Correct event handling and waiting in C#

Stefan Cazacu

Hello. This is a follow up on a previously posted question (http://www.c-sharpcorner.com/Forums/Thread/276805/). I've revised my code and despite it not throwing an exception any more, it doesn't seem to pick up the event. I don't have a lot of documentation on the api but what i'm trying to achieve is: have a form with a white label; after pressing a switch, the label should turn yellow. I've created an event and passed it to the GpioSetupInterruptPin function, configured to trigger on a rising edge.

As soon as i call GpioSetupInterruptPin, the event triggers and doesn't seem to reset. To me, this suggests there is a problem with the event, the waiting for the event or the api doesn't really work. I've managed to contact the developer of the api but he said others have successfully used the api, that he doesn't have experience using C# and offered no additional information on how to use the api or how it works.

Can someone please offer a suggestion on how i could fix my code?

public partial class Form1 : Form

{

// P/Invoke CreateEvent and WaitForSingleObject
private void GPIO_Open() //get handle for gpio
private void GPIO_Output() //output pin declaration
private void button1_Click(object sender, EventArgs e)
{

Interrupt_Setup();

}
private void Interrupt_Setup()
{

hGPIO = GPIOapi.GpioOpenHandle(); //returns a handle to the gpio
GIPO_ON = true;
Debug.WriteLine("Driver open \n" + hGPIO);
GPIO_Output(); //set output pins
GPIO_Interrupt(Trigger); //configure interrupt

}
private void GPIO_Interrupt(string trigger)
{

bool ok;
_Main();
//INTERRUPT DECALRATION
ok = GPIOapi.GpioSetupInterruptPin(hGPIO, port6, 4, GPIOapi.INT_TRIGGER_MODE.TRIGGER_MODE_EDGE,
GPIOapi.INT_TRIGGER_POLARITY.TRIGGER_POL_HIGH_RISING, trigger, true);
if (!ok)

Debug.WriteLine("NO interrupt");

else

Debug.WriteLine("Interrupt set for:" + port6 + "04" + " at " + hGPIO);

}
public static string Trigger = "InputProcessUpdateHandler";
private static InputProcessor inputProcessor = null;
private void _Main()
{

public static IntPtr handle = CreateEvent(IntPtr.Zero, false, false, Trigger); //used P/Invoke
try
{

if (WaitForSingleObject(handle, 0xFFFFFFFF) == false)

label2.BackColor = Color.Red;

}
catch(Exception e)
{Debug.WriteLine("exception: "+e);}

inputProcessor = new InputProcessor();
ShowToggle showToggle = new ShowToggle(inputProcessor);
inputProcessor.Process(label1);

}

}

public class ShowToggle

{

private InputProcessor _inputProcessor = null;
public ShowToggle(InputProcessor inputProcessor)
{

_inputProcessor = inputProcessor;
_inputProcessor.updateHandledBy += InputProcessUpdateHandler;

}
private void InputProcessUpdateHandler(Label label)
{

label.BackColor = Color.Yellow;
Debug.Write("execute");

}

}

public class InputProcessor

{

public delegate void InputProcessUpdateHandler(Label label);
public event InputProcessUpdateHandler updateHandledBy = null;
public void Process(Label label)
{

if (updateHandledBy != null)
updateHandledBy(label);

}

}

9 Replies

Know the answer? Post it — somebody with the same question will find it here.

Vulpes11y ago
Accepted answer
I don't think the code is too bad but I have tried to tidy it up a bit.

As it's difficult to do this when you can't run it, I apologize in advance if I've introduced any bugs.

You'll notice that I've moved the color selection stuff to the ShowToggle class which I felt was the best place for it to be:

public partial class Form1 : Form
{
// STATIC FIELD DECLARATIONS
public static string Trigger = "InputProcessUpdateHandler";
private static InputProcessor inputProcessor = null;
public static IntPtr handle = CreateEvent(IntPtr.Zero, false, false, Trigger); //used P/Invoke

// INSTANCE FIELD DECLARATION
private bool stopped; // flag to show if Stop button pressed

