The Import/Export Library: 3.1 Writing Translation Libraries: How Translation Libraries Work

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A translation library's task is easily stated. It has to do two things: read a native-format file and produce a Metafile translation, and read data in a Metafile and write corresponding data in a native-format file. How easy this is to do depends on the formats involved.

Every translation library specifies what format of Metafile it expects to work with. For example, the FooWrite translation library would translate FooWrite files into IDC_TEXT Metafiles and vice versa. When a user activates the "Import" trigger, he will be presented with a list of formats to use; those formats will correspond to all the libraries which can translate files into formats the application can accept. For example, if the application specified that it could accept text or graphics Metafiles, the user's choice of format would depend on which of the installed libraries could generate text or graphics Metafiles.

The translation library can also suggest a file mask. For example, the FooWrite translation library might specify that FooWrite data files meet the pattern "*.FOO". By default, the Import file selector will show only the files that match the library's mask. However, the user can override this mask, setting a different one or no mask at all.

When a user decides to import a file, the ImportControl opens the source file and creates a temporary transfer VM file. The ImportControl then starts up the appropriate translation library and passes the two file handles to it. The translation library should read the entire source file, translate it to the appropriate Metafile format, and write it to a VM chain in the transfer file. It then returns the VMBlockHandle of the head of the VMChain to the ImportControl. If it was unable to translate the file, it should return a null handle; the ImportControl then displays an appropriate error message. The ImportControl will close the source file automatically.

When a user decides to export a file, the ExportControl creates a temporary transfer file and opens an empty native-format file for the output. The ExportControl then calls the translation library. The library is passed the handles of the two files, as well as the handle of the Metafile VM chain. The library reads the Metafile and writes an appropriate data file. When it is finished, it notifies the ExportControl, which automatically closes the destination file and destroys the temporary transfer file.

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