Archive Magazine CD 1995/Archive Magazine CD 1995.iso/discs/prog_disc/volume_8/issue_06/risc_os/DataTrans

Text File | 1988-12-12 | 18.4 KB | 473 lines

Communication with the Filer: the Data Transfer Protocol
========================================================

These protocols are understood by the Filer, and by applications that load
and save files. In fact, the Filer is not a particularly special case. The
following cases are considered:

(1) The user drags an icon from an application to a Filer window.
The application saves the file.

(2) The user drags an icon from an application to a window of another
application. The transfer is achieved via a temporary file.

(3) The user drags an icon from an application to a window of another
application. The transfer is achieved (if both sides agree) using direct
transfer of memory between the two address spaces.

(4) The user drags an icon from the Filer to the window of an application.
The application inserts or loads the file.

(5) The user double-clicks on an icon in the Filer. An existing application
opens the file.

There are other cases which generalise some of these, with an application
"pretending" to be the Filer. For instance, when a message arrives for
MailMan, it simulates case (5) to cause the editor to display the incoming
message.

This section assumes knowledge of the Wimp program interface. It is separate
because the facilities are actually provided by the Filer rather than the
Wimp. Careful study of the sections on message passing is recommended.

The protocols do not specify whether event codes 17 or 18 are used within
the protocol, as either is acceptable at any stage of the protocols. Type 18
messages are needed when confirmation of non-reply is required, which is
provided by the Wimp. Type 18 messages are recommended when a reply is
expected.

Also, messages may include excess junk at the end of the form. The
precise size of the message is not part of the protocol.

Some of the message formats have unused or copied portions. This is to make
it easier to reply to certain messages by overwriting some fields in the
incoming message and then just sending it back. Note, however, that these
are essential parts of the protocol and must be copied precisely in order to
adhere to the protocol. In some cases acknowledgements and replies are not
used, this makes it especially important to stick to the letter of this document
(rather than just "hacking until it works with the Filer") to prevent surprises
when programs from different sources are brought together.

The following message is sent by an application wishing to save a document (or a
selected part of a document):

1  DataSave

   R0     17 (usually)
   R1!0   size
     !12  0
     !16  1 ; DataSave
     !20  destination window handle             ;
     !24  destination window icon               ; copied from
     !28  destination x coord (screen coords)   ; Wimp_GetPointerInfo
     !32  destination y coord (screen coords)   ;
     !36  estimated size of data, in bytes
     !40  file type of data
     !44  proposed leaf-name of file, 0-terminated

The destination window handle, icon and coordinates are those generated by
Wimp_GetPointerInfo at the end of a drag.

The file type word is a value in the range 0..&fff for typed files, or one of the
following:
    &1000 for a directory
    &2000 for an application directory
    &3000 for a load/exec file

If the target window is a directory viewer (or any other window which wishes
the information to be written only to a file) then it replies as follows:

2  DataSaveAck (save data to file)

   R0     17 (usually)
   R1!0   size
     !12  my-ref field of DataSave message
     !16  2  ; save data to here
     !20..40 ; left unchanged from relevant DataSave
     !44  full path-name of file, 0-terminated

Note that the type and size information are typically ignored when
generating this response. The Filer implements this form of reply for
directory viewer windows.

An editor, on receiving this reply, should save the data specified by the
user in the named file. If this save fails, report using Wimp_ReportError
and the transaction stops.

The editor should also mark the document as not modified, if the whole
document is being saved (rather than some selected portion). An exception to
this is that if the "estimated size" field of the DataSaveAck is negative
then do not mark the document as unmodified: this is a signal from the
recipient that this file is a temporary one, rather than a good repository
for future versions of the file. When marking the document as unmodified,
also remember the full path-name so that future saves can be performed
without the user having to drag icons.

