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Jun 97 - Newton Q and A

Volume Number: 13 (1997)
Issue Number: 6
Column Tag: develop

Newton Q&A: Ask the Llama

by Apple Developer Technical Support Group

Q: I'm probably missing something obvious, but when I try to use an asynchronous output call in some situations, I get an exception, while synchronous calls seem to work fine. For instance, this code fails consistently with bus errors:

   {async: true,
   form: 'string,
   completionScript: func(ep, options, result)
      self:SendData(endpoint, qfCursor)});

On the other hand, this code works fine:

endpoint:output(headerCache, nil, nil);
self:SendData(endpoint, qfCursor);

I'd really prefer to use asynchronous calls, but I keep running into problems. Do you have any suggestions?

A: The key thing you're missing is that self (that is, the receiver) is not what you expect in your asynchronous callback. In your first example, you pass a completion frame as the third argument of the Output message. It's this frame that will be self (that is, that will be the context of the completionScript). This means that you send the sendData message to that completion frame.

What you probably want is to send the message to the ep argument passed to the completionScript. This will be the endpoint to which you sent the Output message. In general, you will have set this endpoint to have a _parent slot that points to your application. Assuming that the sendData method is in either the endpoint or your application, you would modify the body of the completionScript to be

ep:sendData(endpoint, qfCursor);

The same restrictions apply for Input and Option calls as well.

Q: I have a pop-up view that has an icon with some text, and I'd like the user to be able to edit the icon. I call MakeShape on the icon and draw it into a bitmap. I then have an interface for the user to modify the icon. The problem occurs when I try to display the modified icon in the pop-up. It either doesn't show up, has incorrect bounds, or throws an exception. Before I modify the icon, it looks like something that would be returned by GetPictAsBits:

{data: <bits, Length 42>, bounds: {0,0,15,15}, }

Once I call MakeShape on the icon, it looks like this:

{class: bitmap, bounds: <boundsRect, Length 8>,
   data: <pixels, Length 54>, }

The pop-up chokes on this when I try to display it. First it threw an exception because it was expecting a frame instead of a binary object in the bounds frame. So I called SetBounds to set the bounds frame to {0,0,15,15}. This enabled the pop-up to open, but the icon wasn't in the right place. Fiddling with the bounds frame didn't seem to help. So I'm guessing that the next problem is that I have pixels instead of bits in the data slot. I tried calling SetClass on the slot to change it from pixels to bits. That actually didn't throw an exception, but the length is still different and it didn't look right. How can I get my shape or bitmap back into a form that the pop-up will be happy with?

A: The basic problem is that the binary object used by MakeShape to represent the picture (that is, <pixels, Length 54>) is not the same as that used in the bitmap structure (<bits, Length 42>). You have a couple of choices. Since the formats are documented, you can write some code to convert the object returned by MakeShape into a NewtonScript bitmap object. The other option, which seems more complex but is probably easier to implement, is to use the MakeShape representation only for your icon editor. Let the user modify the MakeShape representation, but keep track of the changes and use them to modify the bitmap. So, while the user is editing the icon, there will be two data objects: the original icon representation using the bitmap data structure and the MakeShape representation that the user sees and modifies. As the user changes the MakeShape object, your code is also modifying the bitmap object.

As a related side note, you can use DrawIntoBitmap to get the bits that represent the shape.

Q: In an application I'm building, I want to keep a serial communication session open with a GPS unit 24 hours a day, unattended by the user. Both the MessagePad and the GPS unit are powered by the car. The GPS unit sends out data approximately every two seconds. I'm using the "Basic Serial" Newton DTS sample code as the starting point. The communications code in the application works fine except that occasionally I get a -18003 exception, which results in a kMessage_BufferOverrun message. I've tried turning on useSoftFlowControl and useHardFlowControl in the kCMOInputFlowControlParms parameter frame inside the MBuildConfigOptions method, but that doesn't seem to help.

Can you explain when/why this exception is raised? Can I safely ignore the exception by revising the MNotifyError method? What can I do to prevent the occurrence of the exception in the first place?

A: A -18003 error can indicate several things, including bad parity or a framing error. Usually it indicates that the Newton's UART (SCC chip) buffer has overrun. You can just ignore this error, repost the input specification, and go on your merry way.

To repost the input specification, call the endpoint's SetInputSpec with the relevant input specification. Before reposting the input specification, you may also want to flush the communication tools buffers using the serial discard data option (kCMOSerialDiscard) and flush the NewtonScript buffer with FlushInput. Regardless of whether you flush, you will have lost some data in the process, although for most GPS applications that should be fine. You may also want to try increasing the size of the serial input buffer; this might give you just enough time to process all the data.

A better solution would be to have the GPS receiver support some type of flow control, though this will depend on the manufacturer of the unit.

Q: I have a view with a protoKeyboard button. The input lines within the view all take numeric input, and when the user double-taps, the numeric keypad pops up. But the keyboard button brings up the full QWERTY keyboard instead. How can I make the button open the numeric keypad?

A: The keyboard button is behaving correctly. If you don't specify which keyboard to open, it will always open the global alphanumeric keyboard. There are two ways you can get the behavior you want:

  • Override the 'defaultKeyboard slot in your protoKeyboardButton. This slot contains the keyboard that you want to use. In this case you want the symbol 'numericKeyboard, which specifies the default numeric keyboard.
  • Override the buttonClickScript in the protoKeyboardButton. You can use the OpenKeypadFor function and specify your input line as the view. This will usually do the right thing since calling OpenKeypadFor has just about the same effect as the user's double-tapping in an input line.

Q: I noticed that the NTK project preferences dialog lets you compress your application. How much slower will the application run if I do this?

A: Unfortunately, there's no simple answer to this question. The speed of execution depends on a number of factors, including how many times the system has to uncompress pages from your application into a cache and how full the system heap is.

The best way to find out is to try your application with the three different compression options: none, compressed, or faster compression. Make sure that you perform your tests a number of times and under different memory-loading conditions. If your application will be doing communications, that's a good thing to test. You can also use HeapShow (which comes with NTK) to fill the system and NewtonScript heaps to simulate low-memory conditions.


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