const TextSelection({int baseOffset, int extentOffset, TextAffinity affinity: TextAffinity.downstream, bool isDirectional: false })

Creates a text selection.

The baseOffset and extentOffset arguments must not be null.

Source 

const TextSelection({
  int baseOffset,
  int extentOffset,
  this.affinity: TextAffinity.downstream,
  this.isDirectional: false
}) : baseOffset = baseOffset,
     extentOffset = extentOffset,
     super(
       start: baseOffset < extentOffset ? baseOffset : extentOffset,
       end: baseOffset < extentOffset ? extentOffset : baseOffset
     );