.NET/Java PDF, Tiff, Barcode SDK Library

This ability to continue from where we left off as the consumer iterates through the loop illustrates a subtler benefit of yield return: it doesn t just make the code slightly neater; it lets the code be lazy.

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The AddNumbers method in Example 7-31 creates all of its output before it returns anything. We could describe it as being eager it does all the work it might need to do right up front. But the modified version in Example 7-32, which uses yield return, is not so eager: it generates items only when it is asked for them, as you can see from the output of Example 7-33. This approach of not doing work until absolutely necessary is often referred to as a lazy style. In fact, if you look closely at the output you ll see that the AddNumbers method in Example 7-33 is so lazy, it doesn t seem to run any code at all until we start asking it for items the Starting AddNumbers message printed out at the beginning of the AddNumbers method (before it starts its foreach loop) doesn t appear when we call AddNumbers as you can see, the Starting main loop message appears first, even though Main doesn t print that out until after AddNumbers returns. This illustrates that none of the code in AddNumbers runs at the point when we call AddNumbers. Nothing happens until we start retrieving elements.

Support for lazy collections is the reason that IEnumerable<T> does not provide a Count property. The only way to find out how many items are in an enumeration is to enumerate the whole lot and see how many come out. Enumerable sequences don t necessarily know how many items they contain until you ve asked for all the items.

consumer.wait(); return 0; } Although the results from different executions of the dual producer version differ from time to time, there is a repeating pattern. Listing 12-25 shows a result from one execution. You can see that the lowercase producer takes control first, the uppercase producer cuts in, they shift once or twice, and one of the threads takes the lead. The thread taking the start varies from time to time, and the number of times that the leading thread changes differs from time to time. The repeating pattern each time is that the distribution between the two threads is uneven. One thread always provides the majority of the characters. The reason for this pattern is that threads are scheduled to run for a few milliseconds each before they lose focus. When the buffer has been filled and the producers cannot acquire more free space, either thread can take the lead when there is free space again. Listing 12-25. The characters received by the TextConsumer this text is writTHteIS TEXT nIS WRITTEN USING UPP ER CASE CHARACTEuRS !IT WILL COMPEsTE WITH TEXT WiRITTEN USING LOnWER CASE CHARACTgERS! lower case characters.it will compete with text written using upper case characters.

Lazy enumeration has some benefits, particularly if you are dealing with very large quantities of information. Lazy enumeration makes it possible to start processing data as soon as the first item becomes available. Example 7-34 illustrates this. Its GetAllFilesInDirectory returns an enumeration that returns all the files in a folder, including all those in any subdirectories. The Main method here uses this to enumerate all the files on the C: drive. (In fact, the Directory class can save us from writing all this code there s an overload of Directory.EnumerateFiles that will do a lazy, recursive search for you. But writing our own version is a good way to see how lazy enumeration works.)

It is implemented in Atlas Script using the child tag <layoutTemplate> that specifies the HTML for how the bound controls will appear This specifies the CSS for individual items within the range It is complemented by alternatingItemCssClass Specifies the template for the items that will be rendered within the ItemView control It uses the <itemTemplate> child tag within Atlas Script, and each <itemTemplate> will contain a number of child controls Specifies what to render within the ListView control if there are no records within the data Specifies the CSS class for the selected item Can be used to highlight the selected item against the range of existing items This specifies how columns will be visually separated Defines the HTML code for the column separators Complements separatorCssClass..

class Program { static IEnumerable<string> GetAllFilesInDirectory(string directoryPath) { IEnumerable<string> files = null; IEnumerable<string> subdirectories = null; try {

}

Until now you have relied on shared buffers for passing data between threads. There is also a slightly more costly (but far easier) solution: using signals and slots. Using them avoids having to create and use buffers; instead, you can use the event-driven paradigm throughout the entire application.