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

Using a mutex to protect a variable can sometimes result in a potential performance decrease. Two threads can read the value of a shared variable simultaneously without locking it, but if a third thread enters the scene and tries to update the variable, it has to lock it. To handle this situation, Qt provides the QReadWriteLock class. This class works much like QMutex, but instead of a lock method it provides the methods lockForRead and lockForWrite. Just as when using QMutex, you can use these methods directly or you can use the QReadLocker and QWriteLocker classes that lock a QReadWriteLock when being constructed and unlock it when being destructed. Let s try using a QReadWriteLock in an application. You ll change the behavior of the TextDevice so that the counter is not updated from the write method, but from a new method called increase. The TextThread objects will still be there calling write, but you ll add another thread class for increasing the counter. This class, which is called IncreaseThread, simply calls increase of a given TextDevice object at an even interval. Let s start by having a look at the class declaration of the new TextDevice class, shown in Listing 12-11. Compared with the code in Listing 12-6, the QMutex has been replaced by a QReadWriteLock, and the increase method has been added to the interface. Listing 12-11. The TextDevice class declaration with a QReadWriteLock class TextDevice { public: TextDevice(); void increase(); void write( const QString& );

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That compiles, but we end up losing the Name property that the element in that location previously had, because we overwrote the entire value of the element. Since set doesn t work, that leaves get. The C# compiler could interpret this code:

Returns an array specifying all the bindings of this control. When used in Atlas Script, this is a child tag that contains <binding> tags that define the bindings for this control. Gets or sets the dataContext for the binding. Gets or sets the ID for the ItemView control. Specifies the underlying <div> tag to which this ItemView control will push content.

aai[20].Number = 456;

as being equivalent to the code in Example 7-27.

private: int count; QReadWriteLock lock; }; In the implementation shown in Listing 12-12, you can see the changes made to the TextDevice class. The new method increase creates a QWriteLocker referencing the QReadWriteLock before altering the counter. The updated write method creates a QReadLocker in the same manner before using the counter when creating the text that is sent to the debug console. The code is fairly easy to read and understand, even though the newly implemented protection feature is a fairly complex concept. Listing 12-12. The TextDevice class implementation using the QReadLocker and QWriteLocker to protect the count member variable TextDevice::TextDevice() { count = 0; } void TextDevice::increase() { QWriteLocker locker( &lock ); count++; } void TextDevice::write( const QString& text ) { QReadLocker locker( &lock ); qDebug() << QString( "Call %1: %2" ).arg( count ).arg( text ); } The IncreaseThread class bears many similarities to the TextThread class (the class declaration is shown in Listing 12-13). Because it is a thread, it inherits QThread. The constructor accepts a pointer to the TextDevice object to call increase on, and the class contains a private pointer to such a device (named m_device) for keeping that pointer. Listing 12-13. The IncreaseThread class declaration class IncreaseThread : public QThread { public: IncreaseThread( TextDevice *device ); void run(); private: TextDevice *m_device; };

CanChange elem = aai[20]; elem.Number = 456;

And in fact, that s what it would have done if we were using a reference type. However, it has noticed that CanChange is a value type, and has therefore rejected the code. (The error message says nothing about value types, but you can verify that this is the heart of the problem by changing the CanChange type from a struct to a class. That removes the compiler error, and you ll find that the code aai[20].Number = 456 works as expected.) Why has the compiler rejected this seemingly obvious solution Well, remember that the crucial difference between reference types and value types is that values usually involve copies if you retrieve a value from an indexer, the indexer returns a copy. So in Example 7-27 the elem variable holds a copy of the item at index 20. Setting elem.Number to 456 has an effect on only that copy the original item in the array remains unchanged. This makes clear why the compiler rejected our code the only thing it can do with this:

aai[20].Number = 456;