Unittesting QSyntaxHighlighter

Testing 1, 2, 3 by alisdairI’m using test driven development while building my pet project, DjangoDE. A key part of an IDE is the syntax highlighting of the code in the editor, so that’s one area where I’ve been trying to build up the test suite.

To test the syntax highlighter the obvious approach is to send the right events to write some code into the editor the check the colour of the text. Although the QT documentation is usually excellent, it doesn’t go into enough detail on the implementation of the syntax highlighting framework to enable you to query the colour of the text. In this post I’ll explain how the colour of text is stored, and how you can query it.

A syntax highlighting editor is normally implemented using a QPlainTextEdit widget. This object provides the user interface to the editor and manages the display of the text. The widget contains a QTextDocument instance, which stores the text. To add syntax highlighting you derive a class from QSyntaxHighlighter then instantiate it, passing the document instance as the parameter to the constructor. This is explained in detail in the syntax highlighter example.

The document stores the text as a sequence of QTextBlock objects. These store the text as well as the formatting information used to display it. You might think that you can just call QTextBlock::charFormat to get the colour of the text. Unfortunately it’s not that simple as the format returned by that call is the colour that you’ve explicitly set, not the syntax highlight colour.

Each QTextBlock is associated with a QTextLayout object that controls how the block is rendered. Each layout has a list of FormatRange objects, accessible using the additionalFormats method. It is this list that the QSyntaxHighlighter sets to specify the colour of the text.

Now we know where the colour information is stored, we can find out what colour a particular character will be. Firstly you need to find out which QTextBlock the text you want is. In a plain text document each line is represented by a separate block, so this is quite straightforward. You then get the list of FormatRanges and then iterate through, checking to see if the character you want is between format_range.start and format_range.start + format_range.length

For an example of this you can check out the test file from DjangoDE here


Photo of Testing 1, 2, 3 by alisdair.

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Author: Andrew Wilkinson

I'm a computer programmer and team leader working at the UK grocer and tech company, Ocado Technology. I mostly write multithreaded real time systems in Java, but in the past I've worked with C#, C++ and Python.

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