[gphoto-devel] gp_camera_wait_for_event and JNA - no data returned

[unclebob]

I'm attempting to write a tether in Java, but while the new file event is
triggered correctly, I can't seem to get any data back from it, neither the
eventtype nor the eventdata.

Here's what my JNA binding looks like:

int gp_camera_wait_for_event(Camera camera, int timeout, IntBuffer
eventtype, PointerByReference eventdata, Gphoto2Library.GPContext context);

The code using it looks as follows:

PointerByReference eventdata = new PointerByReference();
IntBuffer eventtype = IntBuffer.allocate(1);

int result = gphoto2.gp_camera_wait_for_event(camera, 1000000000,
eventtype, eventdata, context); //result is always 0, OK
int test = eventtype.get(); //just testing what I got back. test is
always 0, with which it was initialised

CameraFilePath filepath = new CameraFilePath(eventdata.getValue());
//filepath never contains anything


And for completeness, the CameraFilePath Structure (which works without a
hitch when used with gp_camera_capture):


public class CameraFilePath extends Structure
{
/**
* < \brief Name of the captured file.<br>
* C type : char[128]
*/
public byte[] name = new byte[128];
/**
* < \brief Name of the folder of the captured file.<br>
* C type : char[1024]
*/
public byte[] folder = new byte[1024];

public CameraFilePath()
{
super();
}

@Override
protected List getFieldOrder()
{
return Arrays.asList(new String[]{"name", "folder"});
};

/**
* @param name < \brief Name of the captured file.<br>
* C type : char[128]<br>
* @param folder < \brief Name of the folder of the captured file.<br>
* C type : char[1024]
*/
public CameraFilePath(byte name[], byte folder[]) {
super();
if (name.length != this.name.length)
throw new IllegalArgumentException("Wrong array size !");
this.name = name;
if (folder.length != this.folder.length)
throw new IllegalArgumentException("Wrong array size !");
this.folder = folder;
}

public CameraFilePath(Pointer pointer)
{
super(pointer);
}

public static class ByReference extends CameraFilePath implements
Structure.ByReference {

};
public static class ByValue extends CameraFilePath implements
Structure.ByValue {

};
}


I'm still somwhat clumsy in JNA, but for all I know, this should work... ?

Test Camera is Nikon D5300, just in case that might have an influence.





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[Marcus Meissner]

Post by unclebob
I'm attempting to write a tether in Java, but while the new file event is
triggered correctly, I can't seem to get any data back from it, neither the
eventtype nor the eventdata.
int gp_camera_wait_for_event(Camera camera, int timeout, IntBuffer
eventtype, PointerByReference eventdata, Gphoto2Library.GPContext context);
PointerByReference eventdata = new PointerByReference();
IntBuffer eventtype = IntBuffer.allocate(1);

I would try to use com.sun.jna.ptr.IntByReference for eventtype.

Post by unclebob
int result = gphoto2.gp_camera_wait_for_event(camera, 1000000000,
eventtype, eventdata, context); //result is always 0, OK
int test = eventtype.get(); //just testing what I got back. test is
always 0, with which it was initialised
CameraFilePath filepath = new CameraFilePath(eventdata.getValue());
//filepath never contains anything

Can you print out eventdata.getValue() ... Here i think the data converter in
the Structure constructor might not work.

Ciao, MArcus

Post by unclebob
And for completeness, the CameraFilePath Structure (which works without a
public class CameraFilePath extends Structure
{
/**
* < \brief Name of the captured file.<br>
* C type : char[128]
*/
public byte[] name = new byte[128];
/**
* < \brief Name of the folder of the captured file.<br>
* C type : char[1024]
*/
public byte[] folder = new byte[1024];
public CameraFilePath()
{
super();
}
@Override
protected List getFieldOrder()
{
return Arrays.asList(new String[]{"name", "folder"});
};
/**
* C type : char[128]<br>
* C type : char[1024]
*/
public CameraFilePath(byte name[], byte folder[]) {
super();
if (name.length != this.name.length)
throw new IllegalArgumentException("Wrong array size !");
this.name = name;
if (folder.length != this.folder.length)
throw new IllegalArgumentException("Wrong array size !");
this.folder = folder;
}
public CameraFilePath(Pointer pointer)
{
super(pointer);
}
public static class ByReference extends CameraFilePath implements
Structure.ByReference {
};
public static class ByValue extends CameraFilePath implements
Structure.ByValue {
};
}
I'm still somwhat clumsy in JNA, but for all I know, this should work... ?
Test Camera is Nikon D5300, just in case that might have an influence.
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[unclebob]

First let me note that I did solve the problem differently, I wrote a tether
that just checks the file system of the camera for new files at an interval
and downloads them. So I'm not actively solving this problem anymore, but I
did the tests you suggested and post the results here in case it is helpful
for gphoto development.

JNAerator actually produced a method overload of gp_camera_wait_for_event
using IntByReference, but marked it as deprecated.
I can now confirm both ways as working. After initialising the eventtype to
-1, I could confirm that it does indeed change. A timeout event is also
correctly returned as 1.

I can also confirm that the eventdata pointer has indeed been initialised
(it's not null anymore after the call).

The essential problem then seems that I get a GP_EVENT_UNKNOWN back when
pressing the trigger on the camera (eventdata == 0), while I would have been
expecting a GP_EVENT_FILE_ADDED.
This would of course also explain why I was unable to use eventdata to
initialise a CameraFilePath with the received eventdata pointer: If it's not
a GP_EVENT_FILE_ADDED, I'm not getting the data back I thought I should be
getting.

So I assumed that there might be other events triggered in case of a camera
trigger, events that might not be implemented. So I did the following:

do
{
int result = gphoto2.gp_camera_wait_for_event(camera, 1000000,
eventtype, eventdata, context);

System.out.println(eventtype.getValue());
System.out.println(eventdata.getValue().toString());
}
while (eventtype.getValue() == 0);

And sure enough, the GP_EVENT_FILE_ADDED got passed after 7 unknown events
in short order... So basically everything is working properly, but the
documentation could be improved a little (some sort of hint that the
function will return the next event occuring, and that it's very probable
that this will be an as of yet unimplented event. I know it's logical
behaviour after you mull it over some, but it's not /intuitive/).
Or it might be good to change the gp_camera_wait_for_event function in a way
that you can subscribe to a specific event by passing a valid event type
(That's how I interpreted the method right at the beginning before reading
the documentation very carefully).



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