The editor then issues this acknowledgement:

3  DataLoad (drag file from Filer / I have saved data to a file)

   R0     18 (usually)
   R1!0   size
     !12  my-ref field of DataSaveAck message (or 0 if from Filer)
     !16  3 ; load data from here
     !20  destination window handle             ;
     !24  destination icon handle               ; copied from
     !28  destination x coord (screen coords)   ; Wimp_GetPointerInfo
     !32  destination y coord (screen coords)   ;
     !36  must be <= 0 ; size field in other messages
     !40  file type
     !44  full path-name of file, 0-terminated

(This acknowledgement is not used by the Filer, but is used in cases
where the file is a temporary file used to communicate between
two applications).

This message is also used in an entirely independent context. The Filer
sends this message when a file has been dragged into a window belonging to
another application. The application is then free to copy or insert the
file, if it so desires. If it does so successfully it should reply as
follows:

4  DataLoadAck

   R0     17 (usually)
   R1!0   size
     !12  my-ref field of DataLoad message
     !16  4 ; DataLoadAck

This message is only sent if the load was successful.

Thus, the case of a file being saved to the Filer in fact involves 4 messages
being sent:
(1)  DataSave        (application to Filer)
(2)  DataSaveAck     (Filer back to application)
                     (application saves the file)
(3)  DataLoad        (application to Filer, as acknowlegement)
                     (the Filer just turns this around, doing nothing)
(4)  DataLoadAck     (Filer to application)
                     (if this is not received, the application generates an
                     error "Bad Data Transfer, Receiver Dead" and
                     deletes the file that it had saved)
Messages (3) and (4) do not actually lead to significant action in the
save-to-Filer case, but in fact the same code also provides the save
operation to another application, using a temporary file. A fuller discussion
of this appears below, when the possibility of in-memory data transfer has
also been introduced.

5  DataOpen (broadcast for double-clicked file)

This protocol is used to broadcast to all running applications just before
"opening" a file whose icon has been double-clicked. It allows
already-running applications to open the file instead. Typically this will be
used by an editor which is capable of editing several documents of the same
type, so that only one instance of the application runs (using much less
space than if a separate copy of the application were run for each document).

   R0     18 (usually)
   R1!0   size
     !12  0
     !16  5                 ; destination info request
     !20  window handle of dirviewer
     !24  0 ; icon handle not used
     !28  x-offset of icon being opened within viewer
     !32  y                 ; allows for 'zoom' box if implemented
     !36  must be <= 0 ; size field in other messages
     !40  file type
     !44  full path-name of file, 0-terminated.

DataLoadAck is returned by the application which loads the file.
If this is not received, then the Filer will *Run the file.

An application directory that was double-clicked with the shift key held down, is
broadcast as a directory.

If data is to be transferred between two applications without going out to a
file, then the following reply initiates the in-core transfer protocol:

6  RAMFetch (transfer data to buffer in my workspace)

   R0     18 (error message if not acknowledged)
   R1!0   size
     !12  my-ref field of DataSave message
     !16  6 ; RAM fetch.
     !20  buffer address
     !24  buffer size (bytes)

The original sender replies as follows:

7  RAMTransmit (I have transferred some data to a buffer in your workspace)

   R0     18 (error message if not acknowledged)
   R1!0   size
     !12  my-ref field of RAMFetch message
     !16  7 ; RAM transmit.
     !20  buffer address             ; copy of value sent in RAM fetch
     !24  number of bytes written to buffer
          (if buffer not full, send another RAMFetch)

To write the data into the receiver's buffer, use the following call:

Wimp_TransferBlock
Entry:  R0 = task handle of source
        R1 --> source buffer
        R2 = task handle of destination
        R3 --> destination buffer
        R4 = buffer length
        buffer addresses and length are byte-aligned (not nec. word-aligned)
        if the buffer addresses are within application space,
           they are validated to ensure they are within the correct task
Errors: "Invalid task handle"
        "Wimp transfer out of range"

The receiver's buffer is not entirely filled then the receiver will
assume that this is the end of the operation. No further confirmation
is required.

