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Detecting 0-byte files based on extension in Impress and elsewhere

Estimated read time: 3 minutes

Impress (and Writer and Calc) now has support for detecting 0-byte files on open/import based on their extension. This builds on top of the previous language-independent template improvements. This means that e.g. a 0-byte PPTX file will open as an empty Impress presentation, not in Writer.


We regularly see customers wanting minimal templates, which are language independent and have no content. Such files are handy if your workflow is to first name an empty document (create it) and only then edit it (and not the other way around: first create the document, then save it by giving it a name). This is easy for .txt files: if it’s zero bytes, it’s empty. But then this approach is also expected to work for other file formats as well, where our original approach was more technical: if it’s an empty file, that that can be only plain text, so we (almost) always opened it in Writer, not matching the user expectations.

Instead of explaining the problem to people again and again (that a literally empty PPTX file is not a PPTX template), there is value in just adapting the code instead to "do what I mean".

Results so far

An empty PPTX file is now handled like this:
Figure 1. Empty PPTX file opening in Impress

You can see this is no longer opening in Writer as plain text but in Impress, which is clearly a less surprising behavior.

Here is what happens if you open an empty DOTX (template):
Figure 2. Empty DOTX file creates a new Writer document

You can see that it is even recognized that this is a template format, so a new document is created, not the template itself is opened for editing.

How is this implemented?

If you would like to know a bit more about how this works, continue reading… :-)

You can see the code change in this commit. First, we restrict this trick to file URLs, and also to empty files.

Second, we look at the extension of the file and try to match an import filter that usually handles that extension. This helps, because then nominally the correct filter will be used for the import, so save will not ask for a filename (as it happens for new documents), but it will know what target filename and export filter to use.

Finally we need to avoid actually invoking the import filter, because no file content is not something an import filter has to handle if its filter detection would reject the file. (E.g. PPTX is expected to be a valid ZIP file.) This is important, because we want to avoid touching each & every file filter to not fail for empty file content — instead we want to handle this centrally, at a single place.

Want to start using this?

You can get a snapshot / demo of Collabora Office and try it out yourself right now: try unstable snapshot. Collabora intends to continue supporting and contributing to LibreOffice, the code is merged so we expect all of this work will be available in TDF’s next release too (7.1).

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