// P/Invoke CreateEvent and WaitForSingleObject
private void GPIO_Open() //get handle for gpio
private void GPIO_Output() //output pin declaration

private void button1_Click(object sender, EventArgs e)
{
Interrupt_Setup();
}

private void button2_Click(object sender, EventArgs e) // stop button
{
stopped = true;

private void Interrupt_Setup()
{
hGPIO = GPIOapi.GpioOpenHandle(); //returns a handle to the gpio
GIPO_ON = true;
Debug.WriteLine("Driver open \n" + hGPIO);
GPIO_Output(); //set output pins
GPIO_Interrupt(Trigger); //configure interrupt
}

private void GPIO_Interrupt(string trigger)
{
bool ok;
_Main();
//INTERRUPT DECLARATION
ok = GPIOapi.GpioSetupInterruptPin(hGPIO, port6, 4, GPIOapi.INT_TRIGGER_MODE.TRIGGER_MODE_EDGE,
GPIOapi.INT_TRIGGER_POLARITY.TRIGGER_POL_HIGH_RISING, trigger, true);

if (!ok)
Debug.WriteLine("NO interrupt");
else
{
Debug.WriteLine("Interrupt set for:" + port6 + "04" + " at " + hGPIO);
stopped = false;
Thread waitThread = new Thread(WaitForTrigger);
waitThread.Start();
}
}

public void WaitForTrigger()
{
while(true)
{
try
{
if(stopped) break;
if (WaitForSingleObject(handle, 0xFFFFFFFF) == false)
{
inputProcessor.Process(label1);
}
Thread.Sleep(300);
}
catch (Exception e)
{
Debug.WriteLine("exception: " + e);
}
}
}

private void _Main()
{
try
{
if (!WaitForSingleObject(handle, 0xFFFFFFFF))
label2.BackColor = Color.Red;
}
catch(Exception e)
{
Debug.WriteLine("exception: "+ e);
}

inputProcessor = new InputProcessor();
ShowToggle showToggle = new ShowToggle(inputProcessor);
}

private void Form1_FormClosed(Object sender, FormClosedEventArgs e)
{
CloseHandle(handle);
GPIOapi.GpioCloseHandle(hGPIO); // or whatever the code is
}
}

public class ShowToggle
{
private Color[] color =
{
Color.Yellow, Color.Orchid, Color.DarkOrchid, Color.GreenYellow, Color.CornflowerBlue, Color.SteelBlue, Color.Crimson
};
private int i = 0;

private InputProcessor _inputProcessor = null;
public ShowToggle(InputProcessor inputProcessor)
{
_inputProcessor = inputProcessor;
_inputProcessor.updateHandledBy += InputProcessUpdateHandler;
}
private void InputProcessUpdateHandler(Label label)
{
label.BackColor = color[i];
i++;
if (i > 6) i = 0;
Debug.Write("execute");
}
}

public class InputProcessor
{
public delegate void InputProcessUpdateHandler(Label label);
public event InputProcessUpdateHandler updateHandledBy = null;
public void Process(Label label)
{
if (updateHandledBy != null)
updateHandledBy(label);
}
}
Stefan Cazacu11y ago
Thank you Vulpes. You have been incredibly helpful
Stefan Cazacu11y ago
Thank you for the advice regarding closing the handles and infinite loops as it has been very helpful. However, i can't help but feel I have a messy solution using some pointless code.
Vulpes11y ago
If you want the managed event to fire, then you'll need to fire it yourself:

if (WaitForSingleObject(handle, 0xFFFFFFFF) == false)
{
inputProcessor.Process(label1);
BeginInvoke(((System.Action)(() =>label2.BackColor = color[i])));
i++;
if (i > 4) i = 0;
}

You're correct, of course, that closing its handle will destroy the event object so you don't want to do this whilst you're waiting for further signals.

If this is all your application is doing, then you could perhaps close the event handle in the FormClosed event handler in addition to the handle for the gpio.

Possibly, the examples you've seen we're using a manual-reset rather than an auto-reset event and we're only waiting for a single signal which would explain why the handle was closed immediately thereafter.

There's nothing wrong with infinite loops as long as you have a way to (eventually) get out of them which you appear to be lacking at present. One thing you could do is to have a 'stop' button to set a bool flag to true and for your while(true) loop to check the state of this flag and break when its true.
Stefan Cazacu11y ago
Thank you Vulpes.

