Event notification on streams during long reads or writes






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Aug 12, 2003

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An article on event notification for operations on large streams...
Introduction
Sometimes during programming, there are some methods that take a stream and then perform some function on it. Usually, if the stream is fairly large, then the method may not return for an extreme amount of time (e.g. ~20 minutes for SHA1 calculation on 2 GB file across a network). Some applications would simply like to get notified that there was some activity and hopefully the amount of activity. Being notified that a block of bytes is about to, or has been read would solve this problem. Thus, the EventStream
reader attempts to solve this problem. See the example for more information.
Using the code
Below is an example usage of this class, to be notified during a potentially long SHA1 calculation.
class ExampleNotifyClass
{
public int timesNotified = 0 ;
public void SectionWasRead( byte[] buff, int index, int count )
{
Console.WriteLine("Section read: {0}", count ) ;
timesNotified++ ;
}
///
/// Main
///
public static void Main() {
// Create the sample data to read
MemoryStream mem = new MemoryStream() ;
StreamWriter writer = new StreamWriter(mem, Encoding.UTF8) ;
// Populate the stream
for( int i = 0 ; i < 16 * 777 ; i++ )
{
writer.WriteLine( "{0}", i % 10 ) ;
}
// Reset the pointer in the stream
mem.Seek( 0, SeekOrigin.Begin ) ;
// Create a sample class
ExampleNotifyClass toNotify = new ExampleNotifyClass();
EventStream toRead = new EventStream( mem ) ;
toRead.AfterBlockRead += new
EventStream.AfterBlockHandler(toNotify.SectionWasRead ) ;
toRead.BlockEventSize = 8192 ;
// Get notified for every 8192 bytes read
SHA1 sha = new SHA1Managed() ;
// Actual work is done here
byte[] hash = sha.ComputeHash( toRead ) ;
Console.WriteLine( "Our class was notified {0} times.",
toNotify.timesNotified ) ;
}
}