Mac sample code using USB and CFRunLoopSource

Question

I'm trying to (re-)write a program that uses USB controlled scanners. I've got my USBIOInterfaceInterface set up, so I can send and receive messages over the pipes. I am successfully calling WritePipeTO and ReadPipeTO synchronously but I want to read from the scanner asynchronously.

I've tried using ReadPipeAsyncTO, but the callback never gets called until the call timesout. The reason Appears to be that I'm not registering the callback somewhere. If I add a CFRunLoopRun after the async read, then it appears to work, but only by having many nested calls to CFRunLoopRun, which can't be a good idea.

I've seen the functions:

    err = (*usbInterfaceInterface)->CreateInterfaceAsyncEventSource(usbInterfaceInterface, &cfSource);
    CFRunLoopAddSource(CFRunLoopGetCurrent(), cfSource, kCFRunLoopDefaultMode);

but I haven't figured out how to tell the Source what callback should be called when something happens on that interface. When I just create the source, and call the ReadPipeAsyncTO, the callback doesn't get called until timeout.

Is there same sample code somewhere so I can see how these functions are SUPPOSED to work together?

Answer 1

Calling CFRunLoopRun, or its NSRunLoop equivalent, is almost always a sign that your program's architecture is wrong.

It sounds like it would be a terrible pain to do, but your app would be much better—and would work, using the ReadPipeAsyncTO and WritePipeAsyncTO functions—if you rewrote it to use the run loop instead of whatever manual loop you currently have.