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

Instead of letting the TextThread threads write the text to qDebug directly, let the threads operate through a TextDevice object. It s called a text device because it simulates a shared device for printing text. To print text using the device, use the write method, which writes the given text to the debug console. It also enumerates all texts so that you can tell how many times the write method has been called. The TextDevice class declaration can be seen in Listing 12-6. The class contains what you expect from the preceding description of it: a constructor, a write method, a counter for enumerating the calls, and a QMutex for protecting the counter. Listing 12-6. The TextDevice class declaration class TextDevice { public: TextDevice(); void write( const QString& ); private: int count; QMutex mutex; }; The implementation of the TextDevice class demonstrates a new trick. Listing 12-7 shows how the QMutexLocker class is used to lock the mutex. The mutex locker locks the mutex as soon as it is constructed and then unlocks the mutex when it is being destructed. You could have opted for a solution in which you called lock and unlock explicitly, but by using the QMutexLocker you ensure that the mutex is unlocked even if you exit from a return statement in the middle of the method or when reaching the end of the method. The consequence is that the write method cannot be entered twice from different threads the calls will be serialized. Listing 12-7. The TextDevice class implementation TextDevice::TextDevice() { count = 0; }

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aai.TheArray[10].Number = 123; Console.WriteLine(aai.TheArray[10].Number);

That works it prints out 123 as you d expect. But this does not work:

aai[20].Number = 456;

void TextDevice::write( const QString& text ) { QMutexLocker locker( &mutex ); qDebug() << QString( "Call %1: %2" ).arg( count++ ).arg( text ); } The TextThread class run method has not changed much compared with the original Listing 12-2. Now the write method is called instead of qDebug. The change is highlighted in Listing 12-8. The m_device member variable is a pointer to the TextDevice object to use. It is initialized from a given pointer in the constructor. Listing 12-8. The TextThread::run method now calls write instead of outputting directly to qDebug void TextThread::run() { while( !stopThreads ) { m_device->write( m_text ); sleep( 1 ); } } The main function has also been slightly revised, compared with what you saw in Listing 12-3. The new version creates a TextDevice object that is passed on the TextThread thread objects. The new version can be seen in Listing 12-9, in which the changes have been highlighted. Listing 12-9. A TextDevice object is instantiated and passed to the TextThread thread objects int main( int argc, char **argv ) { QApplication app( argc, argv ); TextDevice device; TextThread foo( "Foo", &device ), bar( "Bar", &device ); foo.start(); bar.start(); QMessageBox::information( 0, "Threading", "Close me to stop!" ); stopThreads = true; foo.wait(); bar.wait(); return 0; }

If you try this, you ll find that the C# compiler reports the following error:

You can bind the ItemView control to a <div> element in HTML and update it according to the settings within its <itemTemplate> child tag. This tag defines a series of controls and their bindings to the data property of the ItemView control. It is designed for viewing a single record at a time and is complemented by the ListView control, which is designed for viewing a range of records at a time. The control exposes the properties shown in Table 8-12. Table 8-12. ItemView Properties

error CS1612: Cannot modify the return value of 'ArrayAndIndexer<CanChange>.this[int]' because it is not a variable

That s a slightly cryptic message. But the problem becomes clear when we think about what we just asked the compiler to do. The intent of this code:

aai[20].Number = 456;

Building and executing the application results in a list of numbered "Foo" and "Bar" texts (an example can be seen in Listing 12-10). The order of the output is undefined, but the enumeration always works thanks to the mutex that protects the counter. Listing 12-10. A test run of the counting TextDevice "Call "Call "Call "Call "Call "Call "Call "Call "Call "Call "Call "Call 0: Foo" 1: Bar" 2: Bar" 3: Foo" 4: Bar" 5: Foo" 6: Bar" 7: Foo" 8: Bar" 9: Foo" 10: Bar" 11: Foo"

seems clear we want to modify the Number property of the item whose index is 20. And remember, this line of code is using our ArrayAndIndexer<T> class s indexer. Looking at Example 7-26, which of the two accessors would you expect it to use here Since

we re modifying the value, you might expect set to be used, but a set accessor is an all or nothing proposition: calling set means you want to replace the whole element. But we re not trying to do that here we just want to modify the Number property of the value, leaving its Name property unmodified. If you look at the set code in Example 7-26, it simply doesn t offer that as an option it will completely replace the element at the specified index in the array. The set accessor can come into play only when we re providing a whole new value for the element, as in:

aai[20] = new CanChange { Number = 456 };