If the receiver's buffer is filled then it will send a further RAMFetch
message. This need not specify the same buffer or buffer size. Thus the
operation continues until all data is transferred.

If the other end of the protocol does not answer then cancel the operation
quietly, without generating an error message. The other end (e.g. if it ran
out of space) will have already have complained.

Application code for direct file transfer
-----------------------------------------

To save a file:

1. Transmit DataSave message.

2. If DataSaveAck returned, save the file. If there are errors in this
process, report them to the user and cease the transaction. If the save
succeeds, send a DataLoad.

3. If DataLoadAck is not returned, (because the receiver is dead or badly
written) then the sender should delete the file and report an error message
saying "data transfer failed".  Thus the Filer must acknowledge DataLoad, or
else all files saved to it will be subsequently deleted.

3. If RAMFetch returned, send RAMTransmit and loop until done

NOTE:  all messages in this protocol apart from the DataSave should quote the
       other side's my_ref field in their your_ref fields, to ensure that the
       messages are acknowledged correctly.

NOTE:  In all cases where an unknown message action is received by an
       application, it MUST ignore the message completely.

To receieve a file from another application:

1. Receive DataSave message.

2. If data can be loaded from RAM, send back a RAMFetch
(look at approx data size in DataSave message, but do not
rely on its absolute accuracy, ie. be prepared for MORE
data than that to be sent).

If RAMFetch not acknowledged, load from a file (step 3)

If RAMTransmit received,
  finished if buffer NOT filled,
  else send another RAMFetch and loop.
  If any RAMFetch other than the 1st is not acknowledged,
    report error "data transfer failed".
  (There's no need to check your_ref field if RAMFetch (18) used,
    since the RAMFetch will not be acknowledged.)

3. If data must be in a file, return DataSaveAck "<Wimp$Scrap>". If DataLoad
is received, load the file, delete it and return DataLoadAck. Note that the
your_ref field of the DataLoad tells the receiver whether the file is the
scrap file.

Minimal functionality is for the sender to only cope with DataSaveAck, and
for the receiver to only cope with file-based I/O.  If the receiver wishes to
engage in RAMFetch operations, it should be prepared for the sender to be
ignorant of that protocol - ie. it should be prepared to revert to the scrap
file mechanism.

An Explanation of the Data Transfer Protocols
========================================

Ignoring direct RAM transfer for the moment, all data transfer operations in
the RISC OS desktop world can be accomplished using 4 message types.  These
are:

        DataSave
        DataSaveAck
        DataLoad
        DataLoadAck

The various operations that an application must deal with are as follows:

        1a) Saving data to a file
        1b) Saving data to another application
        2a) Loading data from a file
        2b) Loading data from another application

The protocols involved in each of these cases is described below - note that
it is assumed that all replies carry the myref of the message they are
replying to in their yourref field:


1a) Saving data to a file
-------------------------

Application receives User_Drag_Box event (the file box has been dropped)
Application calls Wimp_GetPointerInfo to find out where the pointer is
Application sends Message_DataSave to the destination, with the leafname
Filer replies with Message_DataSaveAck with "directoryname.leafname"
Application saves data to the file
Application sends Message_DataLoad to the Filer
Filer replies with Message_DataLoadAck
Everyone is happy.


1b) Saving data to another application
--------------------------------------

Application receives User_Drag_Box event (the file box has been dropped)
Application calls Wimp_GetPointerInfo to find out where the pointer is
Application sends Message_DataSave to the destination, with the leaf-name
ExternalTask replies with Message_DataSaveAck with "<Wimp$Scrap>"
Application saves data to the file
Application sends Message_DataLoad to the ExternalTask
ExternalTask loads and deletes the scrap file
ExternalTask replies with Message_DataLoadAck
Everyone is happy.


2a) Loading data from a file
----------------------------

Filer sends Message_DataLoad to the application
Application loads the file
Application replies with Message_DataLoadAck
Everyone is happy.


