One possible way is generating intersection by intersect function, and calculate it's area by area function. The following is an example.
x = 0; y = 0; r = 0.5;
th = 0:pi/50:2*pi;
xunit = r * cos(th) + x;
yunit = r * sin(th) + y;
poly0 = polyshape(xunit(1:end-1),yunit(1:end-1));
poly1 = polyshape([0 0 1 1],[1 0 0 1]);
poly2 = intersect(poly0,poly1);
area(poly2)
The result is:
>> area(poly2)
ans =
0.1962
