FractionalOffset class

An offset that's expressed as a fraction of a Size.

FractionalOffset(1.0, 0.0) represents the top right of the Size.

FractionalOffset(0.0, 1.0) represents the bottom left of the Size.

FractionalOffset(0.5, 2.0) represents a point half way across the Size, below the bottom of the rectangle by the height of the Size.

The FractionalOffset class specifies offsets in terms of a distance from the top left, regardless of the TextDirection.

Design discussion

FractionalOffset and Alignment are two different representations of the same information: the location within a rectangle relative to the size of the rectangle. The difference between the two classes is in the coordinate system they use to represent the location.

FractionalOffset uses a coordinate system with an origin in the top-left corner of the rectangle whereas Alignment uses a coordinate system with an origin in the center of the rectangle.

Historically, FractionalOffset predates Alignment. When we attempted to make a version of FractionalOffset that adapted to the TextDirection, we ran into difficulty because placing the origin in the top-left corner introduced a left-to-right bias that was hard to remove.

By placing the origin in the center, Alignment and AlignmentDirectional are able to use the same origin, which means we can use a linear function to resolve an AlignmentDirectional into an Alignment in both TextDirection.rtl and TextDirection.ltr.

Alignment is better for most purposes than FractionalOffset and should be used instead of FractionalOffset. We continue to implement FractionalOffset to support code that predates Alignment.

See also:

• Alignment, which uses a coordinate system based on the center of the rectangle instead of the top left corner of the rectangle.