The way I was expecting the code to work was for the WaitForTrigger() function to change the colour of label2 once the setup is complete and have the InputProcessUpdateHandler() function change the colour of label1 once an i/o is toggled. The code is largely the same, the only change being:

private void GPIO_Interrupt(string trigger)
{
bool ok;
_Main();
//INTERRUPT DECALRATION
ok = GPIOapi.GpioSetupInterruptPin(hGPIO, port6, 4, GPIOapi.INT_TRIGGER_MODE.TRIGGER_MODE_EDGE,
GPIOapi.INT_TRIGGER_POLARITY.TRIGGER_POL_HIGH_RISING, trigger, true);
Thread waitThread=new Thread(WaitForTrigger);
waitThread.Start();
if (!ok)
Debug.WriteLine("NO interrupt");
else
Debug.WriteLine("Interrupt set for:" + port6 + "04" + " at " + hGPIO);
}

public Color[] color =
{
Color.Orchid, Color.DarkOrchid, Color.GreenYellow, Color.CornflowerBlue, Color.SteelBlue,Color.Crimson
};

public int i = 0;

public void WaitForTrigger()
{
while(true)
{try
{
if (WaitForSingleObject(handle, 0xFFFFFFFF) == false)
{
BeginInvoke(((System.Action)(() =>label2.BackColor = color[i])));
i++;
if (i > 4)
i = 0;
}
Thread.Sleep(300);
}
catch (Exception e)
{ Debug.WriteLine("exception: " + e); }}
}
}

I can see how to get the functionality i want from this code but so many things seem wrong with it, I don't feel that i want to use it. I'm also a bit unclear on the event CloseHandle(). All the examples I've seen call it after the event has been triggered, but if i close the handle, how does the event trigger a second time?
Vulpes11y ago
As the thread hasn't yet reached a satisfactory conclusion, I'd post the code you currently have here and we'll see if we can sort out the remaining issues.
Stefan Cazacu11y ago
Hi. Thank you for the replies and sorry for the long delay on my part (family emergency).

Declaring the handle as public static was just an issue with the code i copied into the post (not sure what i did there). Thank you for the tip of calling WaitForSingleObject() after the call to GpioSetupInterruptPin().

Similarly, thank you to theLizard for the code sample.

I got the code working, however, i still have a few issues with it. I'm not really sure why it's working. It's not the event function that gets called when pressing the button, but whatever i call after WaitForSingleObject(). Also I've also used really messy coding and an infinite loop "while(1)"
Should I post the new code here, edit my original post or start a new thread?
theLizard11y ago
I may be barking up the wrong tree but;

"the event triggers and doesn't seem to reset"

From looking at your code, the event is only ever triggered once and appears to be correct.

If you want the event to trigger @ x interval, you may want to consider using a thread object and in the execute() method

you may want to have somehing like

While(!Terminated)
{
if(WaitForSingleObject(event, INFINITE) == WAIT_OBJECT_0)
{
//do your work here. NOT SURE ABOUT THIS
//if this needs to be called each time, do it here.
//vulpes may now a bit more to help.
ok = GPIOapi.GpioSetupInterruptPin(hGPIO, port6, 4, GPIOapi.INT_TRIGGER_MODE.TRIGGER_MODE_EDGE,

GPIOapi.INT_TRIGGER_POLARITY.TRIGGER_POL_HIGH_RISING, trigger, true);

Synchronize(SetLabel);
ResetEvent(event);
}
}
SetLabel()
{
if(!ok)
form.label2.BackColor = Color.Red;
else
form.label2.BackColor = Color.Blue;
}

You may also need a timer to Do a SetEvent(event) @ say every second or whatever time interval you want on your main form.

NOTE: the code sample is C++,
Vulpes11y ago
I don't understand why you're not getting a compiler error due to the placing of the following line within the _Main() method:

public static IntPtr handle = CreateEvent(IntPtr.Zero, false, false, Trigger);

As it's a public static field, it should be placed at form level.

Also I'd have thought that you should be calling WaitForSingleObject() AFTER the call to GpioSetupInterruptPin() as this presumably sets the state of the auto-reset event to signalled. At the moment, you're calling it (via the _Main() method) BEFORE this point.