computePosition method
计算内容区域
Implementation
Rect computePosition(double left, double top, double right, double bottom) {
double nw = right - left;
double nh = bottom - top;
double leftOffset = _computeLeftOffset(nw);
double topOffset = _computeTopOffset(nh);
double rightOffset = _computeRightOffset(nw);
double bottomOffset = _computeBottomOffset(nh);
return Rect.fromLTRB(left + leftOffset, top + topOffset, right - rightOffset, bottom - bottomOffset);
}