Default Style Alignment

[oleibman]

Fix #3918, sort of. Xlsx cells use the default style when they omit the s tag, or when s="0" is specified. LibreOffice does not honor Alignment in the default Style unless the cell explicitly uses the second form, even though it honors other default Styles (e.g. bold font) when s is omitted. Gnumeric seems to have the same problem. A bug report has been filed with LibreOffice.

In the meantime, this PR adds to Xlsx Writer an optional boolean property explicitStyle0 with setter and getter. Default is false, which will continue the current behavior by adding an s tag only when the cell uses a non-default style (this is how Excel itself behaves). When set to true, Xlsx Writer will explicity write s="0" for all cells using default style. This will allow users to create an Xlsx spreadsheet with default alignment of cells that will show up correctly when the spreadsheet is viewed with LibreOffice. Technically speaking, it is probably safe to always use true, except that the spreadsheet size will be a bit larger. However, my hope is that this is a temporary measure which can go away when the vendors have had a chance to fix their problems, hence the false default.

This is:

• a bugfix (workaround for vendor problem)
• a new feature
• refactoring
• additional unit tests

Checklist:

• Changes are covered by unit tests
  • Changes are covered by existing unit tests
  • New unit tests have been added
• Code style is respected
• Commit message explains why the change is made (see https://github.com/erlang/otp/wiki/Writing-good-commit-messages)
• CHANGELOG.md contains a short summary of the change and a link to the pull request if applicable
• Documentation is updated as necessary

Why this change is needed?

Provide an explanation of why this change is needed, with links to any Issues (if appropriate).
If this is a bugfix or a new feature, and there are no existing Issues, then please also create an issue that will make it easier to track progress with this PR.

[oleibman]

A win for Scrutinizer! I had miscoded some tests, which resulted in a Scrutinizer warning, and which should have resulted in a test failure but didn't. We have always cautioned that it is dangerous to store a Cell in a variable. It appears that the same is true for Style. The problem shows up in the following code adapted from my test:

        $styleA1 = $sheet2->getStyle('A1');
        var_dump($styleA1->getFont()->getItalic());  // false, because A1 is not italic
        $styleA2 = $sheet2->getCell('A2')->getStyle();
        var_dump($styleA1->getFont()->getItalic()); // true, because A2 is talic

I will correct the test, and attempt to update the documentation.

[oleibman]

No concerns with Scrutinizer "complexity" complaint.