2b) Loading data from another application
-----------------------------------------

ExternalTask sends Message_DataSave to the application
Application replies with Message_DataSaveAck to "<Wimp$Scrap>"
ExternalTask saves data to the file
ExternalTask sends Message_DataLoad to the application
Application loads and deletes the scrap file
Application replies with Message_DataLoadAck
Everyone is happy.


Summary
-------

As one would expect, (1b) and (2b) are symmetrical, so that any task can send
data to any other, rather than some tasks being senders and some receivers.
An additional advantage of this method is the overlap between file transfer
between applications and file transfer to and from the Filer.

Note that in protocols (1b) and (2b) it is the loader of the data who is
responsible for deleting the scrap file afterwards, which is logical since it
is the loader who invented the scrap file in the first place!

The following summarises the application code necessary for loading and
saving files:


Loading files
-------------

Received Message_DataSave: respond with Message_DataSaveAck to "<Wimp$Scrap>"
                           make a note of the myref field of your reply

Received Message_DataLoad: load the indicated file
                           if your_ref = scrap my_ref, delete the file
                           reply with Message_DataLoadAck

Saving files
------------

Determine destination using drag operation followed by Wimp_GetPointerInfo
Send Message_DataSave including your proposed leafname

Received Message_DataSaveAck: save data to the indicated file
                              reply with Message_DataLoad for same filename

Received Message_DataLoadAck: usually ignored, but can be useful


Direct RAM transfer
-------------------

For the experts, it is possible to indulge in more efficient data transfer
where two applications are involved (ie. protocols (1b) and (2b)).

Basically, instead of replying with Message_DataSaveAck to "<Wimp$Scrap>", an
application should reply first with Message_RAMFetch, and if that is not
acknowledged, revert to returning the conventional <Wimp$Scrap> message.

    Message_RAMFetch
        R0 = 18 (so that message must be acknowledged)
        R1!0   size
          !12  my-ref field of DataSave message
          !16  6 ; RAM fetch.
          !20  buffer address
          !24  buffer size (bytes)

From the sender's end, it is permissible to totally ignore Message_RAMFetch,
in which case the more usual scrap file method is used.  If the RAM transfer
protocol is understood, however, Wimp_TransferBlock can used to transfer data
into the other task's workspace.

After transferring the data, a RAMTransmit message should be sent.
If the buffer was filled by the sender, the receiver should process the data
and then send another RAMFetch.  If the buffer is not full, this implies that
the file transfer is complete, and there is no need for any further
communication.  Note that this means that if the file length fits into a
whole number of buffers, then the last block must contain 0 bytes of data.

Note that it is possible for an application to support RAM transfer on
loading or saving or neither or both as required - indeed, it may be that an
application can cope with direct RAM transfer for certain filetypes but not
for others, and its capability may be different when loading and saving data.
The protocols described allow for 'cop-out' by either the sender or the
receiver of the data.

New Summary
-----------

Including the RAM transfer protocols, the application code looks like this:


Loading files
-------------

Received Message_DataSave: respond with RAMFetch to appropriate buffer
                           if not acknowledged,
                             respond with Message_DataSaveAck to "<Wimp$Scrap>"
                             make a note of the myref field of your reply

Received Message_RAMTransmit: process data in buffer
                              if buffer was filled, send another RAMFetch
                              otherwise all data has been sent

Received Message_DataLoad: load the indicated file
                           if your_ref = scrap my_ref, delete the file
                           reply with Message_DataLoadAck

Saving files
------------

Determine destination using drag operation followed by Wimp_GetPointerInfo
Send Message_DataSave including your proposed leafname.

Received Message_RAMFetch: call Wimp_TransferBlock to transfer data
                           send Message_RAMTransmit for appropriate amount
                           if buffer filled, expect a RAMFetch in reply

Received Message_DataSaveAck: save data to the indicated file
                              reply with Message_DataLoad for same filename

Received Message_DataLoadAck: usually ignored, but can be useful