the constructor as the first default argument to avoid having to type setParent for each instance created from the class. Listing 1-2. Inheriting QObject and accepting a parent #include <QObject> #include <string> using std::string; class MyClass : public QObject { public: MyClass( const string& text, QObject *parent = 0 ); ... };

barcode font for excel download, barcode generator excel kostenlos, barcode in excel 2017, barcode check digit excel formula, barcode in excel vba, barcodes excel 2010 free, barcode excel 2010 microsoft, random barcode generator excel, barcode data entry excel, how to make barcode in excel 2003,

slow operations don t work synchronously under the covers Take fundamental operations such as reading and writing data from and to devices such as network cards or disks, for example The kernel-mode device drivers that manage disk and network I/O are instructed by the operating system to start doing some work, and the OS expects the driver to configure the hardware to perform the necessary work and then return control to the operating system almost immediately on the inside, Windows is built around the assumption that most slow work proceeds asynchronously, that there s no need for code to progress strictly in sync with the work This asynchronous model is not limited to the internals of Windows there are asynchronous public APIs.

These typically return very quickly, long before the work in question is complete, and you then use either a notification mechanism or polling to discover when the work is finished The exact details vary from one API to another, but these basic principles are universal Many synchronous APIs really are just some code that starts an asynchronous operation and then makes the thread sleep until the operation completes An asynchronous API sounds like a pretty good fit for what we need to build responsive interactive applications# So it seems somewhat ludicrous to create multiple threads in order to use synchronous APIs without losing responsiveness, when those synchronous APIs are just wrappers on top of intrinsically asynchronous underpinnings Rather than creating new threads, we may as well just use asynchronous APIs directly where they are available, cutting out the middle man NET defines two common patterns for asynchronous operations.

There s a low-level pattern which is powerful and corresponds efficiently to how Windows does things under the covers And then there s a slightly higher-level pattern which is less flexible but considerably simpler to use in GUI code..

The Select control has the properties described in Table 4-26. Table 4-26. Select Control Properties

Note To access the QObject class, the header file <QObject> has to be included. This works for most Qt classes; simply include a header file with the same name as the class, omitting the .h, and everything should work fine.

The Asynchronous Programming Model (APM) is a pattern that many asynchronous APIs in the .NET Framework conform to. It defines common mechanisms for discovering when work is complete, for collecting the results of completed work, and for reporting errors that occurred during the asynchronous operation. APIs that use the APM offer pairs of methods, starting with Begin and End. For example, the Socket class in the System.Net.Sockets namespace offers numerous instances of this pattern: BeginAccept and EndAccept, BeginSend and EndSend, BeginConnect and EndConnect, and so on.

#Asynchronous APIs tend to be used slightly differently in server-side code in web applications. There, they are most useful for when an application needs to communicate with multiple different external services to handle a single request.

The exact signature of the Begin method depends on what it does. For example, a socket s BeginConnect needs the address to which you d like to connect, whereas BeginReceive needs to know where you d like to put the data and how much you re ready to receive. But the APM requires all Begin methods to have the same final two parameters: the method must take an AsyncCallback delegate and an object. And it also requires the method to return an implementation of the IAsyncResult interface. Here s an example from the Dns class in System.Net:

public static IAsyncResult BeginGetHostEntry( string hostNameOrAddress, AsyncCallback requestCallback, object stateObject )