.NET/Java PDF, Tiff, Barcode SDK Library

The IDisposable interface has one member, Dispose; typically this interface is used when a class that wraps some unmanaged or limited resource, such as a database connection, calling Dispose ensures that the resource can always be reclaimed in a timely manner and you do not have to wait for garbage collection. The using function is a great way to ensure that the Dispose method of the IDisposable interface is always called when you are finished with the class, even if an exception is thrown. The using function has the type 'a -> ('a -> 'b) -> 'b when 'a :> IDisposable. It therefore has two parameters: the first must implement the IDisposable interface, and the second is a function that must take a parameter of type IDisposable interface and that can

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Direct Current (DC) motors almost always rotate too quickly to be connected directly to wheels and other loads. Some sort of gear train is necessary to lower the speed, which conveniently increases the torque. Both the NXT and RCX motors have built-in gear reduction, as do the newer LEGO Power Functions motors. You might run across several other LEGO motors, but these are the only ones that can be ordered as accessories for the NXT. The NXT motor is an impressive combination of gear reduction and feedback sensing (see Figure 1-28). The gear reduction alone involves eight different gears, and because the Rotation Sensor is way back by the motor end, it has one degree of revolution on the output shaft.

Listing 3-2. innerHTML.xml <table border="1"> <tbody> <tr> <th>Activity Name</th> <th>Location</th> <th>Time</th> </tr> <tr> <td>Waterskiing</td> <td>Dock #1</td> <td>9:00 AM</td> </tr> <tr> <td>Volleyball</td> <td>East Court</td> <td>2:00 PM</td> </tr> <tr> <td>Hiking</td> <td>Trail 3</td> <td>3:30 PM</td> </tr> </tbody> </table> Using responseText and innerHTML greatly simplifies adding dynamic content to the page. Unfortunately, the approach has its drawbacks. As mentioned, the innerHTML property is not a standard property of HTML elements, making its implementation by standards-compliant browsers optional. Most modern browsers, however, support the innerHTML property. Ironically, Internet Explorer, the browser that pioneered the use of innerHTML, has the most limited implementation of it. Many of today s modern browsers expose the innerHTML property as a read/ write property on all HTML elements. Internet Explorer, on the other hand, restricts the innerHTML property to read-only on HTML elements such as tables and table rows, somewhat limiting its usefulness.

return a parameter of any type. The parameter returned from this function is the return type of the using function. The following example illustrates this by opening a file to append to it: #light open System.IO using (File.AppendText("test.txt")) (fun streamWriter -> streamWriter.WriteLine("A safe way to write to a file")) The call to the BCL s method File.AppendText will return a System.IO.StreamWriter, which is a way of writing to a file. A file is managed by the operating system, so it has an unmanaged aspect. This is why System.IO.StreamWriter implements IDisposable. A System.IO.StreamWriter is passed to the function, which itself makes up the second parameter passed to the using function. This is a neat way of ensuring that the streamWriter is available only inside a limited scope of the function and that it is disposed of straightaway when the function ends. If you did not write your file access using the using function, you would have to explicitly call the Dispose method, or your text would not be flushed from memory into the file. Similarly, there would be a risk that an exception could occur. Although the risk is limited, there is a real risk that a file would not be closed properly and the text not written to it. It s worth noting that using returns unit because the call to StreamWriter.WriteLine returns unit. If you were performing an operation that didn t return unit, say reading from a file or database, you could return a value from the using function. The following example illustrates this by reading from a file and then binding the contents of the first line of that file to the identifier text: #light open System.IO let text = using (File.OpenText("test.txt")) (fun streamReader -> streamReader.ReadLine() )

You can see the gears and Rotation Sensor in the breakdown shown in Figure 1-29.

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