.NET/Java PDF, Tiff, Barcode SDK Library

} catch (UnauthorizedAccessException) { Console.WriteLine("No permission to access " + directoryPath); } if (files != null) { foreach (string file in files) { yield return file; } } if (subdirectories != null) { foreach (string subdirectory in subdirectories) { foreach (string file in GetAllFilesInDirectory(subdirectory)) { yield return file; } } }

files = Directory.EnumerateFiles(directoryPath); subdirectories = Directory.EnumerateDirectories(directoryPath);

}

It was not possible to send signals between threads in Qt versions before Qt 4.0. Instead you had to Tip

static void Main(string[] args) { foreach (string file in GetAllFilesInDirectory(@"c:\")) { Console.WriteLine(file); } }

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If you run this, you ll find it starts printing out filenames immediately, even though it clearly won t have had time to discover every single file on the hard disk. (That s why we re not using the overload of Directory.GetFiles that recursively searches subdirectories for us. As you ll see in 8, the Directory class can save us from writing all this code, but it insists on finding all the files before starting to return any of them.) It s possible to chain enumerations together. For example, we can combine Example 7-34 with the AddNumbers function, as shown in Example 7-35.

The Atlas core library (AtlasCore.js) contains the facility to register namespaces and classes using the Type.registerNamespace and Type.registerClass commands. You can use these to build objects in JavaScript and assign them to namespaces for clearer, easier-to-read, and easier-to-debug code. Listing 3-1 shows the definition of the Car class you used earlier. This class is registered to the AtlasBook namespace. Listing 3-1. Creating a Car Namespace Type.registerNamespace("AtlasBook"); AtlasBook.Car = function(strMake, strModel, strYear) { var m_Make = strMake; var m_Model = strModel; var m_Year = strYear; this.getMake = function() { return m_Make; } this.getModel = function() { return m_Model; } this.getMakeandModel = function() { return m_Make + ' ' + m_Model; }

IEnumerable<string> allFiles = GetAllFilesInDirectory(@"c:\"); IEnumerable<string> numberedFiles = AddNumbers(allFiles); foreach (string file in numberedFiles) { Console.WriteLine(file); }

If we re using the version of AddNumbers from Example 7-32 the one that uses yield return this will start printing out filenames (with added numbers) immediately. However, if you try it with the version from Example 7-31, you ll see something quite different. The program will sit there for as many minutes as it takes to find all the filenames on the hard disk it might print out some messages to indicate that you don t have permission to access certain folders, but it won t print out any filenames until it has all of them. And it ends up consuming quite a lot of memory on my system it uses more than 130 MB of memory, as it builds up a huge List<string> containing all of the filenames, whereas the lazy version makes do with a rather more frugal 7 MB. So in its eagerness to do all of the necessary work up front, Example 7-31 actually slowed us down. It didn t return any information until it had collected all of the information. Ironically, the lazy version in Example 7-32 enabled us to get to work much faster, and to work more efficiently.

rely on passing custom events between the threads. This is still supported in Qt 4.0, but using signals and slots is much easier.

This style of enumeration, in which work is done no sooner than necessary, is sometimes called deferred execution. While that s more of a mouthful, it s probably more fitting in cases where the effect is the opposite of what lazy suggests.

Lazy enumeration also permits an interesting technique whereby infinite loops aren t necessarily a problem. A method can yield an infinite collection, leaving it up to the caller to decide when to stop. Example 7-36 returns an enumeration of numbers in the Fibonacci series. That s an infinite series, and since this example uses the BigInteger type introduced in .NET 4, the quantity of numbers it can return is limited only by space and time the amount of memory in the computer, and the impending heat death of the universe, respectively (or your computer s next reboot, whichever comes sooner).

using System.Numerics; // Required for BigInteger ... static IEnumerable<BigInteger> Fibonacci() { BigInteger current = 1; BigInteger previous = 1; yield return 1; while (true) { yield return current; BigInteger next = current + previous; previous = current; current = next; } }

There are some differences between passing signals among threads and using them within a thread. When emitting signals in a single threaded application or within a single thread, the emit call calls all the connected slots directly, and the emitting code is left waiting until the slots are done. When emitting a signal to an object living in another thread, the signal is queued. This means that the emit call will return before or at the same time that the slots are activated. It is possible to use queued signals within a single thread, too. All you need to do is explicitly tell connect that you want to create a queued connection. By default, connect uses direct connections within threads and queued connections between threads. This is the most