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    Step 4. Implementation of OnExecute handler  

On this step we'll add a code to the OnExecute event handler. Note that this event handler is not necessary so if you do not need to add a code to it then do not leave the empty OnExecute handler in your code - it will break normal operation of the service.

There is only one rule that must be followed: the ProcessRequests method should be called from this event handler. This method is an analog of the Application.ProcessMessages in the regular application. The ProcessRequests method has only one boolean parameter. The False value of this parameter means that ProcessRequests should check the message queue for control messages, process it if necessary and return. The True value means that this method should not return until service terminates. So if you need to do something while service runs use the OnExecute event as shown below:




 

 
 

 

procedure TSampleService2.SampleService2Execute(Sender: TObject);
begin
    while not Terminated do
    begin
        ProcessRequests(False);
        DoWhatYouNeed; 
    end;
end;

The Terminated property indicates whether the service should be stopped or it can continue working. In this example our "necessary operation" will be sleeping again. Modify our example as shown below.




 

 
 

 

procedure TSampleService2.SampleService2Execute(Sender: TObject);
begin
    while not Terminated do
    begin
        ProcessRequests(False);
        Sleep(1000); 
    end;
end;

Please note that Terminated property is changed as a result of ProcessRequests call. It means that OnStop event can be fired during this call. If you free some memory in the OnStop handler or destroy some objects that are used in the OnExecute main loop then it is possible to get AV error. There is more safe way to implement OnExecute loop:




 

 
 
 

 

procedure TSampleService2.SampleService2Execute(Sender: TObject);
begin
    while True Terminated do
    begin
        ProcessRequests(False);
        if Terminated then break;
        DoWhatYouNeed; 
    end;
end;

Warning: The final part of this step shows the problems that occur if the ProcessRequests call is lost. It is recommended to use a debug mode to perform this test. In other case you will have to restart your computer to stop uncontrollable service. In addition it is recommended to change the service start mode (StartType property) to stManual and reinstall service. It will prevent the automatic service start after reboot.

Well, if you are ready to continue then save changes, compile your example and run it. It works and can be controlled as usually. Now try to comment the ProcessRequests call and test our service again (in debug mode or not as you wish). You will see that service starts normally but becomes uncontrollable after that. It does not stop or pause because control requests are not processed. If you've started the service in debug mode just close the debug window to terminate it. In other case you will have to restart your computer.

The final conclusion is: call ProcessRequests(False) periodically from the OnExecute event handler.

The next step of this example shows how to use advanced tracing API to simplify the debugging of your application.

 

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