ASP.NET Web PDF Document Viewer/Editor Control Library

Listing 12-20. The run method of the consumer class void TextConsumer::run() { int i = 0; while( !atEnd || availableData.available() ) { availableData.acquire(); qDebug() << buffer[ i ]; i = (i+1) % bufferSize; freeSpace.release(); } } When it comes to synchronizing the producer and consumer and their accesses to the buffer, it is very important to maintain control of the order in which the process occurs. Free space must be acquired before the data is put into the buffer, and available data must be released after the data has been written to the buffer. The same goes for taking data out of the buffer acquire available data before and release free space after. It is also important to update the atEnd flag before releasing free space to avoid the consumer getting stuck waiting for the available data semaphore while the atEnd flag is true. With the atEnd solution, there must also be at least one byte of data to transmit; otherwise, the consumer will hang. One solution is to transmit the length of the data first or an end-of-data token last. Listing 12-21 shows a main function using the TextProducer and TextConsumer classes. It initializes the producer with some contrived Latin text, starts both threads, and then waits for them both to complete. The order in which they are started and the order of the wait calls are irrelevant both threads will synchronize themselves using the semaphores. Listing 12-21. A main function using the TextProducer and TextConsumer classes int main( int argc, char **argv ) { QApplication app( argc, argv ); TextProducer producer( "Lorem ipsum dolor sit amet, " "consectetuer adipiscing elit. " "Praesent tortor." ); TextConsumer consumer; producer.start(); consumer.start(); producer.wait(); consumer.wait(); return 0; }

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What would you expect to happen if you tried to pass this method an IEnumera ble<TraineeFirefighter>, where TraineeFirefighter derives from FirefighterBase It seems like it should work ShowNames expects to get a sequence of FirefighterBase objects, and since TraineeFirefighter derives from FirefighterBase, an IEnumera ble<TraineeFirefighter> will return a sequence of objects that are all of type Firefight erBase (as well as being of type TraineeFirefighter). In C# 4.0, this works as you d expect. But it didn t in previous versions. In general, it s not safe to assume that types are necessarily compatible just because their type arguments happen to be compatible. For example, there s an IList<T> interface which defines an Add method. IList<TraineeFirefighter> cannot safely be converted to IList<FirefighterBase>, because the latter s Add method would allow anything derived from FirefighterBase (e.g., Firefighter, TraineeFirefighter) to be added, but in practice the implementer of IList<TraineeFirefighter> might not allow that it might accept only the TraineeFirefighter type.

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 data context for the binding. Gets or sets the ID for the ListView control. Specifies the underlying <div> tag to which this ListView control will push content. Returns the behaviors associated with this control. When using Atlas Script, you can specify these behaviors using the <behaviors> child tag.

IEnumerable<T> works here because the T type only ever comes out of an enumeration; there s no way to pass instances of T into IEnumerable<T>. The interface definition states this as Example 7-29 shows, the type argument is prefixed with the out keyword. In the official terminology, this means that IEnumerable<T> is covariant with T. This means that if type D derives from type B (or is otherwise type-compatible maybe B is an interface that D implements), IEnumerable<D> is type-compatible with IEnumerable<B>.

Generic arguments can also be prefixed with the in keyword, meaning that the type is only ever passed in, and will never be returned. The IComparable<T> interface we saw earlier happens to work this way. In this case, we say that IComparable<T> is contravariant with T it works the other way around. You cannot pass an IComparable<Train eeFirefighter> to a method expecting an IComparable<FirefighterBase>, because that method might pass in a different kind of FirefighterBase, such as Firefighter. But you can pass an IComparable<FirefighterBase> to a method expecting an ICompara ble<TraineeFirefighter> (even though you cannot pass a FirefighterBase to a method expecting a TraineeFirefighter). An IComparable<FirefighterBase> is capable of being compared to any FirefighterBase, and is therefore able to be compared with a TraineeFirefighter. By default, generic arguments are neither covariant nor contravariant. C# 4.0 introduced support for variance because the absence of variance with collection interfaces just seemed wrong IEnumerable<T> now works like most developers would expect.

Looking at the preceding example, note that there is a performance cost associated with the acquire and release calls. There is a similar cost for using mutexes and read-write locks, so it can sometimes give a performance boost to split the transmitted data into chunks. For example, it might have been quicker to send the string as words instead of character by character, which would mean acquiring space for several characters at once in the producer thread instead of one at a time and doing slightly more processing each time. This would, of course, introduce the performance penalty that the buffer isn t always fully used because the producer would sometimes block even if there is free space in the buffer.

Example 7-30 works much harder than it needs to it creates the enumerator explicitly, and walks through the objects by calling MoveNext in a loop, retrieving the Current value each time around. (A newly created enumerator needs us to call MoveNext before first reading Current. It doesn t automatically start on the first item because there might not be one collections can be empty.) As it happens, that s exactly what foreach does, so we can get that to do the work for us. Example 7-31 does the same thing as Example 7-30, but lets the C# compiler generate the code.

static string[] AddNumbers(IEnumerable<string> names) { List<string> numberedNames = new List<string>(); int i = 0; foreach (string currentName in names) { numberedNames.Add(string.Format("{0}: {1}", i, currentName)); i += 1; } return numberedNames.ToArray(); }