Threading in Delphi for .NET - Events
(Page 12 of 15 )
Although preventing access to a critical block of code is an important task when writing multithreaded applications, there is also the need to be able to signal a waiting thread. It is common to have one or more threads produce work for other threads to manipulate. Threads that create work are referred to as producer threads. Consumer threads are those that do the actual work handed out by the producer threads.
The signal used by the .NET Framework is known as an event. Two types of events are available—AutoResetEvent and ManualResetEvent. When an event is set, it is signaled. Similarly, when an event is reset, the state of the event is not signaled. Events are automatically reset by the .NET Framework when exactly one thread is released when waiting on the event. Manually reset events must be cleared programmatically, and it is likely for multiple threads to be released. Listings 14.12 and 14.13 show the class definition of both types of events.
Listing 14.12 Declaration of the System.Threading.ManualResetEvent Class
System.Threading.ManualResetEvent = class (System.Threading.WaitHandle)
public
constructor Create(initialState: Boolean);
procedure Close; virtual;
function WaitOne: Boolean; virtual; overload;
function WaitOne(timeout: TimeSpan; exitContext: Boolean) :
Boolean; virtual; overload;
function WaitOne(millisecondsTimeout: Integer; exitContext: Boolean) :
Boolean; virtual; overload;
function Reset: Boolean;
function Set: Boolean;
class function WaitAll(waitHandles: WaitHandle[];
millisecondsTimeout: Integer; exitContext: Boolean) : Boolean; overload; static;
class function WaitAll(waitHandles: WaitHandle[]; timeout: TimeSpan;
exitContext: Boolean) : Boolean; overload; static;
class function WaitAll(waitHandles: WaitHandle[]) : Boolean; overload; static;
class function WaitAny(waitHandles: WaitHandle[];
millisecondsTimeout: Integer; exitContext: Boolean) : Integer; overload; static;
class function WaitAny(waitHandles: WaitHandle[]; timeout: TimeSpan;
exitContext: Boolean) : Integer; overload;
class function WaitAny(waitHandles: WaitHandle[]) : Integer; overload; static;
property Handle: System.IntPtr read; write;
end;
Listing 14.13 Declaration of the System.Threading.AutoResetEvent Class
System.Threading.AutoResetEvent = class (System.Threading.WaitHandle,
IDisposable)
public
constructor Create(initialState: Boolean);
function Reset: Boolean;
function Set: Boolean;
end;
Using either type of event is identical. Use the Set() method to signal an event and the Reset() method to clear the signal. Notice that several methods are used for waiting on an event. The overloaded WaitOne() method waits for only one event, whereas the WaitAll() method waits for every entity to be signaled, and the WaitAny() method allows for waiting for any one signal to be fired. Finally, both the WaitAll() and WaitAny() methods provide the capability of waiting for a Mutex or either type of event. Recall that the WaitAll() and WaitAny() methods are inherited from WaitHandle and are listed in the earlier section "The System.Threading.WaitHandle Class."
Find the code on the CD: \Code\Chapter 14\Ex06\prodcons.dpr for an example demonstrating how to use Events. Be sure to define the USE_EVENTS symbol.
 This chapter is from Delphi for .NET Developer's Guide, by Xavier Pacheco (Sams, 2004, ISBN: 0-672-32443-1). Check it out at your favorite bookstore today.
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