How to draw a line at wanted direction through polygon center and trim it at the 2 opposite polygon borders

[MichaelLev]

In my custom widget I need to draw some parallel lines inside polygon,

so that each of them will not go outside the polygon.

If I'll know how to create and draw a line through the polygon center,

I'll know how to draw all of them.

[MichaelLev · Accepted solution]

I solved it - 

I wrote myself all the code needed, using

1) Draw a line from a point knowing the direction and distance from it 

2) the polygon method contains(point)