Android: MetricAffectingSpan and RasterizerSpan

Question

Can someone explain how the following span methods work (for TextView)?

If possible, please briefly.

I have not yet found good explanations or examples on the Internet.

Thank you very much in advance.

(a) MetricAffectingSpan http://developer.android.com/reference/android/text/style/MetricAffectingSpan.html

(b) Span Rasterizer
http://developer.android.com/reference/android/text/style/RasterizerSpan.html