MockCanvas is a utility class for writing tests for canvas operations. It
supports the same API as the regular Canvas class from dart:ui (in theory;
any missing commands can be added as the need arises). In addition, this class
is also a Matcher, allowing it to be used in expect() calls as can be seen
in the example below.
Two mock canvases will match only if they have the same number of commands, and if each pair of corresponding commands matches.
Multiple transform commands (translate(), scale(), rotate() and
transform()) that are issued in a row are always joined into a single
combined transform. Thus, for example, calling translate(10, 10) and
then translate(30, -10) will match a single call translate(40, 0).
Some commands can be partially specified. For example, in drawLine() and
drawRect() the paint argument is optional. If provided, it will be
checked against the actual Paint used, but if omitted, the match will still
succeed.
Commands that involve numeric components (i.e. coordinates, dimensions, etc) will be matched approximately, with the default absolute tolerance of 1e-10.
final canvas = MockCanvas();
// ... draw something on the canvas
// then check that the commands issued were the ones that you'd expect:
expect(
canvas,
MockCanvas()
..translate(10, 10)
..drawRect(const Rect.fromLTWH(0, 0, 100, 100)),
);