Dave Addey wrote:
> Hi NUG,
>
> I have an RB plugin, which has a GetNextDevice function that returns a new
> instance of a class, called HIDDevice. A HIDDevice has one event,
> DataAvailable. The HIDDevice class is defined in the plugin too.
>
> What I need to do in RB code is to subclass the HIDDevice class, so that I
> can enter some custom code in the subclass DataAvailable event. This is
> easy enough. What I also need to do is to assign the output of
> GetNextDevice to an instance of my subclass, and this is causing problems.
>
> For example:
>
> Dim myHIDDevice as HIDDevice_Mouse // a subclass of HIDDevice
>
> myHIDDevice_Mouse = GetNextDevice
>
>
> This second line fails on compilation, with the following error:
>
> Type mismatch error. Expected HIDDevice_Mouse, but got HIDDevice.
>
> Now, surely, HIDDevice_Mouse is a HIDDevice. So, shouldn¹t this work?
>
>
> I also tried casting the output of GetNextDevice, like this:
>
> myHIDDevice = HIDDevice_Mouse(GetNextDevice)
>
> This compiles, but causes an IllegalCastException when this code executes in
> the application. I¹m not sure why.
>
> How should I do this?
You can't convert one instance of a HIDDevice into a HIDDevice_Mouse. They are
different types, even though HIDDevice_Mouse inherits from HIDDevice.
You need to separate your design into two parts. You can, for instance, use a
Factory pattern approach. Your plugin defines a HIDDeviceFactory interface. You
can implement that interface in classes in your REALbasic code, so that you
have a HIDDeviceMouseFactory, HIDDeviceKeyboardFactory, etc. You create
instances of these factories and register the instances with your plugin. The
interface would have one method, "CreateInstance() as HIDDevice". In the case
of the HIDDeviceMouseFactory, you would implement it like so:
Function CreateInstance() as HIDDevice
return new HIDDeviceMouse()
End Function
Your plugin would expose a method for registering factories with it, ie
RegisterFactory( deviceType as String, factory as HIDDeviceFactory )
When the REALbasic calls the RegisterFactory function, the plugin would add the
factory object to an internal list of factories, along with the device type.
When the plugin came across a HID device that matched that type, it would call
the factory object to create an instance of the appropriate HID Device subtype.
Alternatively, you could split the HIDDevice class into two parts. All
instances of HIDDevice would have a device handle property which represented
the actual HID device. You would create instances of HIDDeviceMouse, etc on the
app side, then attach them to the physical HID device return from the plugin.
Created on the REALbasic side: Created on the plugin side:
HIDDevice --------------------------> HIDPhysicalDevice
^
/ \
HIDDeviceMouse HIDDeviceKeyboard
Imagine you had a HIDDeviceManager class in the plugin which has a
DeviceAttached() event. You create a subclass of this in REALbasic. You would
implement this DeviceAttached() event like so:
Class MyHIDDeviceManager
Sub DeviceAttached( theDevice as HIDPhysicalDevice )
select case theDevice.type
case "Mouse"
Dim mouse as HIDDeviceMouse(theDevice)
' Save mouse object somewhere
case "Keyboard"
Dim keyboard as HIDDeviceKeyboard(theDevice)
' Save keyboard object somewhere
end select
End Sub
Hope this helps.
--
Kind regards,
James Milne
_______________________________________________
Unsubscribe or switch delivery mode:
<http://www.realsoftware.com/support/listmanager/>
Search the archives of this list here:
<http://support.realsoftware.com/listarchives/lists.html>
