Simple .NET/ASP.NET PDF document editor web control SDK

foreach (CalendarEvent ev in events) { Console.WriteLine(ev.Title); }

This code works if events is an array CalendarEvent[] but it works equally well if events is a List<CalendarEvent>. And in fact, there are many more specialized collection types in the .NET Framework class library that we ll look at in a later chapter that foreach can work with. You can even arrange for it to work with custom collection classes you may have written yourself. All this is possible because the .NET Framework

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defines some standard interfaces for representing collections of things. The foreach construct depends on a pair of interfaces: IEnumerable<T> and IEnumerator<T>. These derive from a couple of nongeneric base interfaces, IEnumerable and IEnumerator. These interfaces are defined in the class library, and they are reproduced in Example 7-29.

namespace System.Collections.Generic { public interface IEnumerable<out T> : IEnumerable { new IEnumerator<T> GetEnumerator(); } public interface IEnumerator<out T> : IDisposable, IEnumerator { new T Current { get; } }

First a LinkFilter is created and installed as an event filter for the dialog. The linkClicked signal is connected to the showLink slot of the dialog. Notice that the WhatsThisClicked event is passed through the dialog so you can intercept clicked links for all widgets in the dialog here. Since the filter is installed on the dialog it is possible to install the filter from a main window before showing the dialog. After the filter is installed, a QPushButton widget is created and the What s this text is set. To create a link, the <a href='nnn'> ... </a> tag is used. The nnn part is the string passed as the href property of the QWhatsThisClickedEvent and then passed on through the linkClicked signal. The text between the <a href=...> and </a> parts is the text that will be shown as a link. Before the constructor ends, the push button is placed in a layout. Listing 9-8. Setting up a dialog with the LinkFilter event filter LinkDialog::LinkDialog() : QDialog() { LinkFilter *filter = new LinkFilter( this ); this->installEventFilter( filter ); connect( filter, SIGNAL(linkClicked(const QString&)), this, SLOT(showLink(const QString&)) ); QPushButton *button = new QPushButton( "What is this " ); button->setWhatsThis( "This is a <a href='test link'>test link</a>." ); QGridLayout *layout = new QGridLayout( this ); layout->addWidget( button, 0, 0 ); } Figure 9-8 shows the What s this text and the link being shown. When the user clicks the link, a QWhatsThisClickedEvent is triggered, the linkClicked signal is emitted, and the showLink slot is triggered. The source code of the slot is shown in Listing 9-9.

movePrevious()

}

namespace System.Collections { public interface IEnumerable { IEnumerator GetEnumerator(); } public interface IEnumerator { bool MoveNext(); object Current { get; } void Reset(); }

}

Listing 12-18 The variables making a semaphore-monitored, thread-safe buffer const int bufferSize = 20; QChar buffer[ bufferSize ]; QSemaphore freeSpace( bufferSize ); QSemaphore availableData( 0 ); bool atEnd = false; The buffer will be filled from index 0 to bufferSize-1 and then will begin to increase starting from 0 Before putting a character in the buffer, the producer will acquire from the freeSpace semaphore When the character has been put in the buffer, the producer will release to the availableData semaphore This means that if nothing consumes data from the buffer, it will be filled and the availableData semaphore value will be equal to bufferSize, and the producer will not be able to acquire any more free space The producer class in the application is called TextProducer Its constructor expects a QString as argument and stores the string in the private member variable m_text.

The split between the generic and nongeneric interfaces here is a historical artifact. Versions 1.0 and 1.1 of .NET did not support generics, so only the base IEnumerable and IEnumerator interfaces existed. When .NET 2.0 shipped in 2005, generics were introduced, making it possible to provide versions of these interfaces that were explicit about what type of objects a collection contains, but in order to maintain backward compatibility the old version 1.x interfaces had to remain. You will normally use the generic versions, because they are easier to work with. Conceptually, if a type implements IEnumerable<T> it is declaring that it contains a sequence of items of type T. To get hold of the items, you can call the GetEnumerator method, which will return an IEnumerator<T>. An enumerator is an object that lets you work through the objects in an enumerable collection one at a time. The split between enumerables and enumerators makes it possible to have different parts of your program

If you re familiar with C++ and its Standard Template Library, an enumerator is broadly similar in concept to an iterator in the STL.